Ecological Factors Critical to the Exploitation of Entomopathogens in

Chapter 4

Ecological Factors Critical to the Exploitation of Entomopathogens in Pest Control James R. Fuxa

Downloaded by RUTGERS UNIV on May 29, 2018 | https://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0595.ch004

Department of Entomology, Louisiana Agricultural Experiment Station, Louisiana State University Agricultural Center, Baton Rouge, LA 70803

The use o f " b i o r a t i o n a l " agents such as entomopathogens f o r pest c o n t r o l is based i n ecology. Aspects o f environmental r e l e a s e such as t i m i n g and placement must be based on e c o l o g i c a l c o n s i d e r a t i o n s for entomopathogens t o be e f f i c a c i o u s . Timing and placement depend on entomopathogen species c h a r a c t e r i s t i c s (life c y c l e , host specificity, p o r t a l o f entry, site o f attack, s e a r c h i n g ability, v i r u l e n c e , speed o f a c t i o n , r e p r o d u c t i v e c a p a c i t y , transmission) and p o p u l a t i o n c h a r a c t e r i s t i c s (population d e n s i t y , d i s t r i b u t i o n , spread, p e r s i s t e n c e ) . The t a r g e t pest a l s o has s p e c i e s (pest category, r- o r K- s e l e c t i o n , number o f generations, behavior) as w e l l as p o p u l a t i o n c h a r a c t e r i s t i c s ( p o p u l a t i o n d e n s i t y , d i s t r i b u t i o n , age s t r u c t u r e , q u a l i t y ) critical to agent application. Important ecosystem c h a r a c t e r i s t i c s i n c l u d e the h a b i t a t and its stability, the crop o r resource and its value, a g r i c u l t u r a l p r a c t i c e s , the p e s t complex, a b i o t i c and biotic environmental v a r i a b l e s , and environmental r i s k s . Ecological considerations f o r a p p l i c a t i o n o f b i o r a t i o n a l agents can be complex, but not t o a degree t h a t should prevent t h e i r implementation.

C e r t a i n " b i o r a t i o n a l " agents might be u s e f u l i n p e s t management. These b i o r a t i o n a l agents are microorganisms or chemicals found i n nature, o r chemicals s y n t h e s i z e d by man t o mimic n a t u r a l chemicals. Microorganisms i n c l u d e v i r u s e s , b a c t e r i a , protozoa, f u n g i , nematodes, and some o f their by-products (toxins). Biochemicals include 0097-6156/95/0595-0042$13.50/0 © 1995 American Chemical Society

Hall and Barry; Biorational Pest Control Agents ACS Symposium Series; American Chemical Society: Washington, DC, 1995.


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semiochemicals, hormones, natural plant regulators, n a t u r a l i n s e c t growth r e g u l a t o r s , and enzymes. The major r a t i o n a l e f o r development of such agents f o r pest c o n t r o l i s t h e i r environmental s a f e t y . For example, the i n s e c t pathogens are h o s t - s p e c i f i c t o the degree t h a t they have caused v i r t u a l l y no environmental harm upon r e l e a s e (1). On the other hand, these " b i o r a t i o n a l " agents, though having excellent potential for suppressing pest p o p u l a t i o n s , can be i n h e r e n t l y d i f f i c u l t t o use. One reason agents such as the entomopathogens can be d i f f i c u l t t o use i s due t o problems i n d e l i v e r y of the agent i n a t i m e l y manner and s u i t a b l e l o c a t i o n such t h a t the agent i s i n a p o s i t i o n t o exert i t s p e s t - s u p p r e s s i v e action. For example, i t has been estimated t h a t a v i r u s p r e p a r a t i o n sprayed f o r i n s e c t c o n t r o l can l o s e 70% of i t s a c t i v i t y before i t even impacts the t a r g e t f o l i a g e (2) . The requirements f o r such d e l i v e r y are based h e a v i l y i n ecology. Pathogens, l i k e other l i v i n g organisms, must f i t i n t o an e c o l o g i c a l niche i f they are t o s u r v i v e and function. A d d i t i o n a l l y , the concept of pest c o n t r o l , i n t e g r a t e d pest management, i s based i n ecology. The purpose of t h i s paper i s t o outline the e c o l o g i c a l c o n s i d e r a t i o n s — the pathogen p o p u l a t i o n , pest p o p u l a t i o n , and ecosystem or environmental f a c t o r s — t h a t might affect the use of entomopathogens in pest management, with emphasis on d e l i v e r y , or t i m i n g and placement of r e l e a s e . Basic

Ecology

In order t o d i s c u s s ecology r e l a t i v e t o pest management with entomopathogens, i t i s necessary t o e s t a b l i s h c e r t a i n d e f i n i t i o n s and concepts. "Ecology" i s d i f f i c u l t t o d e f i n e to everyone's s a t i s f a c t i o n . A d e f i n i t i o n proposed by Andrewartha (see 3) i s s u i t a b l e : "ecology i s the s c i e n t i f i c study of the d i s t r i b u t i o n and abundance of organisms." Regardless of the exact d e f i n i t i o n , an important concept i n ecology i s t h a t every i n d i v i d u a l i n a p o p u l a t i o n of a given s p e c i e s i s p a r t of the environment of other i n d i v i d u a l s of t h a t s p e c i e s (4). The i n d i v i d u a l organism i s a b a s i c u n i t f o r study (5) . A group of i n d i v i d u a l s comprises a p o p u l a t i o n , d e f i n e d by Mayr as "the group of p o t e n t i a l l y i n t e r b r e e d i n g i n d i v i d u a l s a t a given l o c a l i t y , " which r e s u l t s i n a s i t u a t i o n whereby a l l members of a l o c a l p o p u l a t i o n share i n a s i n g l e gene p o o l (5) . Another concept that has been adopted by a g r i c u l t u r a l s c i e n t i s t s i n general and b i o l o g i c a l c o n t r o l s p e c i a l i s t s i n p a r t i c u l a r i s t h a t of the "ecosystem" (6). The ecosystem i s comprised of the community (a l e v e l of o r g a n i z a t i o n higher than the p o p u l a t i o n , c o n s i s t i n g of c o e x i s t i n g interdependent populations) and i t s p h y s i c a l



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environment (5). An organism's environment o f t e n i s separated i n t o b i o t i c and a b i o t i c , or p h y s i c a l , f a c t o r s (e.g., 7), though environmental f a c t o r s can be d i v i d e d i n other ways (4) . The l i n k a g e s among p o p u l a t i o n s and other factors i n an ecosystem are complex and lead to fluctuations i n population d e n s i t i e s . E p i z o o t i o l o g y of entomopathogens and pest management, i n c l u d i n g m i c r o b i a l c o n t r o l of i n s e c t s , are h e a v i l y based i n e c o l o g i c a l p r i n c i p l e s . E p i z o o t i o l o g y can be d e f i n e d as the s c i e n c e of causes and forms of the mass phenomena of d i s e a s e a t a l l l e v e l s of i n t e n s i t y i n a host p o p u l a t i o n (8) . In other words, i t i s the study of animal d i s e a s e a t the p o p u l a t i o n l e v e l . E p i z o o t i o l o g y encompasses the t o t a l environment i n c l u d i n g the host and pathogen p o p u l a t i o n s and thus is heavily allied with ecology. If entomopathogens and other " b i o r a t i o n a l " agents are t o be used f o r pest p o p u l a t i o n suppression, they must be integrated into pest management. Pest management i n c o r p o r a t e s a wide v a r i e t y of approaches aimed a t maintaining pest populations below economic injury t h r e s h o l d s , or the numbers of i n s e c t s t h a t w i l l cause damage equal t o the c o s t of a r t i f i c i a l c o n t r o l . Thus, a prime purpose of m i c r o b i a l c o n t r o l i s t o i n c r e a s e d i s e a s e l e v e l s i n i n s e c t pest p o p u l a t i o n s . The time and p l a c e where these two populations (pest and pathogen) i n t e r a c t , and subsequent p o p u l a t i o n dynamics, depend h e a v i l y on t h e i r ecology. The approach by which an entomopathogen i s used t o suppress a pest p o p u l a t i o n i s important t o the manner i n which the pathogen i s u t i l i z e d . There have been t h r e e such approaches i n which entomopathogens are a r t i f i c i a l l y produced and r e l e a s e d (9). In the m i c r o b i a l i n s e c t i c i d e approach, r e l a t i v e l y l a r g e numbers of pathogen u n i t s are r e l e a s e d f o r quick suppression of the pest p o p u l a t i o n . R e s i d u a l e f f e c t s are not s i g n i f i c a n t , and subsequent i n c r e a s e s i n the pest p o p u l a t i o n t o damaging l e v e l s r e q u i r e a d d i t i o n a l r e l e a s e s of the pathogen. The seasonal c o l o n i z a t i o n approach amounts t o a "booster shot;" the r e l e a s e r e s u l t s i n r e p l i c a t i o n of the pathogen and suppression of more than one pest generation. T h i s may or may not be aimed a t immediate knockdown of the pest p o p u l a t i o n , and subsequent r e l e a s e s are r e q u i r e d , u s u a l l y i n each new growing season. Introduction-establishment r e s u l t s i n permanent pest p o p u l a t i o n suppression; the entomopathogen s p e c i e s or s t r a i n becomes a permanent p a r t of the ecosystem i n which i t i s r e l e a s e d . E c o l o g i c a l c o n s i d e r a t i o n s f o r the a p p l i c a t i o n of entomopathogens i n pest management i n c l u d e pathogen c h a r a c t e r i s t i c s , pest c h a r a c t e r i s t i c s , and the ecosystem. Pathogen A t t r i b u t e s D i f f e r e n t entomopathogens have c h a r a c t e r i s t i c s t h a t a f f e c t every phase of developing and u t i l i z i n g them f o r m i c r o b i a l


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control. Some c h a r a c t e r i s t i c s , such as t h e i r c a p a b i l i t y to be produced i n e x p e n s i v e l y i n l a r g e q u a n t i t i e s f o r r e l e a s e , r e l a t e only i n d i r e c t l y t o the a p p l i c a t i o n and e f f i c a c y of pathogens i n the ecosystem and w i l l not be d i s c u s s e d i n t h i s paper. The " e c o l o g i c a l " f a c t o r s t h a t relate to application f a l l i n t o two major groups: characteristics of the species or strain and c h a r a c t e r i s t i c s of the p o p u l a t i o n . Pathogen Species o r S t r a i n . C e r t a i n c h a r a c t e r i s t i c s o f an entomopathogen t h a t r e l a t e t o ecology of a p p l i c a t i o n are inherent t o the pathogen s p e c i e s or s t r a i n and thus f u n c t i o n on t h e l e v e l of an i n d i v i d u a l pathogen u n i t ' s a t t a c k on the i n s e c t host. Most c h a r a c t e r i s t i c s of pathogen s p e c i e s o r s t r a i n s (subspecies) a f f e c t t i m i n g o f a p p l i c a t i o n , though some a f f e c t placement. Perhaps t h e most obvious t i m i n g c o n s i d e r a t i o n f o r an entomopathogen i s t h a t i t s l i f e c y c l e be temporally synchronized with t h a t o f i t s host i n s e c t i n a way s u i t a b l e f o r pest management. This i s p o t e n t i a l l y a g r e a t e r problem with r e s p e c t t o the p e r s i s t e n t approaches to microbial control — seasonal c o l o n i z a t i o n and, e s p e c i a l l y , introduction-establishment. The pathogen might be unable t o f u n c t i o n , f o r example, i f i t i s quiescent while the pest i s a c t i v e . There a r e examples o f poor synchrony i n attempted b i o l o g i c a l c o n t r o l with p a r a s i t o i d s (10). One example i n n a t u r a l pest p o p u l a t i o n s u p r e s s i o n by an entomopathogen i s t h a t o f the fungus Nomuraea rileyi i n c a t e r p i l l a r p e s t s o f soybean i n the southeastern United S t a t e s . T h i s fungus i s c o n s i s t e n t l y a major m o r t a l i t y agent i n t h i s soybean system, but the high m o r t a l i t y u s u a l l y occurs t o o l a t e i n t h e season t o prevent crop damage (11). T h i s example represents a l a c k of synchrony of the pathogen with r e s p e c t t o pest management r a t h e r than with r e s p e c t t o t h e host life cycle, but i t presents an opportunity f o r fungal a p p l i c a t i o n t o c o r r e c t f o r the asynchrony. Host s p e c i f i c i t y has a s u b t l e e f f e c t on a p p l i c a t i o n . A l l f i v e major pathogen groups have a t l e a s t a few s p e c i e s t h a t are r e l a t i v e l y host s p e c i f i c , though such s p e c i f i c i t y i s perhaps most common among the v i r u s e s and protozoa. Host s p e c i f i c i t y l i m i t e d t o pest i n s e c t s i s , o f course, a major advantage f o r pest c o n t r o l due t o environmental s a f e t y . However, a high degree of s p e c i f i c i t y i s a severe problem i n commercial development, because narrow host ranges r e s t r i c t market s i z e as w e l l as u s e f u l n e s s i n pest complexes (12) . I f an entomopathogen has a wide host range, then "secondary" hosts can support viral replication and, t h e r e f o r e , i t s persistence i n the environment. The a v a i l a b i l i t y o f a g r e a t e r number of hosts, due t o the g r e a t e r number o f s u s c e p t i b l e s p e c i e s , simply i n c r e a s e s the inoculum t h a t can r e s u l t . There i s c i r c u m s t a n t i a l evidence t h a t t h i s can occur with c e r t a i n v i r u s e s with r e l a t i v e l y wide host ranges (3). In such a



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case, long-term c o n t r o l , f o r example through seasonal colonization, might r e q u i r e l e s s frequent and l e s s extensive a p p l i c a t i o n s o f t h e pathogen. Another inherent c h a r a c t e r i s t i c o f entomopathogens i s the " p o r t a l o f e n t r y " i n t o t h e host i n s e c t . Bacteria, v i r u s e s , and protozoa v i r t u a l l y always must be i n g e s t e d when they a r e a r t i f i c i a l l y a p p l i e d f o r i n s e c t c o n t r o l . Nematodes and f u n g i invade p r i m a r i l y through the e x t e r n a l integument o r through body openings other than the mouth, though many can invade a f t e r being ingested and a few must be ingested i n order t o i n f e c t the i n s e c t . A t h i r d p o r t a l has been hypothesized t o be important i n t h e longer term approaches t o c o n t r o l (13-14); t h i s i s by v e r t i c a l , o r p a r e n t - t o - o f f s p r i n g , t r a n s m i s s i o n , which i s thought t o occur i n many o f t h e v i r u s e s and protozoa. A critical problem t o any organism with an i n t e r n a l l y p a r a s i t i c l i f e s t y l e i s i n v a s i o n o f the host o r , i n other words, having the opportunity t o contact a host and t h e c a p a b i l i t y t o surmount t h a t host's e x t e r n a l b a r r i e r s (e.g., the integument) t o i n v a s i o n . P o r t a l o f entry i s c r i t i c a l t o t i m i n g and placement of a p p l i c a t i o n f o r m i c r o b i a l c o n t r o l , and i t i n t e r a c t s with the l i f e s t y l e and feeding h a b i t s o f t h e i n s e c t host as w e l l as c e r t a i n environmental f a c t o r s (discussed below) . For a pathogen t h a t must be ingested, a p p l i c a t i o n must be synchronized t o t h e i n s e c t ' s f e e d i n g so t h a t t h e pathogen w i l l be ingested before t h e i n s e c t i s t o o o l d t o be i n f e c t e d and before t h e pathogen can be i n a c t i v a t e d by environmental f a c t o r s such as s u n l i g h t . For example, t i m i n g o f a p p l i c a t i o n i s important t o t h e e f f i c a c y o f t h e bacterium Bacillus thuringiensis (15-16). Timing and placement a r e c r i t i c a l f o r an i n s e c t t h a t feeds i n p r o t e c t e d l o c a t i o n s , such as a f r u i t feeder l i k e Heliothis spp., because t h e pathogen must be ingested while t h e i n s e c t feeds on the s u r f a c e o f the f r u i t . After i t burrows, d e l i v e r y o f the pathogen so t h a t i t can be ingested becomes v i r t u a l l y impossible. Synchrony with t h e host's h a b i t s i s not as c r i t i c a l f o r pathogens with an e x t e r n a l p o r t a l o f entry, though other f a c t o r s can s t i l l make t i m i n g important. Entry through t h e integument a l s o a f f e c t s the host i n s e c t s t h a t can be t a r g e t e d f o r c o n t r o l ; p l a n t - s u c k i n g i n s e c t s a r e v i r t u a l l y immune t o c o n t r o l attempts by pathogens t h a t must be ingested. I t has been hypothesized t h a t a v e r t i c a l t r a n s m i s s i o n p o r t a l o f entry lowers the t h r e s h o l d o f host p o p u l a t i o n d e n s i t y f o r s u c c e s s f u l i n t r o d u c t i o n - e s t a b l i s h m e n t (17-18); t h i s lower t h r e s h o l d i n t u r n can allow g r e a t e r f l e x i b i l i t y i n t i m i n g and placement o f r e l e a s e . In a d d i t i o n t o p o r t a l of entry, t h e s i t e o f i n v a s i o n by pathogens can be l i m i t e d t o c e r t a i n stages, ages, and t i s s u e s o f t h e host i n s e c t , which i n t u r n can a f f e c t t i m i n g and placement o f a p p l i c a t i o n . The v a s t m a j o r i t y o f entomopathogens i n nature a t t a c k only one stage, u s u a l l y the feeding stage o f t h e host (e.g., l a r v a e ) , and not



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others (e.g., eggs or adults). In fact, most entomopathogens are even more l i m i t e d ; i n s e c t l a r v a e u s u a l l y become more d i f f i c u l t t o i n f e c t as they age ( i . e . , "maturation immunity") (3) . Greater r a t e s of f e e d i n g compensate t o some degree f o r t h i s reduced s u s c e p t i b i l i t y (3), but not enough t o completely counteract i t . T h i s again a f f e c t s t i m i n g of a p p l i c a t i o n . For best r e s u l t s , the pathogen must be a p p l i e d t o maximize contact between i t and the most s u s c e p t i b l e ages of the t a r g e t i n s e c t . The importance of t i m i n g due t o maturation immunity has been best demonstrated f o r B. thuringiensis (e.g., 15-16) . T i s s u e s of the host i n s e c t t h a t are attacked by the pathogen can a f f e c t a p p l i c a t i o n . I f midgut i s a s i t e of i n f e c t i o n and l a r g e q u a n t i t i e s of the pathogen are voided through the gut, t h i s can c o n t r i b u t e t o widespread d i s t r i b u t i o n of the pathogen. For example, c e r t a i n v i r u s e s can be r e l e a s e d a t very l i m i t e d l o c a t i o n s and then spread throughout a pest i n s e c t ' s geographical range by t h i s mechanism (3). A few pathogens, p a r t i c u l a r l y c e r t a i n nematodes and f u n g i , have l i m i t e d searching a b i l i t y . Nematodes such as Steineraejna carpocapsae and Romanomermis culicivorax, and a q u a t i c f u n g i such as Lagenidium giganteum can move s h o r t d i s t a n c e s and a c t i v e l y contact the host i n s e c t p r i o r t o the i n f e c t i o n process. T h i s c a p a b i l i t y , p a r t i c u l a r l y with nematodes t a r g e t e d a g a i n s t c e r t a i n t e r r e s t r i a l i n s e c t s , can be an important f a c t o r i n placement of a f i e l d application. Most entomopathogens must be timed and p l a c e d a c c u r a t e l y i n order t o i n f e c t an i n s e c t p r e p a r i n g t o burrow i n t o a p l a n t p a r t or s o i l . Once an i n s e c t has burrowed, i t can be v i r t u a l l y impossible t o d e l i v e r most pathogens t o a s u i t a b l e contact p o i n t with the host, p a r t i c u l a r l y i f the pathogen must be ingested. If a pathogen such as a nematode can search even a d i s t a n c e of 2-3 cm., i t only has t o be d e l i v e r e d i n t o the general v i c i n i t y of the host i n s e c t f o r i n f e c t i o n t o take p l a c e . For example, h e t e r o r h a b d i t i d nematodes have a tendency t o move downward i n s o i l , g i v i n g them good p o t e n t i a l f o r c o n t r o l of such " c r y p t i c " i n s e c t s as Japanese b e e t l e s , b i l l b u g s , and r o o t weevils (19). V i r u l e n c e i s the disease producing power of a pathogen, a c h a r a c t e r i s t i c o f t e n a s s o c i a t e d with s t r a i n s w i t h i n a pathogenic s p e c i e s . V i r u l e n c e i s sometimes measured i n terms of time r e q u i r e d t o k i l l a host but i s more o f t e n measured as number of pathogen u n i t s r e q u i r e d t o k i l l a c e r t a i n p o r t i o n of a group of host i n s e c t s . V i r u l e n c e can a f f e c t a p p l i c a t i o n i n two ways. First, a more v i r u l e n t pathogen r e q u i r e s fewer pathogen u n i t s a t the p o i n t of contact with a host i n s e c t i n the f i e l d than a l e s s v i r u l e n t pathogen. T h i s i n t u r n a f f e c t s placement of a p p l i c a t i o n . Thus, increased v i r u l e n c e i s o f t e n an o b j e c t i v e of recombinant-DNA research (9). Second, v i r u l e n c e requirements can change with the approach t o m i c r o b i a l c o n t r o l ; i t has been hypothesized t h a t v i r u l e n t



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s t r a i n s may not be i d e a l f o r the longer-term approaches, p a r t i c u l a r l y i n t r o d u c t i o n - e s t a b l i s h m e n t (20). For example, h i g h l y v i r u l e n t s t r a i n s of the bacterium Bacillus popilliae do not e s t a b l i s h and c o n t r o l Japanese b e e t l e s as w e l l as s t r a i n s with moderate v i r u l e n c e (16). Thus, virulence can affect placement indirectly, since a p p l i c a t i o n requirements are l e s s c r i t i c a l f o r r e l a t i v e l y permanent c o n t r o l by i n t r o d u c t i o n - e s t a b l i s h m e n t . One of the simplest way to categorize entomopathogens, and one t h a t can a f f e c t t i m i n g of a p p l i c a t i o n , i s by speed of a c t i o n . In t h i s r e s p e c t , entomopathogens f a l l i n t o two c a t e g o r i e s : "quick damage" and "slow" pathogens. The quick pathogens stop i n s e c t f e e d i n g w i t h i n 24 h, though death may take s e v e r a l days. These mostly i n c l u d e those t h a t produce t o x i n s (e.g., B. thuringiensis) or those that initiate a bacterial septicemia (e.g., S. carpocapsae). The slow pathogens d e b i l i t a t e t h e i r hosts a f t e r 3-4 d or more by more t y p i c a l " p a r a s i t i c " a c t i o n . These comprise the g r e a t m a j o r i t y of n a t u r a l s t r a i n s of entomopathogens, i n c l u d i n g v i r t u a l l y a l l the v i r u s e s , f u n g i , and protozoa, and many of the nematodes and b a c t e r i a . A major emphasis of recombinantDNA r e s e a r c h of entomopathogens i s t o improve t h i s slow a c t i o n , because, u n t i l the i n s e c t i s d e b i l i t a t e d by a slow pathogen, i t continues t o feed (cause damage) and be observed by users who expect quick a c t i o n . For example, slow a c t i o n has been c i t e d as a reason f o r f a i l u r e of c e r t a i n pathogens i n the p e s t i c i d e market (21-23). In c e r t a i n cases, t i m i n g of a p p l i c a t i o n can a l l e v i a t e t h i s problem. The nuclear p o l y h e d r o s i s v i r u s (NPV) of Anticarsia gemmatalis, a soybean pest, was s i g n i f i c a n t l y more e f f e c t i v e i n reducing pest numbers and damage when i t was sprayed s e v e r a l days before pest numbers reached thresholds developed for a p p l i c a t i o n of chemical p e s t i c i d e s (24). One of the b a s i c c h a r a c t e r i s t i c s of a s p e c i e s from an e c o l o g i c a l viewpoint i s i t s innate c a p a c i t y f o r i n c r e a s e , which r e l a t e s p a r t l y t o i t s r e p r o d u c t i v e r a t e . The " p a r a s i t i c " l i f e s t y l e of many entomopathogens, due t o i t s r i s k y nature, p a r t i c u l a r l y i n host-to-host t r a n s f e r , o f t e n r e s u l t s i n a high r e p r o d u c t i v e r a t e . For example, as many as t h r e e generations of an entomopathogenic v i r u s may be produced i n the f i e l d w i t h i n one generation of the host i n s e c t (25), with numbers of v i r a l p o l y h e d r a l i n c l u s i o n bodies commonly exceeding 10 per host i n s e c t (26). High r a t e s of pathogen r e p l i c a t i o n can a f f e c t a p p l i c a t i o n i n the long-term approaches t o c o n t r o l ; the extensive environmental contamination t h a t can r e s u l t from high pathogen r e p r o d u c t i o n or r e p l i c a t i o n r a t e s can reduce frequency and amount of pathogen a p p l i c a t i o n . Transmission has been c a l l e d "one of the key ecological f a c t o r s t h a t must be understood before entomopathogens can be manipulated" (25) . In nature, host-to-host t r a n s f e r can be a "weak l i n k " i n the l i f e 9



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c y c l e of these pathogens, p r o v i d i n g a n a t u r a l p o i n t t o enhance t h e i r a c t i v i t y through t i m e l y and accurate delivery. In the m i c r o b i a l i n s e c t i c i d e approach t o control, transmission is replaced by artificial application. Thus, i t i s c r i t i c a l t h a t pathogen t i m i n g and placement account f o r the ecology of natural transmission. Most n a t u r a l s t r a i n s of entomopathogens are used f o r seasonal c o l o n i z a t i o n or i n t r o d u c t i o n - e s t a b l i s h m e n t . For these approaches, n a t u r a l t r a n s m i s s i o n i s c r i t i c a l (9, 25, 27-28). Most i n s e c t pathogens have some stage s p e c i a l i z e d f o r s u r v i v a l o u t s i d e the host so t h a t they can t r a n s f e r t o a new host. These pathogens are t r a n s p o r t e d by a v a r i e t y of a b i o t i c and b i o t i c f a c t o r s (29). Transmission can a f f e c t placement of a pathogen f o r microbial control. The best examples come from a technique c a l l e d " a u t o d i s p e r s a l , " whereby the pathogen i s a p p l i e d over a l i m i t e d area and i s then d i s p e r s e d by i n f e c t e d or contaminated i n s e c t s . An example i s the d i s p e r s a l of a b a c u l o v i r u s f o r long-term c o n t r o l of palm r h i n o c e r o s b e e t l e , Oryctes rhinoceros (30-31). V e r t i c a l , or p a r e n t - t o - o f f s p r i n g , t r a n s m i s s i o n , found i n many protozoa and v i r u s e s , might provide a method f o r pathogen a p p l i c a t i o n and d i s p e r s a l f o r m i c r o b i a l c o n t r o l (28). For these approaches t o c o n t r o l , t r a n s m i s s i o n might lower the t h r e s h o l d of host p o p u l a t i o n d e n s i t y f o r s u c c e s s f u l i n t r o d u c t i o n and lower the minimum i n t r o d u c t i o n r a t e of the pathogen (the amount t h a t must be d e l i v e r e d t o the p o i n t of contact with the i n s e c t ) (17-18, 32). Vertical t r a n s m i s s i o n i s perhaps the most e f f i c i e n t route of t r a n s f e r of a pathogen between hosts (3). Pathogen Population C h a r a c t e r i s t i o s . The p o p u l a t i o n i s a b a s i c u n i t of ecology of a s p e c i e s , and the pathogen population i s a unit basic to a p p l i c a t i o n f o r microbial control. Basic population characteristics include density, distribution, and spread. Persistence i s b a s i c a l l y a c h a r a c t e r i s t i c of the taxon but u s u a l l y i s measured as a p o p u l a t i o n parameter. Population density i s one of the most basic characteristics i n ecology, and pathogen population d e n s i t y i s one of the most important f a c t o r s i n d i s e a s e e p i z o o t i c s . Due t o the i n t e r e s t i n m i c r o b i a l c o n t r o l and e p i z o o t i o l o g y , there are numerous examples of d o s e - r e l a t e d response by i n s e c t s t o entomopathogens both i n the f i e l d and l a b o r a t o r y (e.g., 33-37). However, t h e r e i s v i r t u a l l y no information about a c t u a l dosages d e l i v e r e d t o i n s e c t s in the field (38), though there t h e o r e t i c a l l y are t h r e s h o l d s f o r pathogen p o p u l a t i o n d e n s i t y t o develop e p i z o o t i c s (39). I t i s i n t u i t i v e l y obvious t h a t , a l l other f a c t o r s being equal, a g r e a t e r pathogen p o p u l a t i o n d e n s i t y simply i n c r e a s e s the chance of contact between a pathogen and an u n i n f e c t e d host. Thus, the pathogen p o p u l a t i o n d e n s i t y d e l i v e r e d a t the a p p r o p r i a t e time and



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p l a c e through m i c r o b i a l c o n t r o l technology w i l l impact h e a v i l y on success of the c o n t r o l e f f o r t . The d i s t r i b u t i o n of an organism i s another b a s i c e c o l o g i c a l parameter. The e f f e c t of pathogen d i s t r i b u t i o n i n m i c r o b i a l c o n t r o l i s not c l e a r . Pathogens most l i k e l y have clumped d i s t r i b u t i o n s i n nature (40); t h i s may e x p l a i n how i n s e c t s encounter high doses i n nature and why poor i n f e c t i o n r a t e s o f t e n r e s u l t when a pathogen i s sprayed evenly i n t o an ecosystem. Harper (25) p o i n t e d out t h a t much of a spray a p p l i c a t i o n i s wasted. Of course, t h i s i s a l s o the case i n nature; p a r a s i t i s m i s a r i s k y l i f e s t y l e , and most pathogen t r a n s m i s s i v e u n i t s never encounter a host, which i s a major reason t h a t they r e p l i c a t e i n such l a r g e numbers. This implies that pathogen clumping might b e n e f i t m i c r o b i a l c o n t r o l (3) . Degree of clumping, as determined by spray d r o p l e t s i z e , has a f f e c t e d c o n t r o l experiments (37). On the other hand, "thorough" coverage, presumably meaning even d i s t r i b u t i o n , has been c a l l e d an important goal for application technology (16) . Thus, there i s a good p o s s i b i l i t y t h a t d i s t r i b u t i o n i s a c r i t i c a l parameter f o r placement of m i c r o b i a l s , y e t i t a l s o i s c l e a r t h a t much research i s necessary i n t h i s area. Spread of a pathogen p o p u l a t i o n i s c l o s e l y r e l a t e d t o the intrinsic c a p a b i l i t y of that pathogen to be transmitted. Fuxa (29) reviewed the a b i o t i c and b i o t i c environmental agents t h a t t r a n s p o r t pathogens, as w e l l as the e f f e c t of biotechnology on t r a n s p o r t . However, d i s p e r s a l and t r a n s p o r t of pathogens a f t e r environmental release i s only p o o r l y understood (29, 41). Yet c a p a b i l i t y f o r spread c l e a r l y can a f f e c t placement of application. For example, the NPV of A. gemmatalis spreads at a r a t e of approximately 1 m per day a f t e r i t s r e l e a s e i n t o soybean. I t has been estimated t h a t t h i s v i r u s can be sprayed a t i n t e r v a l s of 22 m and 100 m t o provide s a t i s f a c t o r y i n s e c t c o n t r o l through seasonal c o l o n i z a t i o n and introduction-establishment, r e s p e c t i v e l y (42). Environmental persistence is a characteristic i n t r i n s i c t o pathogen species and s t r a i n s , but i t u s u a l l y i s measured as a population parameter. P e r s i s t e n c e i s one of the few f a c t o r s c r i t i c a l t o a l l the approaches t o m i c r o b i a l c o n t r o l (20). The pathogen must p e r s i s t a t the p o i n t of contact with the i n s e c t long enough t o be encountered; f o r the long-term approaches t o c o n t r o l , persistence somewhere i n the h a b i t a t i s an obvious prerequisite. Entomopathogens g e n e r a l l y p e r s i s t only s h o r t time p e r i o d s , measured i n terms of one or a few days, on exposed surfaces (41); s u n l i g h t q u i c k l y k i l l s a l l pathogens, and moisture i s c r i t i c a l t o nematodes and f u n g i . I t i s i n t u i t i v e l y obvious, t h e r e f o r e , t h a t t i m i n g and persistence are critically interrelated in entomopathogen a p p l i c a t i o n ; the r e l e a s e must be timed so t h a t the i n s e c t encounters the pathogen before i t i s


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i n a c t i v a t e d . For the longer-term approaches, p e r s i s t e n c e through r e c y c l i n g reduces the number of times t h a t an entomopathogen must be r e l e a s e d . Pest A t t r i b u t e s


The pest i n s e c t i s a component of the environment, one as critical as any t o the timing and a p p l i c a t i o n of entomopathogens. A pest can be d e f i n e d simply as "an organism d e t r i m e n t a l t o man" (43). No organism i s intrinsically a pest, but i t becomes one when i t s l i f e s t y l e somehow c o n f l i c t s with man. Pest a t t r i b u t e s , l i k e those of a pathogen, can be d i v i d e d i n t o s p e c i e s (or s t r a i n ) and p o p u l a t i o n c h a r a c t e r i s t i c s . Pest Species or S t r a i n * Pests can be d i v i d e d i n t o c a t e g o r i e s which can impact timing f o r m i c r o b i a l c o n t r o l . Key pests are those organisms t h a t appear y e a r l y a t such high l e v e l s t h a t c o n t r o l measures are necessary i f economic l o s s e s are t o be avoided; populations of o c c a s i o n a l pests grow t o damaging l e v e l s on an o c c a s i o n a l b a s i s , when n a t u r a l r e g u l a t i n g f a c t o r s do not keep the p o p u l a t i o n r e s t r a i n e d (44). These c a t e g o r i e s i n f l u e n c e a p p l i c a t i o n of m i c r o b i a l c o n t r o l agents i n much the same way as other control agents, such as chemicals. A p p l i c a t i o n f o r key pests can almost be timed on a scheduled b a s i s , as, f o r example, with Heliothis spp. i n cotton, though pest s c o u t i n g i s s t i l l r e q u i r e d . Sampling f o r pests and t i m i n g c o n t r o l measures i n response t o p o p u l a t i o n increases i s u s u a l l y the best procedure f o r occasional pests. Another way t o c o n c e p t u a l i z e pests populations i s according t o the theory of r-K s e l e c t i o n . T h i s theory holds t h a t there i s a continuum of s p e c i e s based on t h e i r l i f e histories. At one end are the r - s e l e c t e d s p e c i e s , those t h a t take advantage of temporary h a b i t a t s and usually characterized by swift development, early breeding, high r e p r o d u c t i v e r a t e , r e l a t i v e l y small s i z e , and polyphagy. K - s e l e c t e d species s p e c i a l i z e i n f u l l y e x p l o i t i n g s p e c i a l niches i n s t a b l e h a b i t a t s , and they are c h a r a c t e r i z e d by low reproductive r a t e s , good competitive a b i l i t i e s , low numbers of h i g h - q u a l i t y progeny, l a r g e s i z e , e f f e c t i v e defenses against n a t u r a l enemies, and long l i f e - s p a n s (45-46). Several authors have suggested t h a t the r-K continuum has a bearing on m i c r o b i a l c o n t r o l (25, 46-48). Anderson (32) suggested t h a t a p p l i c a t i o n s of a m i c r o b i a l once every few years could s u f f i c e f o r c o n t r o l of c e r t a i n f o r e s t pests (K-selected), whereas frequent a p p l i c a t i o n s g e n e r a l l y are necessary i n row crops ( r selected pests). E n t w i s t l e (46) proposed t h a t the r-K continuum should be considered i n a p p l i c a t i o n s t r a t e g i e s f o r v i r u s e s i n m i c r o b i a l c o n t r o l . For r - s e l e c t e d p e s t s , autodissemination and e a r l y i n t r o d u c t i o n s ( i . e . , seasonal c o l o n i z a t i o n ) are p o s s i b l e approaches; l a t t i c e



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i n t r o d u c t i o n s (spot a p p l i c a t i o n a t i n t e r v a l s t o lower c o s t s , depending on v i r a l spread t o evenly d i s t r i b u t e i t s e l f ) and s i n g l e sprays are not l i k e l y t o be u s e f u l ; and m u l t i p l e a p p l i c a t i o n s are o f t e n necessary. For K - s e l e c t e d pests, early introductions are unlikely to work; autodissemination, lattice introductions, and single sprays o f t e n are v a l u a b l e ; and m u l t i p l e sprays are sometimes e s s e n t i a l . Another pest species c h a r a c t e r i s t i c i s i t s number of generations. U n i v o l t i n e i n s e c t s produce one generation per year, and m u l t i v o l t i n e produce more than one. M u l t i v o l t i n e pests may r e q u i r e e i t h e r p e r s i s t e n t agents or m u l t i p l e a p p l i c a t i o n s . For example, b r i q u e t s d i s p e n s i n g B. thuringiensis over a prolonged time p e r i o d were developed f o r mosquitoes with a continuous succession of generations (15). M u l t i p l e a p p l i c a t i o n s of pathogens, or pathogens t h a t can r e c y c l e and c o n t r o l pests f o r the remainder of a growing season, would be necessary f o r other m u l t i v o l t i n e pests, whereas i n s e c t s with f e e d i n g stages present only a few days, such as western corn rootworm (Diabrotica virgifera virgifera) , present only a narrow window of opportunity f o r t i m i n g an a p p l i c a t i o n (19). Behavior i s , arguably, the most important host species c h a r a c t e r i s t i c with respect to timing and placement of entomopathogen a p p l i c a t i o n . Behavior can be age-related. For example, young l a r v a e of many i n s e c t s , such as Pieris rapae and Trichoplusia ni, have a very slow f e e d i n g r a t e (and, t h e r e f o r e , feed over a smaller area) compared with the o l d e r l a r v a e (3) . Thus, placement w i l l be critical f o r these i n s e c t s . Some i n s e c t s are gregarious; there are several examples of disease spreading through a colony of gregarious c a t e r p i l l a r s or sawfly l a r v a e i f a r e l a t i v e l y few can be i n f e c t e d (3) . Insects that feed on plant surfaces usually are comparatively easy t o t a r g e t f o r pathogen a p p l i c a t i o n , whereas t i m i n g as w e l l as placement i s much more d i f f i c u l t f o r an i n s e c t t h a t burrows i n t o a f r u i t or some other s t r u c t u r e (49). For example, B. thuringiensis products have had r e l a t i v e l y l i t t l e success f o r c o n t r o l of f r u i t feeders i n cotton, corn, and apple, but have provided good c o n t r o l of l e a f or s u r f a c e feeders i n cabbage, avocado, s t o r e d products, and f o r e s t s i t u a t i o n s (50). Plant s u r f a c e feeding can be broken down another step; c e r t a i n i n s e c t s , such as Agrotis ipsilon, feed on hypogeal p a r t s of the p l a n t and on leaves c l o s e t o the ground, where spray p e n e t r a t i o n can be poor (50). Sucking i n s e c t s l i m i t the pathogens a v a i l a b l e f o r p o s s i b l e c o n t r o l t o those t h a t can invade through the e x t e r n a l integument; however, such "contact" a c t i o n can somewhat s i m p l i f y placement of a p p l i c a t i o n (49). Insects t h a t i n h a b i t s o i l o b v i o u s l y can cause d e l i v e r y problems i f they are not a t or near the s u r f a c e , though t h i s disadvantage i s perhaps counteracted t o some degree by the f a c t t h a t many entomopathogens



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s u r v i v e w e l l and cause e p i z o o t i c s i n s o i l (49). Aquatic i n s e c t s such as mosquito l a r v a e pose s p e c i a l problems i n placement o f a pathogen because these l a r v a e u s u a l l y frequent and feed i n c e r t a i n s p e c i f i c depths o f water, ranging from the surface t o the bottom s u b s t r a t e (15, 49) . On the other hand, water i s a medium conducive t o the activity of c e r t a i n pathogens, certain fungi and nematodes, t h a t can search f o r a host (pest) i n s e c t , l e s s e n i n g the importance o f d e l i v e r i n g the pathogen t o the exact l o c a t i o n o f t h e i n s e c t . S o c i a l i n s e c t s have a v a r i e t y of behaviors t h a t can make d e l i v e r y d i f f i c u l t , p a r t i c u l a r l y i f one or more queen i n s e c t s must be k i l l e d . Such behaviors were r e c e n t l y reviewed f o r ants (51) . Pest behavior i s p o t e n t i a l l y more important t o placement and t i m i n g o f m i c r o b i a l s than conventional p e s t i c i d e s , many o f which work by contact and p o s s i b l y fumigant a c t i o n i n a d d i t i o n t o i n g e s t i o n (52). Pest P o p u l a t i o n C h a r a c t e r i s t i c s . L i k e the pathogen, the pest has p o p u l a t i o n as w e l l as taxon c h a r a c t e r i s t i c s t h a t a f f e c t m i c r o b i a l c o n t r o l . Insect c o n t r o l with chemical pesticides concentrates on pest population c h a r a c t e r i s t i c s , p a r t i c u l a r l y with r e s p e c t t o t i m i n g o f a p p l i c a t i o n , so i t i s not s u r p r i s i n g t h a t t h i s a l s o i s a consideration f o r microbials. Population d e n s i t y i s a key f a c t o r i n d e f i n i n g a pest; many species of i n s e c t s compete with man f o r v a r i o u s resources, but, i n most cases, only the ones t h a t become s u f f i c i e n t l y numerous t o cause economically significant damage a r e considered p e s t s . I t f o l l o w s t h a t d e n s i t y a l s o i s a key f a c t o r i n determining the t i m i n g o f a p p l i c a t i o n of a c o n t r o l agent. Insect pest management i s based h e a v i l y on such t i m i n g i n the framework o f economic i n j u r y levels. An economic i n j u r y l e v e l i s the pest p o p u l a t i o n d e n s i t y a t which economic l o s s e s begin t o surpass t h e c o s t s of c o n t r o l . The economic t h r e s h o l d , a l s o known as the a c t i o n t h r e s h o l d , i s the pest p o p u l a t i o n d e n s i t y a t which a c e r t a i n c o n t r o l a c t i o n must be taken t o prevent the pest p o p u l a t i o n from reaching the economic i n j u r y l e v e l . I f a c t i o n t h r e s h o l d s are adhered t o by growers o r resource managers, then p e s t i c i d e s a r e a p p l i e d only when needed, thus reducing t h e i r harmful s i d e - e f f e c t s . The problem f o r m i c r o b i a l s i s t h a t a c t i o n t h r e s h o l d s are dynamic and depend on a v a r i e t y o f f a c t o r s , i n c l u d i n g the a c t i v i t y and speed of the c o n t r o l agent. The decision-making process f o r a p p l i c a t i o n o f m i c r o b i a l s , i n c l u d i n g t h e i r t i m i n g of r e l e a s e , i s h e a v i l y dependent on a l a r g e body o f research o f the r e l a t i v e l y q u i c k - a c t i n g chemical p e s t i c i d e s . The a c t i o n t h r e s h o l d s developed f o r chemical p e s t i c i d e s w i l l not always work f o r m i c r o b i a l s . T h i s i s p a r t i c u l a r l y t r u e o f the slow pathogens. For example, A. gemmatalis NPV sprayed a few days before i t s host p o p u l a t i o n reaches t h e a c t i o n t h r e s h o l d f o r chemical i n s e c t i c i d e s was s i g n i f i c a n t l y more e f f e c t i v e i n reducing



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pest numbers and damage t o soybean than v i r u s sprayed at the a c t i o n t h r e s h o l d (24). For d i r e c t pests (those t h a t d i r e c t l y damage the f i n a l product, such as a f r u i t ) , the t i m i n g r e q u i r e d t o apply a slow pathogen i n r e l a t i o n t o an a c t i o n t h r e s h o l d may be so p r e c i s e as t o be i m p r a c t i c a l , because the economic i n j u r y l e v e l i s very low. Another problem with action thresholds, even for chemical i n s e c t i c i d e s , i s t h a t they are d i f f i c u l t t o determine f o r i n s e c t s t h a t v e c t o r diseases or are simply annoying t o man (53). Another aspect of host d e n s i t y i s the concept of host d e n s i t y dependence. Populations of b i o l o g i c a l c o n t r o l agents g e n e r a l l y i n c r e a s e and decrease i n response t o s i m i l a r i n c r e a s e s or decreases i n the d e n s i t y of t h e i r host or prey p o p u l a t i o n s . Such d e n s i t y dependence i s almost c e r t a i n l y t r u e f o r entomopathogens i n general (47) . Density dependence has an i n d i r e c t bearing on t i m i n g of a p p l i c a t i o n of a m i c r o b i a l p e s t i c i d e . Pathogens r e l e a s e d i n the longer-term approaches t o c o n t r o l cannot be r e l e a s e d when host p o p u l a t i o n d e n s i t y i s too low, because t h e r e w i l l l i k e l y be a t h r e s h o l d of host p o p u l a t i o n d e n s i t y below which the pathogen would not be able t o r e p l i c a t e t o a s u f f i c i e n t degree t o c o n t r o l subsequent generations of the pest. There i s evidence f o r such t h r e s h o l d s i n r e l e a s e s of NPV f o r suppression of A. gemmatalis populations i n soybean (42). Pest p o p u l a t i o n d i s t r i b u t i o n i s not as w e l l s t u d i e d as pest d e n s i t y , though d i s t r i b u t i o n can be an important consideration i n application. Pest d i s t r i b u t i o n or d i s p e r s i o n can a f f e c t pathogen a p p l i c a t i o n i n two ways. The first i s i n t u i t i v e l y obvious but has not been researched: if a host population has a clumped d i s t r i b u t i o n , then as much of the pathogen p o p u l a t i o n as p o s s i b l e must be d e l i v e r e d t o those clumps t o c o n t a c t the t a r g e t i n s e c t s and avoid wastage. The second has only an i n d i r e c t e f f e c t on t a r g e t i n g ; clumped host p o p u l a t i o n s can be conducive t o pathogen t r a n s m i s s i o n , a t l e a s t w i t h i n the clumps. For seasonal c o l o n i z a t i o n and i n t r o d u c t i o n establishment, t h i s can reduce the pathogen inoculum t h a t must be d e l i v e r e d t o i n f e c t a number of i n s e c t s . For example, host clumping was conducive t o prevalence of disease caused by an entomopathogenic fungus (54), protozoan (55) and v i r u s e s (3, 56). Pest d i s t r i b u t i o n a l s o can a f f e c t pathogen d e l i v e r y due t o the i n s e c t s ' location i n the habitat. For example, vertical d i s t r i b u t i o n of a t a r g e t i n s e c t i n s o i l would a f f e c t the s e l e c t i o n of the type of pathogen t o use (e.g., 19) and/or the way i n which a pathogen i s d e l i v e r e d (e.g., s o i l s u r f a c e spray, or use of a seeder or l i g h t t i l l i n g f o r inoculation into s o i l ) . Age s t r u c t u r e of a pest p o p u l a t i o n i s another f a c t o r i n t i m i n g of a p p l i c a t i o n . As d i s c u s s e d above (see "Pathogen Species and S t r a i n C h a r a c t e r i s t i c s " ) , the g r e a t m a j o r i t y of pathogens r a p i d l y decrease i n e f f e c t i v e n e s s as



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the host i n s e c t becomes o l d e r . Thus, i t f o l l o w s t h a t the age s t r u c t u r e of a host p o p u l a t i o n w i l l a f f e c t t i m i n g ; the pathogen must be a p p l i e d when a s u f f i c i e n t p r o p o r t i o n of the host p o p u l a t i o n i s young enough t o be i n f e c t e d and a t a s u f f i c i e n t dosage t o i n f e c t the hosts as they age. For example, s i n g l e a p p l i c a t i o n s of B. thuringiensis and c e r t a i n v i r u s e s have c o n t r o l l e d u n i v o l t i n e pests with relatively uniform age structure (57). Repeated applications of B. thuringiensis and viruses were necessary f o r i n s e c t s with overlapping generations and more complex age s t r u c t u r e . Pest p o p u l a t i o n q u a l i t y i s a major c o n s i d e r a t i o n i n a p p l i c a t i o n , not only i n the sense of p o p u l a t i o n q u a l i t y at the time of a p p l i c a t i o n q u a l i t y , but also in maintaining a c e r t a i n q u a l i t y . " Q u a l i t y " i n t h i s sense p r i m a r i l y r e f e r s t o s u s c e p t i b i l i t y of the i n s e c t s t o the pathogen. Insects can develop r e s i s t a n c e t o pathogens j u s t as they do t o chemical i n s e c t i c i d e s . A l s o , the s t a t e of n u t r i t i o n as w e l l as p h y s i c a l and b i o t i c s t r e s s o r s i n f l u e n c e the susceptibility of insects to various pathogens (58-59). Stress usually increases s u s c e p t i b i l i t y and thus decreases the number of pathogen u n i t s t h a t must be d e l i v e r e d t o the i n s e c t i n order t o i n i t i a t e disease. Resistance can be d e f i n e d as the development of an a b i l i t y i n a s t r a i n of i n s e c t s t o t o l e r a t e doses of pathogens t h a t would prove l e t h a l or cause disease i n the m a j o r i t y of i n d i v i d u a l s i n a normal p o p u l a t i o n of the same s p e c i e s . Thus, i t i s a " s t r a i n " c h a r a c t e r i s t i c . However, i n terms of a p p l i c a t i o n of an entomopathogen f o r i n s e c t control, i t i s better discussed as a population c h a r a c t e r i s t i c . Resistance has been demonstrated f o r a l l f i v e pathogen groups (60-62). Resistance has developed t o B. thuringiensis i n the f i e l d (63-64), and r e s i s t a n c e might be developing i n the f i e l d t o a v i r a l i n s e c t i c i d e (65) . The p r o p o r t i o n of r e s i s t a n t versus s u s c e p t i b l e i n s e c t s i n the pest p o p u l a t i o n w i l l impact the amount t h a t must be d e l i v e r e d t o the t a r g e t i n s e c t ; r e s i s t a n t i n s e c t s w i l l r e q u i r e a heavier dosage. Furthermore, r e s i s t a n c e impacts a p p l i c a t i o n i n the sense t h a t a c e r t a i n q u a l i t y ( s u s c e p t i b i l i t y ) of the pest p o p u l a t i o n be maintained. I t has been w e l l accepted t h a t , i n the case of chemicals, a p p l i c a t i o n can increase the degree of r e s i s t a n c e i n pest populations i n the f i e l d (66-68). P e s t i c i d e s t h a t are a p p l i e d i n a manner t h a t w i l l k i l l (or render incapable of reproducing) a high p r o p o r t i o n of a pest p o p u l a t i o n over a wide area and long time p e r i o d exert a g r e a t s e l e c t i v e pressure on t h a t population, i n many cases l e a d i n g t o r a p i d development of r e s i s t a n c e and reduced e f f i c a c y of f u t u r e a p p l i c a t i o n s of t h a t chemical and perhaps others. There i s concern t h a t c e r t a i n a p p l i c a t i o n s t r a t e g i e s f o r microbials, such as engineering the gene for B. thuringiensis ^-endotoxin i n t o crop p l a n t s or other b a c t e r i a l species (47), amounts t o i n d i s c r i m i n a t e


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application of a persistent i n s e c t i c i d e that w i l l r a p i d l y lead t o resistance.


The

Ecosystem

The pathogen p o p u l a t i o n i n m i c r o b i a l control is a biological entity interacting as p a r t o f i t s own environment with numerous environmental components which can g r e a t l y impact when and where a pathogen i s a p p l i e d f o r i n s e c t c o n t r o l . One way such f a c t o r s can be analyzed i s according t o t h e i r i n t r i n s i c nature: a b i o t i c , b i o t i c , and interacting (ecosystem) f a c t o r s . An a d d i t i o n a l c o n s i d e r a t i o n f o r a p p l i c a t i o n i s t h a t o f environmental r i s k assessment. There can be problems i n t h e d i s t i n c t i o n between a b i o t i c and b i o t i c f a c t o r s and c o n s i d e r a t i o n o f t h e i r i n d i v i d u a l e f f e c t s on pathogen p o p u l a t i o n s (4, 69); f o r example, food and s h e l t e r can be d i f f i c u l t t o c l a s s i f y i n such a manner, p a r t i c u l a r l y f o r o p e r a t i o n a l ecology (4) . Nevertheless, c l a s s i f i c a t i o n i n t o a b i o t i c and b i o t i c f a c t o r s has been u s e f u l i n d i s c u s s i o n s o f entomopathogen ecology. Other environmental f a c t o r s a r e comprised o f both l i v i n g and n o n - l i v i n g components, such as h a b i t a t s t a b i l i t y o r the type of ecosystem. I t must be emphasized t h a t a l l these f a c t o r s , i n c l u d i n g t h e host and pathogen p o p u l a t i o n s , i n t e r a c t i n a complex manner. In terms of pathogen a p p l i c a t i o n , non-host, non-pathogen environmental f a c t o r s have t h e g r e a t e s t e f f e c t on pathogen s u r v i v a l and t r a n s p o r t between host i n s e c t s , a l e s s e r e f f e c t on host s u s c e p t i b i l i t y , and an i n t e r a c t i n g e f f e c t on many o f t h e pathogen and host parameters, p a r t i c u l a r l y p o p u l a t i o n l e v e l f a c t o r s , already d i s c u s s e d . A b i o t i c Factors. Sunlight i s one of t h e most c r i t i c a l environmental f a c t o r s . V i r t u a l l y every entomopathogen i s k i l l e d o r i n a c t i v a t e d q u i c k l y when f u l l y exposed t o s u n l i g h t (7, 69-70). Pathogens l o s e t h e i r a c t i v i t y w i t h i n days o r even hours i n many s i t u a t i o n s , most notably on a e r i a l p l a n t s u r f a c e s exposed t o s u n l i g h t (28, 71). On the other hand, s u n l i g h t can s t i m u l a t e t h e growth o f a few pathogens, such as c e r t a i n f u n g i (7) . The e f f e c t of s u n l i g h t on a p p l i c a t i o n i s obvious; the pathogen must be a p p l i e d i n a manner such t h a t i t c o n t a c t s t h e host i n s e c t before i t i s i n a c t i v a t e d . Pathogens o f t e n have been formulated t o increase t h e i r p e r s i s t e n c e i n s u n l i g h t (28) , but the b e n e f i t s of such formulations g e n e r a l l y have not been worth t h e a d d i t i o n a l cost (9) . Timing o f a p p l i c a t i o n t o t h e dusk-to-dawn time p e r i o d has o f t e n been recommended as an inexpensive method t o delay exposure of the pathogen t o s u n l i g h t (28), though t h i s might not be u s e f u l with an ingested pathogen a p p l i e d against a pest t h a t feeds only d u r i n g d a y l i g h t hours. Temperature can a f f e c t entomopathogens i n two ways: i t can a f f e c t pathogen s u r v i v a l before i t invades t h e i n s e c t , and i t can then a f f e c t the r e l a t i o n s h i p between



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pathogen and host. High temperatures can i n a c t i v a t e pathogens before t h e i r contact with the pest i n s e c t (70) as w e l l as decrease the s u s c e p t i b i l t y of t h e p e s t ( 7 ) . Low temperatures can decrease feeding r a t e s o f mosquitoes, which i n t u r n reduces consumption o f B. thuringiensis israelensis t o x i n (15) . On t h e other hand, once t h e i n s e c t i s i n f e c t e d , high temperature a l s o can i n t e n s i f y o r a c c e l e r a t e disease development o r simply a f f e c t the l i f e c y c l e o f a pathogen (7, 54, 72). Thus, i t i s sometimes h e l p f u l t o apply a pathogen i n t h e e a r l y morning o r evening i n order t o avoid high temperatures (e.g., 73). Humidity o r a s u r f a c e f i l m o f moisture a f f e c t s t h e survival and a c t i v i t y of certain entomopathogens, p a r t i c u l a r l y f u n g i and nematodes (7, 69-70). Although humidity l e v e l can be a c o n s t r a i n t f o r such pathogens, t h i s o f t e n i s a t the "microhabitat" l e v e l r a t h e r than a t the o v e r a l l atmospheric l e v e l . For example, f u n g a l i n f e c t i o n s can be i n i t i a t e d i n i n s e c t s a t r e l a t i v e l y low macrohumidities i f microhumidity a t the s u r f a c e o f t h e host integument o r f o l i a g e i s s u f f i c i e n t l y high (70). The nematodes i n p a r t i c u l a r a r e known f o r t h e i r f a i l u r e s when pest c o n t r o l i s attempted i n s i t u a t i o n s where t h e l a r v a e are s u b j e c t t o d e s i c c a t i o n ; a major reason f o r t h e r e c e n t success o f c e r t a i n nematodes i s simply t h a t they a r e t a r g e t e d against i n s e c t s t h a t l i v e i n moist m i c r o h a b i t a t s , such as s o i l o r burrows i n p l a n t s t r u c t u r e s . Thus, c e r t a i n pathogens must be a p p l i e d a t a time and p l a c e when humidity i s high enough f o r them t o s u r v i v e and i n f e c t t h e host i n s e c t . P r e c i p i t a t i o n can be d e t r i m e n t a l t o p e r s i s t e n c e o r advantageous t o d i s p e r s a l o f entomopathogens and thus i s a f a c t o r i n t i m i n g o f a p p l i c a t i o n . R a i n f a l l washes some pathogens from p l a n t surfaces before they can i n f e c t i n s e c t s , though other pathogens a r e not washed away by p r e c i p i t a t i o n (69, 74). On the other hand, r a i n f a l l can d i s p e r s e pathogens, f o r example throughout a t r e e canopy or i n t o s o i l (7, 28-29). Thus, t h e o r e t i c a l l y , a p p l i c a t i o n over a r e l a t i v e l y l i m i t e d p o r t i o n of a canopy might r e s u l t i n more widespread t a r g e t i n g of a pest p o p u l a t i o n though t i m i n g might be d i f f i c u l t due t o u n p r e d i c t a b i l i t y o f precipitation. I t i s c l e a r t h a t i t i s important t o know which pathogens a r e washed o f f p l a n t surfaces by p r e c i p i t a t i o n and which a r e not. Effects of a i r o r water c u r r e n t s on pathogen a p p l i c a t i o n a r e s i m i l a r t o those o f p r e c i p i t a t i o n i n t h e sense t h a t they a r e l a r g e l y negative but have some p o t e n t i a l t o c o n t r i b u t e t o pathogen d i s p e r s a l (29). D r i f t of inoculum during a p p l i c a t i o n i s a problem w i t h any type of c o n t r o l agent; wind can cause a c o n t r o l agent t o miss the t a r g e t o r be a p p l i e d i n an uneven manner. Thus, spray a p p l i c a t i o n s o f t e n a r e r e l e a s e d a t a time of day when wind v e l o c i t y i s diminished. A s l i g h t wind can be b e n e f i c i a l by increasing thouroughness of pathogen dispersal throughout a p l a n t canopy. Though p r a c t i c a l a p p l i c a t i o n



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might be d i f f i c u l t , i t has been proposed t h a t p r e v a i l i n g wind c u r r e n t s or even some s o r t of fan might be used t o widely d i s t r i b u t e pathogens a f t e r a c t u a l a p p l i c a t i o n l i m i t e d t o a r e l a t i v e l y small area (28). Water c u r r e n t s play a r o l e s i m i l a r to a i r currents. Water t r a n s p o r t s b a c t e r i a l pathogens (29), which can a f f e c t a p p l i c a t i o n e i t h e r by d i l u t i n g the pathogen a t the intended p o i n t of contact with the host or by d i s p e r s i n g the pathogen from a r e l a t i v e l y l i m i t e d p o i n t of r e l e a s e . S o i l u s u a l l y p r o t e c t s entomopathogens and o f t e n i s a r e s e r v o i r f o r long-term c o n t r o l (49, 74). Due to r e l a t i v e l y long p e r s i s t e n c e i n s o i l (some pathogens l i v e f o r y e a r s ) , timing of s o i l a p p l i c a t i o n o f t e n i s l e s s c r i t i c a l than i n more exposed s i t u a t i o n s . P h y s i c a l s o i l or substrate structure can affect placement of application. For example, baculoviruses adsorb t o c l a y p a r t i c l e s , which can a f f e c t t h e i r movement i n s o i l (3, 74). S i m i l a r l y , B • thuringiensis israelensis crystal t o x i n adsorbs t o p a r t i c l e s of mud and organic m a t e r i a l s , l e s s e n i n g i t s chance of i n g e s t i o n by mosquito l a r v a e (75) . B i o t i c F a c t o r s . The crop or resource being p r o t e c t e d from the pest can a f f e c t a p p l i c a t i o n i n two ways. The f i r s t i s i t s economic value. I f the p l a n t p a r t s u b j e c t t o damage has a low economic i n j u r y l e v e l , such as a f r u i t s o l d f o r produce, then l i t t l e damage can be t o l e r a t e d . Pathogen a p p l i c a t i o n must be timed so t h a t s u f f i c i e n t pathogen u n i t s are placed at the p o i n t of contact with the pest i n s e c t before even "cosmetic" damage can occur. T h i s , of course, a l s o r e l a t e s t o pest behavior, h a b i t a t s t a b i l i t y , and other f a c t o r s discussed p r e v i o u s l y . I f the l e a f of an apple t r e e i s damaged, or i f the crop i s f e d t o domestic animals r a t h e r than humans, timing and placement are not as c r i t i c a l , and repeat a p p l i c a t i o n s ( f o r m u l t i v o l t i n e pests) may not be r e q u i r e d . A p p l i c a t i o n of a r e l a t i v e l y expensive m i c r o b i a l i n s e c t i c i d e t o low-value crops, such as pasture grasses, usually is not justifiable economically; however, such crops can be very amenable t o long-term approaches t o c o n t r o l , p a r t i c u l a r l y i f the pathogen can be a p p l i e d i n a c o s t - e f f e c t i v e manner, such as l a t t i c e i n t r o d u c t i o n . Franz (7) and Burges (49) extensively discussed type of crop i n r e l a t i o n to microbial control. The second manner i n which the crop or resource being protected a f f e c t s a p p l i c a t i o n i s b i o l o g i c a l . Several aspects of a host p l a n t a f f e c t pathogen a p p l i c a t i o n , i n c l u d i n g i t s p h y s i c a l s t r u c t u r e , growth c h a r a c t e r i s t i c s , and chemistry. P h y s i c a l s t r u c t u r e can a f f e c t pathogens i n s e v e r a l ways. Closed canopies as opposed t o more open growth p a t t e r n s , i n c l u d i n g d i f f e r e n t shapes or s i z e s of leaves, a f f e c t p e n e t r a t i o n and d e p o s i t of a sprayed pathogen on v a r i o u s s u r f a c e s (e.g., 16). Rapid growth of a p l a n t can r e s u l t i n r a p i d d i l u t i o n of an applied pathogen, n e c e s s i t a t i n g more frequent a p p l i c a t i o n (74).



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P l a n t a l l e l o c h e m i c a l s can enhance o r i n h i b i t a c t i v i t y o f pathogens against host i n s e c t s , which might a f f e c t t h e dosage t h a t must be d e l i v e r e d t o the t a r g e t (16, 27, 50, 69). A l s o , d i f f e r e n t host p l a n t s can a f f e c t pathogen p e r s i s t e n c e (74), though i t i s not always known whether t h i s i s due t o a l l e l o c h e m i c a l s o r t o m i c r o s c o p i c o r macroscopic p l a n t s t r u c t u r e , perhaps by shading t h e pathogen from s u n l i g h t . The pH of dew on c e r t a i n types of p l a n t s decreases p e r s i s t e n c e of v i r u s e s (74), which i n t u r n can a f f e c t t i m i n g and dosage a p p l i e d t o t h e p l a n t . The pest complex i s an important but l a r g e l y unknown f a c t o r i n the a p p l i c a t i o n of both chemical and m i c r o b i a l p e s t i c i d e s . Very o f t e n , more than one pest s p e c i e s damage a crop o r resource simultaneously. In such cases, t i m i n g of a p p l i c a t i o n becomes confused, because economic damage might occur before the p o p u l a t i o n o f any one pest s p e c i e s reaches i t s t h r e s h o l d f o r i n s e c t i c i d e a p p l i c a t i o n . T h i s i s not as s e r i o u s a f a c t o r i n t i m i n g a p p l i c a t i o n s o f entomopathogens, p r i m a r i l y because many such pathogens a r e so h o s t - s p e c i f i c t h a t they w i l l not suppress populations of more than one of t h e pest s p e c i e s . However, problems i n t i m i n g and dosage d e l i v e r e d t o t a r g e t p e s t s can a r i s e with those entomopathogens with r e l a t i v e l y broad host ranges, such as B. thuringiensis o r t h e fungus Beauveria bassiana, p a r t i c u l a r l y when two pests i n a complex have d i f f e r e n t l e v e l s o f s u s c e p t i b i l i t y (76). Various b i o t i c agents i n t h e environment can a f f e c t a p p l i c a t i o n by t r a n s p o r t i n g a pathogen. Predatory and p a r a s i t i c arthropods, v a r i o u s scavengers, b i r d s , and mammals have transported v a r i o u s entomopathogens a f t e r t h e i r release (3, 29, 77-78). The entomopathogenic v i r u s e s i n p a r t i c u l a r are known f o r d i s p e r s a l i n t h i s manner. Generally, t h e animals e a t i n f e c t e d o r dead host i n s e c t s ; the pathogen s u r v i v e s passage through t h e gut and i s deposited i n a new l o c a t i o n i n t h e animal's f e c e s . Though t h i s type of t r a n s p o r t i s not i n p r a c t i c a l use i n pathogen a p p l i c a t i o n , i t has t h e p o t e n t i a l t o reduce t h e amount of inoculum and the area t r e a t e d with a pathogen for microbial control. For example, t h e NPV o f A. gemmatalis can be r e l e a s e d a t i n t e r v a l s of >20 m and s t i l l provide shorto r long-term i n s e c t c o n t r o l due t o t r a n s p o r t by predatory arthropods (42). E f f e c t s of other b i o t i c agents a r e not as w e l l s t u d i e d as t r a n s p o r t agents. The 5-endotoxin o f B. thuringiensis i s degraded by c e r t a i n s o i l microbes and, p o t e n t i a l l y , l e a f c o l o n i z i n g b a c t e r i a (79). Nematodet r a p p i n g f u n g i might reduce the e f f i c a c y o f nematodes (70) . Another unknown i s the outcome of r e l e a s i n g new pathogen strains (whether natural or g e n e t i c a l l y engineered) f o r insect control i n locations with indigenous s t r a i n s of t h e same pathogen s p e c i e s (9); t h e r e i s v i r t u a l l y no information about whether such new s t r a i n s could compete f o r a niche i n such a s i t u a t i o n . A l l o f these instances might r e q u i r e an a p p l i c a t i o n s t r a t e g y t o


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avoid antagonism with such b i o t i c agents. On the p o s i t i v e s i d e , there i s evidence t h a t , i n c e r t a i n s i t u a t i o n s , a pathogen can be a p p l i e d i n a l i m i t e d manner t o i n f e c t a non-pest i n s e c t and b u i l d a g r e a t e r , more widespread inoculum before i n f e s t a t i o n of the crop by the pest i n s e c t (80). Ecosystem F a c t o r s . Several authors have proposed the idea t h a t h a b i t a t s t a b i l i t y a f f e c t s the success of c l a s s i c a l b i o c o n t r o l with p a r a s i t o i d s , predators (45) , and pathogens (e.g., 20, 25). Stable h a b i t a t s are those t h a t do not undergo v a r i o u s kinds of upheaval. For example, a permanent body of water i s more s t a b l e than one t h a t d r i e s p e r i o d i c a l l y ; temperate c l i m a t e s , with t h e i r d i f f e r e n t seasons, are less stable than the tropics; and, p a r t i c u l a r l y relevant to b i o l o g i c a l control, forests, grasslands, orchards, and v a r i o u s p e r e n n i a l crops o f f e r a more s t a b l e h a b i t a t than annual crops, p a r t i c u l a r l y rowcrop a g r i c u l t u r e i n which even the s o i l i s d i s t u r b e d . S t a b i l i t y g e n e r a l l y i s f a v o r a b l e f o r pathogen p e r s i s t e n c e , whether i n an a b i o t i c environmental component, such as s o i l , or i n a b i o t i c component, such as a host p o p u l a t i o n t h a t i s present f o r much of the year. Two s u c c e s s f u l , long-term examples of c o n t r o l with v i r u s e s have been a t t r i b u t e d p a r t l y t o h a b i t a t s t a b i l i t y (25). Stable h a b i t a t s are thought t o o f f e r advantages f o r the seasonal c o l o n i z a t i o n approach as w e l l as i n t r o d u c t i o n s (7) . In a d d i t i o n , unstable h a b i t a t s tend t o f o s t e r r - s e l e c t e d pest species (48) , which can indirectly a f f e c t pathogen application. Thus, s t a b l e h a b i t a t s are thought t o be conducive t o the long-term approaches t o c o n t r o l , which, i n t u r n , o f t e n r e q u i r e r e l e a s e of fewer pathogen u n i t s due t o the p o t e n t i a l f o r pathogen p e r s i s t e n c e and spread. C u l t u r a l p r a c t i c e s , or, i n other words, normal a g r i c u l t u r a l operations, can i n d i r e c t l y a f f e c t pathogen application. For example, modifying p l a n t i n g p r a c t i c e s , such as row spacing or p l a n t i n g date, can l e a d t o an e a r l y c l o s u r e of the p l a n t canopy i n a crop such as soybean, which i n t u r n can increase the e f f e c t i v e n e s s of a fungus f o r i n s e c t c o n t r o l by i n c r e a s i n g intracanopy humidity and shading (81) or, possibly, increase difficulty in d e l i v e r i n g a pathogen t o an i n s e c t i n s i d e the canopy. Thus, canopy c l o s u r e , p a r t i a l l y dependent on farming p r a c t i c e s , might be a f a c t o r i n d e c i d i n g when t o apply the fungus. Use of chemical p e s t i c i d e s a l s o can be a f a c t o r . The fungus B. bassiana was not harmed by a f u n g i c i d e commonly used i n potatoes provided t h a t c o n i d i a were a p p l i e d a t l e a s t one day a f t e r the f u n g i c i d e (82) . I r r i g a t i o n can a f f e c t e f f i c a c y of entomopathogens f o r i n s e c t c o n t r o l (28), f o r example, by r a i s i n g humidity or moisture l e v e l s , and thus might d i c t a t e when a pathogen i s applied.



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Environmental R i s k s . Environmental r i s k s a r e becoming a concern i n pathogen a p p l i c a t i o n . Entomopathogens a r e s a f e t o almost a l l non-target organisms (1), which i s the major reason f o r t h e i r r e s e a r c h and development f o r i n s e c t c o n t r o l . One concern i s t h a t the a c t i v i t y o f g e n e t i c a l l y engineered entomopathogens might not be p r e d i c t a b l e i n the environment; t h e r e f o r e , they c a r r y a somewhat g r e a t e r r i s k (83) . A p p l i c a t i o n techniques have been proposed f o r f i e l d - t e s t i n g t o g r e a t l y r e s t r i c t any p o s s i b l e t r a n s p o r t of the microorganism o u t s i d e the r e l e a s e s i t e (29) . Concern over n a t u r a l s t r a i n s o f entomopathogens i s t h a t they might reduce populations o f arthropods c l o s e l y r e l a t e d t o the t a r g e t pest s p e c i e s ; most such non-target arthropods would not be s e v e r e l y a f f e c t e d , but a few might be endangered species or might p l a y a c r u c i a l r o l e i n the ecosystem (84). Timing and placement of a p p l i c a t i o n are p o t e n t i a l means t o reduce r i s k s t o these non-target organisms without hampering pest p o p u l a t i o n suppression. For example, a p p l i c a t i o n o f a pathogen w i t h a broad i n s e c t host range (e.g., B. bassiana) might be delayed d u r i n g a blossom p e r i o d when p o l l i n a t i n g i n s e c t s a r e p r e s e n t ( 7 ) . Summary o f E c o l o g i c a l F a c t o r s i n A p p l i c a t i o n I t i s c l e a r t h a t entomopathogens and other b i o r a t i o n a l agents whose a c t i v i t y i s c l o s e l y r e l a t e d t o the p e s t i n s e c t ' s l i f e system must be i n t e g r a t e d i n t o a wide v a r i e t y of e c o l o g i c a l f a c t o r s as w e l l as a g r i c u l t u r a l and resource-management p r a c t i c e s . T h i s f i t s i n t o the concept of i n t e g r a t e d pest management (IPM), whereby a l l s u i t a b l e c o n t r o l techniques are i n t e g r a t e d w i t h one another and with other crop production p r a c t i c e s t o suppress (not e l i m i n a t e ) pest p o p u l a t i o n s below economic i n j u r y l e v e l s w h i l e maintaining the i n t e g r i t y o f the ecosystem. This c o n t r o l concept i s h e a v i l y based i n ecology. The f a c t o r s t h a t a f f e c t t i m i n g and placement o f entomopathogen a p p l i c a t i o n are summarized i n Table 1. I t should be emphasized t h a t t h i s c a t e g o r i z a t i o n i s somewhat a r b i t r a r y , depending not only on one's p o i n t o f view but also on the s p e c i f i c pest-pathogen system under consideration. For example, s o i l i s a somewhat minor f a c t o r f o r a p p p l i c a t i o n i n many entomopathogen-pest-crop systems, but i t might be c r i t i c a l t o placement f o r c o n t r o l of a s o i l p e s t . S i m i l a r l y , environmental r i s k s a r e a major c o n s i d e r a t i o n i n m i c r o b i a l c o n t r o l f o r r e g u l a t o r y reasons, but they c u r r e n t l y have only secondary importance i n a p p l i c a t i o n f o r reasons of e f f i c a c y . The list of f a c t o r s i n Table 1 i s somewhat overwhelming at f i r s t glance, but t h i s will not n e c e s s a r i l y be the case i n r e s e a r c h and development of a p a r t i c u l a r entomopathogen. T h i s summary was developed from a review o f many pest-pathogen systems, and i t covers f a c t o r s r e l a t e d t o a l l t h r e e major approaches t o m i c r o b i a l c o n t r o l . Any one pest-pathogen system c e r t a i n l y w i l l not



Table 1. Categories and Specific Ecological Factors Affecting Entomopathogen Timing and Placement of Application for Microbial Control

Environmental Component

Timing

Factors A f f e c t i n a : Placement


IMPORTANCE Pathogen species or s t r a i n

l i f e cycle s i t e of attack speed o f a c t i o n

p o r t a l of entry searching a b i l i t y virulence transmission

Pathogen population

persistence

density distribution spread after release

Pest species or s t r a i n

pest category behavior

behavior

Pest

density age s t r u c t u r e

distribution quality

sunlight humidity resource (crop): economic resource (crop): biological

sunlight humidity a i r currents water depth, currents resource (crop): economic resource (crop): biological

population

Ecosystem

SECONDARY IMPORTANCE Pathogen species or s t r a i n

host range p o r t a l of entry reproduction r a t e transmission

host range s i t e of attack reproduction r a t e

Pest species or s t r a i n

r-K continuum generations/yr.

r-K continuum

Pest

quality

population

Ecosystem

temperature precipitation precipitation soil soil b i o t i c agents: transport pest complex management b i o t i c agents: practices antagonists environmenta1 or risks synergists habitat s t a b i l i t y management p r a c t i c e s environmental r i s k s


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i n c l u d e a l l these c o m p l e x i t i e s t o a s i g n i f i c a n t degree. Nevertheless, one lesson i s c l e a r . A p p l i c a t i o n technology has been based p r i m a r i l y on a great d e a l of r e s e a r c h of pathogen f o r m u l a t i o n ; breakthroughs c e r t a i n l y are p o s s i b l e i n f o r m u l a t i o n and r e s e a r c h should be continued. However, t i m i n g and placement of a p p l i c a t i o n have been r e l a t i v e l y ignored, and success i n m i c r o b i a l c o n t r o l w i l l depend t o a l a r g e degree on e c o l o g i c a l f a c t o r s i n r e l a t i o n t o a p p l i c a t i o n technology.


Acknowledgments Approved f o r p u b l i c a t i o n by the D i r e c t o r of the L o u i s i a n a A g r i c u l t u r a l Experiment S t a t i o n as manuscript number 9417-8132. Literature Cited 1. 2.

3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

L a i r d , M; Lacey, L. A.; Davidson, E. W., Eds. Safety of Microbial Insecticides; CRC Press: Boca Raton, FL, 1990. Lewis, F. B.; E t t e r , D. O., J r . In Microbial Control of Insect Pests: Future Strategies in Pest Management Systems; A l l e n , G. E., Ignoffo, C. M., Jaques, R. P., Eds.; NSF-USDA-Univ. F l o r i d a , G a i n e s v i l l e , 1978, pp. 261-272. Evans, H. F. In The Biology of Baculoviruses; Granados, R. R., F e d e r i c i , B. A., Eds.; CRC Press, Boca Raton, FL, 1986, V o l . 2; pp. 89-132. Andrewartha, H. G.; B i r c h , L. C. The Distribution and Abundance of Animals; Univ. Chicago Press: Chicago, IL, 1954. P r i c e , P. W. Insect Ecology; Wiley & Sons: New York, 1975. Price, P. W. In Biological Control in Crop Production; Papavizas, G. C.; Ed.; BARC Symposium V; A l l a n h e l d , Osmun Publ.: London, 1981; pp. 3-19. Franz, J . M. In Microbial Control of Insects and Mites; Burges, H. D., Hussey, N. W., Eds.; Academic Press: London, 1971; pp. 407-444. Fuxa, J . R.; Tanada, Y. In Epizootiology of Insect Diseases; Fuxa, J . R., Tanada, Y., Eds.; Wiley & Sons: New York, 1987; pp. 3-21. Fuxa, J . R.; In IPM Systems in Agriculture; Rajak, R. L., Upadhyay, R. K., Mukerji, K. G., Eds.; A d i t y a Book P r i v a t e L t d . : India, In Press. Hågvar, B. Acta Entomol. Bohemoslov. 1991, 88, 111. Fuxa, J . R. In ESA Handbook of Soybean Insects; H i g l e y , L. G., Boethel, D. J . , Eds.; Entomol. Soc. Amer.: Lanham, MD, In Press. Fuxa, J . R. Biotech. Adv. 1991, 9, 425-442. Andreadis, T. G. In Epizootiology of Insect Diseases; Fuxa, J . R., Tanada, Y., Eds.; Wiley & Sons: New York, 1987; pp. 159-176.


64

14. 15.

16. 17. 18.


19. 20. 21. 22. 23. 24. 25. 26. 27.

28. 29. 30. 31. 32. 33. 34. 35. 36.

BIORATIONAL PEST CONTROL AGENTS Fuxa, J . R.; R i c h t e r , A. R. Environ. Entomol. 1991, 20, 603-609. Becker, N; M a r g a l i t , J . In Bacillus thuringiensis, an Environmental Pesticide: Theory and Practice; E n t w i s t l e , P. F., Cory, J . S., B a i l e y , M. J . , Higgs, S., Eds.; Wiley & Sons: New York, 1993; pp. 147-170. Tanada, Y; Kaya, H. K. I n s e c t Pathology; Academic Press: San Diego, 1993. Anderson, R. M.; May, R. M. Nature. 1979, 280, 361367. Anderson, R. M.; May, R. M. Science. 1980, 210, 658-661. Georgis, R. In Entomopathogenic Nematodes in Biological Control; Gaugler, R., Kaya, H. K., Eds.; CRC Press: Boca Raton, FL, 1990; pp. 173-191. Fuxa, J . R. Mem. J n s t . Oswaldo Cruz. 1989, 84 (Supl. III), 81-88. Huber, J . In The Biology of Baculoviruses; Granados, R. R., F e d e r i c i , B. A., Eds.; CRC Press, Boca Raton, FL, 1986, V o l . 2; pp. 181-202. Jaques, R. P.; Laing, J . E.; Laing, D. R.; Yu, D. S. K. Can. Entomol. 1987, 119, 1063-1067. Jutsum, A. R. Phil. Trans. R. Soc. Lond. B. 1988, 318, 357-373. R i c h t e r , A. R.; Fuxa, J . R. J. Econ. Entomol. 1984, 77, 1299-1306. Harper, J . D. In Epizootiology of Insect Diseases; Fuxa, J . R., Tanada, Y., Eds.; Wiley & Sons: New York, 1987; pp. 473-496. Shapiro, M. In The Biology of Baculoviruses; Granados, R. R., F e d e r i c i , B. A., Eds.; CRC Press, Boca Raton, FL, 1986, V o l . 2; pp. 31-61. Tanada, Y. In Regulation of Insect Populations by Microorganisms; B u l l a , L. A., J r . , Ed.; Ann. N.Y. Acad. S c i . ; N.Y. Acad. S c i . : 1973, V o l . 217; pp. 120130. Ignoffo, C. M. In Biological Control in Agricultural IPM Systems; Hoy, M. A., Herzog, D. C., Eds.; Academic Press, Orlando, FL, 1985; pp. 243-261. Fuxa, J . R. In Risk Assessment in Genetic Engineering; L e v i n , M. A., Strauss, H. S., Eds.; McGraw-Hill: New York, 1991; pp. 83-113. Bedford, G. O. Agric. Ecosystems Environ. 1986, 15, 141-147. Zelazny, B.; Lolong, A.; Pattang, B. J. Invertebr. Pathol. 1992, 59, 61-68. Anderson, R. M. Parasitology. 1982, 84, 3-33. Hagen, K. S.; van den Bosch, R. Annu. Rev. Entomol. 1968, 13, 325-384. Ferron, P. Annu. Rev. Entomol. 1978, 23, 409-442. Kalmakoff, J . ; Crawford, A. M. In Microbial and Viral Pesticides; Kurstak, E., Ed.; Dekker: New York, 1982; pp. 435-448. M o r r i s , O. N. In Microbial and Viral Pesticides; Kurstak, E., Ed.; Dekker: New York, 1982; pp. 239287. Hall and Barry; Biorational Pest Control Agents ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

4. FUXA 37. 38.

39. 40. 41. Downloaded by RUTGERS UNIV on May 29, 2018 | https://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0595.ch004

42. 43. 44. 45. 46. 47. 48. 49. 50.

51. 52. 53. 54. 55. 56.


Tanada, Y.; Fuxa, J . R. In Epizootiology of Insect Diseases; Fuxa, J . R., Tanada, Y., Eds.; Wiley & Sons: New York, 1987; pp. 113-157. Pinnock, D. E. In Baculoviruses for Insect Pest Control: Safety Considerations; Summers, M., Engler, R., Falcon, L. A., V a i l , P. V., Eds.; Am. Soc. M i c r o b i o l . : Washington, DC, 1975; pp. 145-154. Onstad, D. W. Biological Control. 1993, 3, 353-356. Burges, H. D.; Hussey, N. W. In Microbial Control of Insects and Mites; Burges, H. D., Hussey, N. W., Eds.; Academic Press: London, 1971; pp. 687-709. Fuxa, J . R. Bull. Entomol. Soc. Am. 1989, 35, 1224. Fuxa, J . R.; R i c h t e r , A. R. Environ. Entomol. 1994, 23, i n p r e s s . Rabb, R. L. In Concepts of Pest Management; Rabb, R. L., Guthrie, F. E., Eds.; North C a r o l i n a S t . Univ.: R a l e i g h , NC, 1970; pp. 1-5. Woods, A. Pest Control: a Survey; Wiley & Sons: New York, 1974. Hokkanen, H. In CRC Critical Reviews in Plant Sciences; CRC Press: Boca Raton, FL, 1985, V o l . 3; pp. 35-72. E n t w i s t l e , P. F. In Biological Plant and Health Protection; Franz, J . M., Lindaur, M., Eds.; G. F i s c h e r V e r l a g : S t u t t g a r t , 1986; pp. 257-278. Fuxa, J . R. Annu. Rev. Entomol. 1987, 32, 225-251. Evans, H. F.; E n t w i s t l e , P. F. In Epizootiology of Insect Diseases; Fuxa, J . R., Tanada, Y., Eds.; Wiley & Sons: New York, 1987; pp. 257-322. Burges, H. D. In Microbial Control of Pests and Plant Diseases 1970-1980; Burges, H. D., Ed.; Academic Press: London, 1981; pp. 797-836. Navon, A. In Bacillus thuringiensis, an Environmental Pesticide: Theory and Practice; E n t w i s t l e , P. F., Cory, J . S., B a i l e y , M. J . , Higgs, S., Eds.; Wiley & Sons: New York, 1993; pp. 125-146. O i , D. H.; P e r e i r a , R. M. Flor. Entomol. 1993, 76, 63-74. Falcon, L. A. Annu. Rev. Entomol. 1976, 21, 305324. Lacey, L. A.; Harper, J . D. J. Entomol. Sci. 1986, 21, 206-213. Carruthers, R. I . ; Soper, R. S. In Epizootiology of Insect Diseases; Fuxa, J . R., Tanada, Y., Eds.; Wiley & Sons: New York, 1987; pp. 357-416. Maddox, J . V. In Epizootiology of Insect Diseases; Fuxa, J . R., Tanada, Y., Eds.; Wiley & Sons: New York, 1987; pp. 417-452. Bedford, G. O. In Microbial Control of Pests and Plant Diseases 1970-1980; Burges, H. D., Ed.; Academic Press: London, 1981; pp. 409-426.


65

66 57.

58. 59. 60.


61.

62.

63. 64. 65. 66. 67. 68. 69. 70.

71. 72. 73. 74.

75.

BIORATIONAL PEST CONTROL AGENTS Jaques, R. P. In Regulation of Insect Populations by Microorganisms; B u l l a , L. A., J r . , Ed.; Ann. N.Y. Acad. S c i . ; N.Y. Acad. S c i . : 1973, V o l . 217; pp. 109119. B r i e s e , D. T. In The Biology of Baculoviruses; Granados, R. R., F e d e r i c i , B. A., Eds.; CRC Press, Boca Raton, FL, 1986, V o l . 2; pp. 237-263. Watanabe, H. In Epizootiology of Insect Diseases; Fuxa, J . R., Tanada, Y., Eds.; Wiley & Sons: New York, 1987; pp. 71-112. B r i e s e , D. T. In Pathogenesis of Invertebrate Microbial Diseases; Davidson, E. W., Ed.; A l l a n h e l d , Osmun Publ.: Totowa, NJ, 1981; pp. 511-545. E n t w i s t l e , P. F.; Cory, J . S.; B a i l e y , M. J . ; Higgs, S. Bacillus thuringiensis, an Environmental Biopesticide: Theory and Practice; Wiley & Sons: New York, 1993. Fuxa, J . R. In Parasites and Pathogens of Insects; Beckage, N. E., Thompson, S. N., F e d e r i c i , B. A., Eds.; Academic Press: San Diego, 1993, V o l . 2; pp. 197-209. McGaughey, W. H. Science. 1985, 229, 193-195. Tabashnik, B. E.; Cushing, N. L.; Finson, N.; Johnson, M. J. Econ. Entomol. 1990, 83, 1671-1676. Fuxa, J . R.; Abot, A. R.; Moscardi, F.; Sosa-Gómez, D. R.; R i c h t e r , A. R. Resistant Pest Management. 1993, 5, 40-41. Brown, A. W. A.; P a l , R. Insecticide Resistance in Arthropods; World Health O r g a n i z a t i o n : Geneva, 1971. C u r t i s , C. F.; Cook, L. M.; Wood, R. J . Ecol. Entomol. 1978, 3, 273-287. Georghiou, G. P.; T a y l o r , C. E. Proc. Int. Congr. Entomol., 15th, 1976; 1977, pp. 759-785. Benz, G. In Epizootiology of Insect Diseases; Fuxa, J . R., Tanada, Y., Eds.; Wiley & Sons: New York, 1987; pp. 177-214. Roberts, D. W.; Fuxa, J . R.; Gaugler, R.; G o e t t e l , M.; Jaques, R.; Maddox, J . In Handbook of Pest Management in Agriculture; Pimentel, D., Ed.; CRC Press: Boca Raton, FL, 1991, V o l . 2; pp. 243-278. Podgwaite, J . D. In Viral Insecticides for Biological Control; Maramorosch, K., Sherman, K. E., Eds.; Academic Press: Orlando, FL, 1985; pp. 775-797. Weiser, J . In Microbial and Viral Pesticides; Kurstak, E., Ed.; Dekker: New York, 1982; pp. 531557. Schmiege, D. C. J. Econ. Entomol. 1963, 56, 427431. Young, S. Y. III; Yearian, W. C. In The Biology of Baculoviruses; Granados, R. R., F e d e r i c i , B. A., Eds.; CRC Press, Boca Raton, FL, 1986, V o l . 2; pp. 157-179. Ohana, B.; M a r g a l i t , J . ; Barak, Z. Appl. Env. Microbiol. 1987, 53, 828-831.


4.

FUXA

76. 77. 78.


79.

80. 81. 82. 83. 84.


67

McEwen, F. L.; Glass, E. H.; Davis, A. C.; S p l i t t s t o e s s e r , C. M. J. Insect Pathol. 1960, 2, 152-164. Harper, J . D. In The Biology of Baculoviruses; Granados, R. R., F e d e r i c i , B. A., Eds.; CRC Press, Boca Raton, FL, 1986, V o l . 2; pp. 133-155. L e i s y , D. J . ; Fuxa, J . R. In Opportunities from Natural Sources in Crop Protection; Copping, L. G., Ed.; Critical Reviews in A p p l i e d chemistry; E l s e v i e r : Oxford, In Press. G e l e r n t e r , W.; Schwab, G. E. In Bacillus thuringiensis, an Environmental Pesticide: Theory and Practice; E n t w i s t l e , P. F., Cory, J . S., B a i l e y , M. J . , Higgs, S., Eds.; Wiley & Sons: New York, 1993; pp. 89-104. Ignoffo, C. M. J. Invertebr. Pathol. 1978, 31, 1-3. Sprenkel, R. K.; Brooks, W. M.; Van Duyn, J . W.; D i e t z , L. L. Environ. Entomol. 1979, 8, 334-339. L o r i a , R.; G a l a i n i , S.; Roberts, D. W. Environ. Entomol. 1983, 12, 1724-1726. Fuxa, J . R. In Safety of Microbial Insecticides; L a i r d , M., Lacey, L. A., Davidson, E. W., Eds.; CRC Press: Boca Raton, FL, 1990; pp. 203-207. Lockwood, J . A. Environ. Entomol. 1993, 22, 503518.

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