Plant Natural Products as Parasitoid Cuing Agents - American

Chapter 25

Plant Natural Products as Parasitoid Cuing Agents 1

Douglas W. Whitman

Downloaded by UNIV LAVAL on April 23, 2018 | https://pubs.acs.org Publication Date: November 28, 1988 | doi: 10.1021/bk-1988-0380.ch025

Insect Biology and Population Management Research Laboratory, Agricultural Research Service, U.S. Department of Agriculture, Tifton, GA 31793

Plant natural products play an important role in the orientation of insect parasitoids to their herbivorous insect hosts. Few of these stimulating or attractive plant compounds have been identified, but terpenoids, thiocyanates, and sugars are represented. Another group of synomonal plant natural products is the "green leaf odors," six carbon alcohols, aldehydes, and their derived esters, found in many plants. A better understanding of how plant volatiles and contact stimulants influence the behaviors of insect parasitoids, such as the parasitic Hymenoptera, could aid in the application of biological control. The herbivorous i n s e c t s t h a t attack our food and f i b e r crops are humankind's g r e a t e s t competitors, causing b i l l i o n s o f d o l l a r s i n damage and c o n t r o l costs annuall y . Most o f these p e s t i f e r o u s species are themselves attacked by microorganisms, predators, and p a r a s i t o i d s , c o l l e c t i v e l y known as n a t u r a l enemies. One of the l a r g e r and more important groups o f n a t u r a l enemies i s the p a r a s i t i c wasps (order Hymenoptera), which number about 100,000 species worldwide. These u s e f u l p a r a s i t o i d s make t h e i r l i v i n g by seeking out other i n s e c t s which serve as food f o r t h e i r young. U s u a l l y only one wasp egg i s l a i d on or i n the i n s e c t host, and the wasp l a r v a e completes i t s e n t i r e development on t h a t p a r t i c u l a r host, e v e n t u a l l y k i l l i n g i t . The p o t e n t i a l value o f using n a t u r a l enemies such as p a r a s i t i c wasps t o c o n t r o l crop pests has long been !

Current address: Department of Biology, Illinois State University, Normal, IL 61761 This chapter not subject to U.S. copyright Published 1988 American Chemical Society

Cutler; Biologically Active Natural Products ACS Symposium Series; American Chemical Society: Washington, DC, 1988.


25.

WHITMAN

Plant Natural Products as Parasitoid Cuing Agents

387

recognized by crop p r o t e c t i o n s p e c i a l i s t s . Indeed, the advantages of such " b i o l o g i c a l c o n t r o l " are manifold. S u c c e s s f u l b i o l o g i c a l c o n t r o l decreases p e s t i c i d e use, thereby reducing p o t e n t i a l environmental contamination as w e l l as chemical d i s r u p t i o n of the agroecosystem. The g r e a t e s t b e n e f i t of using n a t u r a l enemies to c o n t r o l p l a n t pests occurs when the b i o l o g i c a l c o n t r o l agent becomes permanently e s t a b l i s h e d i n the environment, thus p r o v i d i n g a p e r e n n i a l low-cost means of crop p r o t e c t i o n . When i t works, b i o l o g i c a l c o n t r o l can be r e l a t i v e l y inexpensive, l o n g - l a s t i n g , and environmentally sound. Indeed, there have been many v i c t o r i e s i n the a p p l i c a t i o n of t h i s technique (1) , successes which continue to save m i l l i o n s of d o l l a r s annually. Unfortunately, however, the majority of attempts to u t i l i z e b i o l o g i c a l c o n t r o l have f a i l e d (2, 3). Predators and p a r a s i t o i d s which work w e l l i n the laboratory are o f t e n dismal f a i l u r e s i n the f i e l d . The reasons f o r such mishaps are v a r i e d and complex, {4), but o f t e n can be a t t r i b u t e d to what seems to be an i n a b i l i t y or unwillingness of released b i o l o g i c a l c o n t r o l agents to l o c a t e and attack t h e i r hosts. Many b i o l o g i c a l c o n t r o l p r a c t i t i o n e r s , who have spent months c a r e f u l l y c u l t u r i n g large numbers of n a t u r a l enemies i n the l a b o r a t o r y , are f r u s t r a t e d to see the subjects of t h e i r research simply f l y away f o l l o w i n g f i e l d l i b e r a t i o n (5^7) · solve t h i s problem, i t i s e s s e n t i a l to gain an understanding of the f a c t o r s which i n i t i a t e and maintain host searching behavior, and a knowledge of the cues used by n a t u r a l enemies to l o c a t e t h e i r hosts. Simply put, once we know what turns p a r a s i t o i d s on and how p a r a s i t o i d s f i n d and choose t h e i r prey, then we can manipulate these systems f o r our own advantage. The f a c t o r s which govern p a r a s i t o i d o r i e n t a t i o n to prey are best understood i n the p a r a s i t i c wasps, where host l o c a t i o n and i d e n t i f i c a t i o n i s accomplished l a r g e l y v i a the chemical modality (Figure 1). Various studies have shown that p a r a s i t i c Hymenoptera can be stimulated by kairomones o r i g i n a t i n g from such d i v e r s e sources as the host's body, f r a s s , s c a l e s , h a i r s , mandibular gland s e c r e t i o n s , s i l k , and even sexual and aggregation pheromones (8-10). I t i s becoming i n c r e a s i n g l y c l e a r , however, that p l a n t n a t u r a l products a l s o play an important r o l e i n t h i s process, serving to a t t r a c t and stimulate Hymenopterous p a r a s i t o i d s as w e l l as other c l a s s e s of n a t u r a l enemies {8, 1Λ)· The idea that p a r a s i t o i d s u t i l i z e p l a n t v o l a t i l e s and contact stimulants to l o c a t e t h e i r prey i s not new, but was suggested at l e a s t 50 years ago (1^2, 1_3) · Since then, numerous researchers have documented the a t t r a c t i o n of p a r a s i t o i d s to odors of a great v a r i e t y of p l a n t s (Vinson 1981). I t i s t h e r e f o r e s u r p r i s i n g that i n only f i v e studies have the a c t u a l s t i m u l a t i n g p l a n t synomones been i d e n t i f i e d . Table I l i s t s these studies T o



388

BIOLOGICALLY ACTIVE NATURAL PRODUCTS

Figure 1. Female M i c r o p l i t i s c r o c e i p e s wasp o r i e n t i n g to v o l a t i l e s r e l e a s e d as i t s c a t e r p i l l a r host feeds upon a bean l e a f .


Cutler; Biologically Active Natural Products ACS Symposium Series; American Chemical Society: Washington, DC, 1988. 19 Laboratory: Y-tube Olfactometer

Noctuidae: Heliothis virescens

Malvaceae: Gossypium hirsutum

Sesquiterpene: q-humulene 7-bisabolene β-caryophyllene oxide spathulenol β-bisabolol gossonorol

Antennation, Ovipositon, Walking attraction

17, 18 Field: Baited trap

Flight Pteromalidae: attraction Cheirophachus colon Eulophidae: Entedon leucogramma Braconidae: Spathius benefactor, Dendrosoter protuberans

Scolytidae: Scolytus multistriatus

Ulmaceae: Ulmus americana

Terpene: q-cubebene

Ichneumonidae: Campoletis sonorensis

ï 16 Field: Baited trap

Flight attraction

Pteromalidae: Heydenia unica

Scolytidae: Dendroctonus frontalis

Pinaceae: Various Pinus spp.

Terpene: a-pinene

00

I' I

s:

8·"

?s 15

Laboratory: Walking olfactometer

Walking attraction

Braconidae: Diaeretiella

Aphididae: Myzus persicae

Brassicaceae: Brassica oleracea

Thicyanate: allyl isothiocyanate rapae

14

Laboratory: Contact bioassay

Oviposition stimulant

*««»

H

Tachinidae: Cyzenis a l b i c a n s

Reference

Bioassay

Activity

Parasitoid

Geometridae: Operophtera brumata

Herbivore

I

Fagaceae: Quercus robur

Plant

Plant Natural Products Which Function as Synomones for P a r a s i t i c Insects

en

Sugar sucrose fructose

Plant Compound

Table I .



390

BIOLOGICALLY ACTIVE NATURAL PRODUCTS

and shows the great heterogeneity which c h a r a c t e r i z e s them. I t i s s i g n i f i c a n t to note that s e v e r a l f a m i l i e s of p a r a s i t i c wasps as w e l l as a t a c h i n i d f l y are represented. These p a r a s i t o i d s attack a d i v e r s i t y of prey ranging from sap-sucking aphids and l e a f - f e e d i n g c a t e r p i l l a r s to bark-feeding b e e t l e s . The s t i m u l a t i n g n a t u r a l products are produced by a d i v e r s i t y of p l a n t s representing c u l t i v a t e d and w i l d taxa, herbs and t r e e s , and deciduous and evergreen species, and include sugars, thiocyanates, and terpenoids (Figure 2), which act as long or short range o l f a c t o r y a t t r a c t a n t s , contact a r r e s t a n t s , or o v i p o s i t i o n stimulants. Notice a l s o that whereas some compounds were tested i n the laboratory with olfactometers, others were examined i n the f i e l d with b a i t e d s t i c k y t r a p s . The d i v e r s i t y of p l a n t , herbivore, and p a r a s i t o i d taxa, and the v a r i e t y of a c t i v e p l a n t components, which are represented i n these s t u d i e s , s t r o n g l y suggest that p l a n t n a t u r a l product s t i m u l a t i o n of p a r a s i t o i d s i s a widespread phenomenon, perhaps a major f a c t o r i n the o r i e n t a t i o n of i n s e c t parasitoids. In our l a b o r a t o r y , we r e c e n t l y began a s e r i e s of studies to e l u c i d a t e the r o l e of v o l a t i l e p l a n t n a t u r a l products i n o r i e n t i n g and a t t r a c t i n g the braconid wasp M i c r o p l i t i s croceipes (Figure 1). Female M. croceipes attack and l a y t h e i r eggs i n H e l i o t h i s c a t e r p i l l a r s . These nefarious c a t e r p i l l a r s feed upon a great number of important crops i n c l u d i n g cotton, corn, bean, tomato, and tobacco, causing hundreds of m i l l i o n s of d o l l a r s i n annual damage worldwide. We chose to study a s e r i e s of s i x carbon a l c o h o l s , aldehydes, and t h e i r d e r i v i t i v e e s t e r s (Figure 3), which are c o l l e c t i v e l y known as "green l e a f odors," because of t h e i r common occurrence i n p l a n t s (20-22). I t i s these substances, along with p l a n t a l l e l o c h e m i c a l s such as terpenoids and mustard o i l s , which give each p l a n t group i t s c h a r a c t e r i s t i c odor. The exact f u n c t i o n served by the green l e a f compounds i s unknown. They may have some metabolic purpose, or perhaps deter or harm p l a n t comp e t i t o r s or herbivores. However, data s u b s t a n t i a t i n g these r o l e s are scarce. I t i s known that damaged p l a n t s r e l e a s e r e l a t i v e l y l a r g e q u a n t i t i e s of these v o l a t i l e s (e.g. the odor of a f r e s h l y mowed lawn) (23, .24) · In f a c t , some green l e a f v o l a t i l e s appear to be produced only when p l a n t t i s s u e i s damaged (22). To t e s t the a t t r a c t i v e n e s s of these substances to M. cropeipes wasps, we employed a 75x75x200 cm wind tunnel. Young mated laboratory-reared female wasps were placed i n the downwind end of the tunnel and a m i c r o c a p i l l a r y tube c o n t a i n i n g the t e s t compound was place 100 cm upwind (preliminary experiments had shown that males d i d not respond to these substances). By using v a r i o u s - s i z e d c a p i l l a r y tubes, we obtained a wide range of r e l e a s e r a t e s f o r our t e s t compounds. I f the substance and the


25. WHITMAN

Plant Natural Products as Parasitoid Cuing Agents

391

CHX>H HOHjC

^ / 0 \ 0 H

"Ο.

HOH,C

HO/] I^CHjOH

CH.OH OH

HO

HO sucrose

fructose

CH =CH-CH -N=C=S :

2


allyl isothiocyanate

( — )-a-cubebene

α-pinene

y-bisabolene

a-humulene

β-caryophyllene oxide

gossonorol

β-bisabolol

spathulenol

Figure 2 . P l a n t n a t u r a l products insect parasitoids.

known t o stimulate

/r

Plant Natural Products as Parasitoid Cuing Agents - American

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