The Effects of Plant Biochemicals on Insect Growth and Nutritional

9 The Effects of Plant Biochemicals on Insect Growth and Nutritional Physiology J O H N C.

REESE

Downloaded by CORNELL UNIV on May 13, 2017 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0062.ch009

Department of Entomology, 237 Madison, W I 53706

Russell Labs, University of Wisconsin,

In terms of their effects on insects, plant biochemicals may be divided into nutrients and non-nutrients. Nutrients have received a great deal of attention over the years. They were even thought to be the basis for host plant specificity at one time. Research in recent decades has shown, however, that most species of insects do not differ greatly in their qualitative requirements for nutrients. Thus, although the host plant obviously has to satisfy the nutritional requirements of the insect, it does not seem likely that the insects' nutritional requirements play more than a minor role in host plant specificity (1). Non-nutrients, or allelochemics (non-nutritional chemicals produced by an organism of one species, and which affect the growth, health, behavior, or population biology of another species (2)), can be extremely important factors in host plant resistance. I will concentrate my discussion of the effects of these allelochemics on insect growth and nutritional physiology, how these non-nutrients may be interacting with nutrients, and on recent work I have done in Dr. Beck's laboratory at the University of Wisconsin on the effects of various allelochemics and dietary moisture levels on insect nutritional physiology. Current Areas of Active Research. Plant Apparency. Recent investigations have indicated that evolutionary strategies in plant defense mechnisms may be based on host plant specificity as well as on the population densities and successional status of the plant species (3-5). Within a particular ecosystem, some species of plants will be predictable or apparent in both time and space (i.g. an oak tree). These plants can be easily found by herbivores. Other species are less predictable (and thus less apparent) and so are less likely to be found by herbivores. The predictable species are probably subjected to greater feeding pressure by herbivores and tend to contain high concentration of dosage-dependent (quantitative) inhibitors of digestion and assimilation. These substances are 129

Hedin; Host Plant Resistance to Pests ACS Symposium Series; American Chemical Society: Washington, DC, 1977.


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u s u a l l y not h i g h l y t o x i c , but slow the growth of i n s e c t s . The l e s s apparent species tend t o contain lower concentrations of more h i g h l y t o x i c ( q u a l i t a t i v e ) a l l e l o c h e m i c s . Insects tend to be more l i k e l y to evolve d e t o x i f y i n g mechanisms f o r these substances, and so to become adapted or s p e c i a l i z e d f o r feeding on c e r t a i n s p e c i e s . Such p l a n t species r e l y mainly on escape as a defense a g a i n s t the adapted species of i n s e c t s . The t o x i n s and d e t e r r e n t s i n these p l a n t s are most e f f e c t i v e a g a i n s t the nonadapted g e n e r a l i z e d feeders. Unfortunately, our a g r i c u l t u r a l p r a c t i c e s have taken many p l a n t s t h a t probably evolved under the unapparent category (and so contain the chemical defenses charact e r i s t i c o f t h i s group) and have made these p l a n t s h i g h l y p r e d i c t a b l e and t h e r e f o r e apparent by p l a n t i n g them i n huge f i e l d s and by p l a n t i n g the same species year a f t e r year on the same ground. Perhaps a p r o d u c t i v e d i r e c t i o n f o r host p l a n t r e s i s t a n c e research would be to attempt to i n c r e a s e concentrations o f dosage-dependent f a c t o r s c h a r a c t e r i s t i c of apparent p l a n t s i n agronomically accept a b l e v a r i e t i e s o f our crop p l a n t s . Metabolic E f f e c t s . Such hypotheses as the p l a n t apparency hypothesis are examples of a general s h i f t i n emphasis from b e h a v i o r a l e f f e c t s toward metabolic e f f e c t s . For a number of years the token s t i m u l i (6) theory o f host s e l e c t i o n formed the b a s i s o f a g r e a t d e a l o f research. A rough i d e a o f the research emphasis i n p a s t years can be gained by comparing the approximately 400 references c i t e d by Hedin e t a l Ç 7 ) i n t h e i r compil a t i o n o f b e h a v i o r a l chemicals, to the approximately 100 r e f e rences c i t e d by Beck and Reese (1) i n t h e i r compilation of chemicals having metabolic e f f e c t s . Despite t h i s approximately four-to-one r a t i o o f work on b e h a v i o r a l aspects to work on metabolic e f f e c t s , there are r e a l l y very few good examples o f a r e s i s t a n c e v a r i e t y u t i l i z i n g a b e h a v i o r a l chemical f o r i t s r e s i s tance mechanism (8). Chronic E f f e c t s Hypothesis. For s e v e r a l years I have been t e s t i n g the hypothesis that some a l l e l o c h e m i c s may have, i n a d d i t i o n to immediate e f f e c t s on s u r v i v a l and feeding behavior, s u b t l e c h r o n i c e f f e c t s , even at low concentrations, on r a t e of growth, u t i l i z a t i o n of food and pupation (1, 9-12). In other words, I have been i n t e r e s t e d i n the things Feeny would c a l l "dosage-dependent" f a c t o r s . The i n h i b i t i o n o f growth may be due t o an i n h i b i t i o n o f i n g e s t i o n , a s s i m i l a t i o n , or e f f i c i e n c y of conversion o f a s s i m i l a t e d o r ingested food. The more b i o l o g i c a l l y a c t i v e compounds r e l i e d on i n h i b i t i o n of v a r i o u s combinat i o n s o f the above processes f o r t h e i r e f f e c t s . The r e s u l t s o f these experiments w i l l be d i s c u s s e d i n more d e t a i l i n a l a t e r section. Insect D i e t e t i c s . Over the years, each o f the major ideas concerning host p l a n t s p e c i f i c i t y ( n u t r i t i o n , token s t i m u l i , and metabolic e f f e c t s ) have played r o l e s i n broadening our understanding of the biochemical b a s i s (or b e t t e r , bases) of p l a n t -



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insect interactions. In general, each idea was considered more or l e s s separate from each of the others, and c e r t a i n l y most research p r o j e c t s have been l a r g e l y based on only one of these ideas. We are now s t a r t i n g to recognize that any given p l a n t i n s e c t i n t e r a c t i o n must s u r e l y depend upon a l l of these f a c t o r s (If 13, 14, Γ7). Under the concept of i n s e c t d i e t e t i c s (15, 16) i t i s recognized that the feeding i n s e c t must i n g e s t food "that not only meets i t s n u t r i t i o n a l requirements, but t h a t i s a l s o capable of being a s s i m i l a t e d and converted i n t o the energy and s t r u c t u r a l substances required f o r normal a c t i v i t y and develop ment" (1). Therefore, i n s e c t d i e t e t i c s encompasses i n s e c t n u t r i t i o n i n the c l a s s i c a l sense, any a l l e l o c h e m i c e f f e c t s on feeding behavior, the e f f e c t s of any a l l e l o c h e m i c s on s u r v i v a l , and the e f f e c t s of any compounds which have chronic a f f e c t s on growth, development, or reproduction. A l l of these aspects are c r u c i a l to a given p l a n t - i n s e c t i n t e r a c t i o n . A d e l e t e r i o u s e f f e c t i n any one o f these areas could cause a p l a n t to become more r e s i s t a n t to the attack of i n s e c t s . These d i f f e r e n t areas of i n s e c t d i e t e t i c s undoubtedly i n t e r a c t with each other i n ways unknown to us a t present. Some of these i n t e r a c t i o n s may prove to be very important i n host p l a n t r e s i s t a n c e research of future years. In a d d i t i o n t o p h y s i o l o g i c a l i n t e r a c t i o n s between d i f f e r e n t aspects of i n s e c t d i e t e t i c s , there may be some important b i o l o g i c a l i n t e r a c t i o n s i n r e l a t i o n to the permanence of a r e s i s t a n t variety. There are some outstanding examples of r e s i s t a n t v a r i e t i e s r e t a i n i n g t h e i r r e s i s t a n c e over long periods of time. Such e x c e p t i o n a l l y s t a b l e r e s i s t a n t v a r i e t i e s must s u r e l y have a number of r e s i s t a n c e mechanisms such that the l i k e l i h o o d of a r e s i s t a n t i n s e c t biotype a r i s i n g i s lessened. A r e s i s t a n t c u l t i var probably has a greater p r o b a b i l i t y of r e l a t i v e permanence, i f i t contains a l l e l o c h e m i c s which a f f e c t both behavior and metabolism. Thus b e h a v i o r a l errancy i s punished by metabolic e f f e c t s (17, 18). Such e f f e c t s may come from the same or d i f f e r e n t compounds. For an i n s e c t to counteradapt to such a s i t u a t i o n would r e q u i r e at l e a s t two mutations, one b e h a v i o r a l and one metabolic. I n t e r a c t i o n s between A l l e l o c h e m i c s and N u t r i e n t s . Allelo chemics may i n t e r a c t with e s s e n t i a l n u t r i e n t s of i n s e c t food. Indeed, many of the d e l e t e r i o u s metabolic or chronic e f f e c t s of p l a n t a l l e l o c h e m i s may be due p r i m a r i l y to v a r i o u s i n t e r a c t i o n s between a l l e l o c h e m i c s and n u t r i e n t s . Except f o r a few examples that w i l l be discussed l a t e r , l i t t l e work has been done on the i n t e r a c t i o n s between a l l e l o c h e mics and n u t r i e n t s i n i n s e c t s and the p o s s i b l e u t i l i z a t i o n of such knowledge i n host p l a n t r e s i s t a n c e . More i n v e s t i g a t i o n s have been conducted on v e r t e b r a t e s . A b r i e f review of some of t h i s work may prove to be u s e f u l i n g i v i n g us clues as to how c e r t a i n a l l e l o c h e m i c s i n h i b i t i n s e c t growth. H a t f i e l d (21) has reviewed s e v e r a l examples from the v e r t e brate l i t e r a t u r e i n which there seemed to be i n t e r a c t i o n s between



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a l l e l o c h e m i c s and n u t r i e n t s . Chestnut tannins have been found to reduce the a v a i l a b i l i t y of l y s i n e i n various d i e t s . I n t e r a c t i o n s between gossypol, cyclopropene f a t t y a c i d s , and p r o t e i n have been reported. A l l e l o c h e m i c s may serve as antioxidants and thus p r o t e c t c e r t a i n n u t r i e n t s from o x i d a t i o n (22), so i n t e r a c t i o n s are not always negative. Various t o x i c f a c t o r s , e s p e c i a l l y enzyme i n h i b i t o r s , from soybeans have been known f o r many years (23). Some of these f a c t o r s render c e r t a i n elements such as nitrogen or s u l f u r unavailable. Since s e v e r a l of these f a c t o r s are heat l a b i l e , and so i n many cases are r e a l l y not much of a problem i n human n u t r i t i o n (24), such f a c t o r s might prove e s p e c i a l l y u s e f u l i n r e s i s tance programs. U n l i k e many p o t e n t i a l r e s i s t a n c e f a c t o r s , t h e i r d e l e t e r i o u s e f f e c t s might be f e l t p r i m a r i l y by the i n s e c t p e s t s , r a t h e r than by the human consumer. This i s an area that c e r t a i n l y warrants f u r t h e r research. Phytate i s a compound which can be found i n r e l a t i v e l y high amounts i n cottonseed meal (23). I t may form complexes with such minerals as z i n c , thus rendering them unavailable to monogastric animals. C e r t a i n p r o t e i n s may a l s o be complexed by phytate. Experiments i n which cottonseed meal was t r e a t e d with the enzyme phytase suggest t h a t there are i n t e r a c t i o n s between phytate and such things as phosphorous and gossypol. The i n t e r a c t i o n s between dicoumarol i n sweet c l o v e r hay and vitamin Κ have been known f o r a number of years. Supplementing the d i e t with vitamin Κ can lessen the hemorrhagic e f f e c t s of the dicoumarol (23). From the preceding d i s c u s s i o n of i n t e r a c t i o n s between a l l e l o chemics and n u t r i e n t s i n v e r t e b r a t e s , i t i s apparent that ento mologists may p r o f i t from t e s t i n g some o f the same types o f hypo theses t h a t have been t e s t e d with v e r t e b r a t e s . C e r t a i n l y , some o f the d e l e t e r i o u s e f f e c t s of p l a n t a l l e l o c h e m i c s may be due,to i n t e r a c t i o n s between the a l l e l o c h e m i c and some n u t r i e n t . Several such i n t e r a c t i o n s have already been demonstrated. For example, c e r t a i n a l l e l o c h e m i c s resemble c e r t a i n n u t r i e n t s so c l o s e l y t h a t they may compete m e t a b o l i c a l l y . L-Canavanie i s q u i t e s i m i l a r to L-arginine. I t s t o x i c i t y (25-29) may be due to the formation of d e f e c t i v e canavanyl p r o t e i n s . L-Canavanine and various other amino a c i d analogues may a l s o a c t as i n h i b i t o r s o f i n s e c t repro d u c t i o n (30), p o s s i b l y through t h i s same mechanism. C e r t a i n a l l e l o c h e m i s may i n t e r f e r e with n u t r i e n t s by b l o c k i n g their availability. This seems to be the case f o r oak l e a f tan n i n s . The tannins apparently form a complex with p r o t e i n s such t h a t the p r o t e i n s are l e s s a v a i l a b l e to winter moth (Operophthera brumata) l a r v a e (31, 32). S i m i l a r l y , the " d i g e s t i b i l i t y - r e d u c i n g " f a c t o r s i n creosote r e s i n s seem to somehow block d i g e s t i b i l i t y (5). Recently, i n t e r a c t i o n s between c e r t a i n diterpene acids and c h o l e s t e r o l have been demonstrated (20). The p a r t i a l r e v e r s a l of growth i n h i b i t i o n i n the presence of r e l a t i v e l y l a r g e amounts o f c h o l e s t e r o l suggested to these workers that these diterpene a c i d s (e.g. levopimaric acid) a f f e c t the i n s e c t s ' hormonal system.



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As i n the vertebrate l i t e r a t u r e discussed above, protease i n h i b i t o r s have been shown to have d e l e t e r i o u s e f f e c t s on various i n s e c t s . Most have d e a l t with t r y p s i n i n h i b i t o r s . Ryan (33) and Ryan and Green (34), have reviewed t h i s subject i n d e t a i l . B i r k and Applebaum (35) s t u d i e d the e f f e c t s of soybean t r y p s i n i n h i b i t o r s on development and p r o t e i o l y t i c a c t i v i t y of T r i b o l i u m castaneum. Su e t a l (36) found that r e l a t i v e l y high doses of soybean t r y p s i n i n h i b i t o r caused i n c r e a s e d a d u l t m o r t a l i t y i n S i t o p h i l u s oryzae. A p a r t i c u l a r l y e x c i t i n g aspect of t h i s area o f research i s the p o s s i b i l i t y that the p l a n t may be able t o produce much higher l e v e l s of enzyme i n h i b i t o r s a f t e r being attacked. T h i s feature gives the p l a n t an adaptive advantage (and an agronomic advantage) i n that i t does not expend energy f o r the synthesis o f such m a t e r i a l s u n t i l they are a c t u a l l y needed f o r defense. Green and Ryan (37.) made the e x c i t i n g d i s c o v e r y t h a t the wounding of the leaves of potato o r tomato p l a n t s by a d u l t Colorado potato b e e t l e s induces a r a p i d accumulat i o n o f protease i n h i b i t o r . Further, t h i s response was not confined t o immediate area o f the attack, but spread to other p a r t s o f the p l a n t . Recently, some work has been done on the e f f e c t s of s p e c i f i c a l l e l o c h e m i c s on a s s i m i l a t i o n of food (passage across the gut w a l l ) , e f f i c i e n c y of conversion of a s s i m i l a t e d food i n t o i n s e c t t i s s u e , and e f f i c i e n c y o f conversion of ingested food. A reduct i o n i n n u t r i t i o n a l i n d i c e s , such as those mentioned above, r e s u l t i n g from the i n g e s t i o n o f d e l e t e r i o u s a l l e l o c h e m i c s may be due to a number o f f a c t o r s , most of which r e l a t e i n one way or another to i n t e r a c t i o n s between a l l e l o c h e m i c s and n u t r i e n t s . For example, some d e l e t e r i o u s a l l e l o c h e m i c s may b i n d to a s p e c i f i c n u t r i e n t . They may a l s o bind to and i n a c t i v a t e d i g e s t i v e enzymes o r membrane c a r r i e r p r o t e i n s . A l l e l o c h e m i c s with hydroxyl groups on adjacent carbon atoms may chelate c e r t a i n e s s e n t i a l minerals. The l i t e r a t u r e d e a l i n g with the e f f e c t s o f s p e c i f i c a l l e l o chemics on these n u t r i t i o n a l i n d i c e s reamins sparse, but i s i n c r e a s i n g . Shaver e t a l (19) found t h a t gossypol decreased a s s i m i l a t i o n by bollworm l a r v a e , H e l i o t h i s zea, but had no measurable e f f e c t on u t i l i z a t i o n by tobacco budworm l a r v a e , H_. v i r e s c e n s . E r i c k s o n and Feeny (18) t e s t e d the hypothesis t h a t the l a r v a l feeding niche of P a p i l i o polyxenes a s t e r i u s i s p a r t i a l l y bounded by a l l e l o c h e m i c s which are not r e q u i r e d f o r p e r c e p t i o n o f host or non-host p l a n t s . They demonstrated t h a t s i n i g r i n , or one o f i t s breakdown products, reduced a s s i m i l a t i o n but d i d not reduce the e f f i c i e n c y with which a s s i m i l a t e d food was converted i n t o t i s s u e . Dr. Beck and I have examined the e f f e c t s of a number of p l a n t a l l e l o c h e m i c s on growth and development o f black cutworm (Agrotis i p s i l o n ) l a r v a e (1, 9-12). We have shown that growth i n h i b i t i o n may be due to i n h i b i t i o n of e i t h e r a s s i m i l a t i o n , or e f f i c i e n c y of conversion of a s s i m i l a t e d food, or a combination of both. I n h i b i t i o n of e i t h e r o f these processes w i l l , o f course, i n h i b i t the e f f i c i e n c y of conversion of ingested food. The s p e c i f i c r e s u l t s o f these experiments w i l l be summarized i n a l a t e r s e c t i o n .


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P o t e n t i a l of N u t r i t i o n a l Index Techniques As discussed i n the previous s e c t i o n , n u t r i t i o n a l index techniques have been used i n f r e q u e n t l y to demonstrate p o s s i b l e n u t r i e n t - a l l e l o c h e m i c i n t e r a c t i o n s or mechanisms by which d e l e t e r i o u s p l a n t compounds may i n h i b i t herbivore growth. P l a n t feeding i n s e c t s c e r t a i n l y do not grow e q u a l l y w e l l on a l l p l a n t s o r p l a n t t i s s u e s , even when there are no apparent b e h a v i o r a l b a r r i e r s to t h e i r feeding. The d i f f e r e n c e s are much greater than can be explained i n terms o f p o s s i b l e d i f f e r e n c e s i n n u t r i e n t content as such o f the d i f f e r e n t p l a n t t i s s u e s . I t seems l i k e l y , t h e r e f o r e , that a l l e l o c h e m i c s c h a r a c t e r i s t i c s of various p l a n t species exert strong i n f l u e n c e s on the growth and development of insects. The n u t r i t i o n a l i n d i c e s I found most u s e f u l are a s s i m i l a t i o n (AD), e f f i c i e n c y o f conversion of a s s i m i l a t e d food (ECD), and e f f i c i e n c y o f conversion o f ingested food (ECI). _ amount ingested (mg) - feces amount ingested (mg) ECD

=

ECI =

— weight gain (mg) amount ingested (mg) - feces

(mg)

χ

1 Q Q

χ

1 Q Q

χ

1 Q Q

(mg)

weight gain (mg) amount ingested (mg)

AD (approximate d i g e s t i b i l i t y ) measures the a s s i m i l a t i o n of food and i s termed "approximate" because i t does not s u b t r a c t the weight o f waste products i n the feces or such metabolic products as the p e r i t r o p h i c membrane (38) and exuviae from the t o t a l weight o f the feces. ECD measures the e f f i c i e n c y with which a s s i m i l a t e d food i s converted i n t o i n s e c t t i s s u e . This index w i l l decrease as the p r o p o r t i o n of a s s i m i l a t e d food metabolized f o r energy increases (38). ECI measures the o v e r a l l a b i l i t y of the i n s e c t to convert ingested food i n t o t i s s u e (38). N u t r i t i o n a l i n d i c e s f o r i n s e c t s on host and non-host p l a n t s vary a great d e a l . Using the southern armyworm, Prodenia e r i d a n i a , Soo Hoo and Fraenkel (39, 40) compared the feeding behavior, growth, s u r v i v a l , and n u t r i t i o n a l i n d i c e s o f the i n s e c t on p l a n t t i s s u e s r e p r e s e n t i n g 32 f a m i l i e s . Most of the p l a n t s t e s t e d were fed upon, but host s u i t a b i l i t y was q u i t e v a r i e d , ranging from normal r a p i d larvae growth to complete m o r t a l i t y . N u t r i t i o n a l i n d i c e s were determined f o r l a r v e feeding on f r e s h p l a n t t i s s u e s representing 12 d i f f e r e n t p l a n t f a m i l i e s . AD ranged from 36% on a "poor" host t o 73% on a "good" host. ECD values ranged from 16% to 57%; ECI values ranged from 10% to 38%. Waldbauer (41) determined n u t r i t i o n a l i n d i c e s o f the tobacco homworm, Manduca s ex t a , l a r v a e on both host p l a n t s and on non-



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host p l a n t s . In the l a t t e r case, the larvae were induced to feed on non-host p l a n t t i s s u e s by removing the sense organs that enable the i n s e c t to d i s t i n g u i s h host p l a n t s from non-host p l a n t s . The use o f n u t r i t i o n a l i n d i c e s as a b a s i s f o r comparing host p l a n t u t i l i z a t i o n by a phytophagous i n s e c t being reared on d i f f e r e n t host p l a n t s has some l i m i t a t i o n s (38). V a r i a t i o n s i n water and f i b e r contents may r e s u l t i n v a r i a t i o n s i n the i n d i c e s (39-42) (see l a t e r s e c t i o n o f t h i s paper). A r e l a t i v e l y low AD might be the r e s u l t of high f i b e r content of the d i e t , but might a l s o be due t o wound-induced p r o t e o l y t i c enzyme i n h i b i t o r s (34, 37) which would reduce the d i g e s t i b i l i t y of the food. A low ECD c o u l d r e s u l t from a n t i m e t a b o l i t e s present i n the d i e t a r y m a t e r i a l , but could a l s o be produced by unfavorable amino a c i d r a t i o s t h a t would prevent the synthesis of normal s t r u c t u r a l p r o t e i n s . Plant m a t e r i a l c o n t a i n i n g acute t o x i n s preclude the determination of meaningful n u t r i t i o n a l i n d i c e s because the larvae would sicken and/or d i e during the experimental p e r i o d . Accordingly, nu t r i t i o n a l i n d i c e s are u s e f u l i n a p r e l i m i n a r y assessment o f host p l a n t u t i l i z a t i o n , but are not s u f f i c i e n t t o i d e n t i f y the s p e c i f i c f a c t o r s i n f l u e n c i n g the e f f i c i e n c y o f that u t i l i z a t i o n . The e f f e c t s o f chemical p l a n t f a c t o r s , i n c l u d i n g both a l l e l o c h e m i c s and n u t r i t i o n a l f a c t o r s , on the e f f i c i e n c y of d i e t a r y u t i l i z a t i o n can best be i n v e s t i g a t e d by i n c o r p o r a t i n g them i n t o a standardized a r t i f i c i a l d i e t and then determining n u t r i t i o n a l i n d i c e s . In t h i s way chemical f a c t o r s can be studied with much l e s s equivoca t i o n . A l s o , by using the technique o f p e r f u s i o n o f known amounts o f a compound i n t o p l a n t m a t e r i a l , most o f the l i m i t a t i o n s discussed above can be avoided. E r i c k s o n and Feeny (18) were q u i t e s u c c e s s f u l i n t h e i r use o f t h i s technique. They demonstrated t h a t P a p i l i o polyxenes a s t e r i u s larvae ate j u s t as much wen fed on p l a n t t i s s u e perfused with s i n i g r i n as when fed c o n t r o l t i s s u e . The s i n i g r i n i n h i b i t e d a s s i m i l a t i o n , though. In a d d i t i o n t o the n u t r i t i o n a l index experiments d i s c u s s e d above, I a l s o s t u d i e d the e f f e c t s o f a l l e l o c h e m i c s on some other aspects o f d i e t e t i c s o f the black cutworm. The i n s e c t s were reared on an a r t i f i c i a l d i e t (43). A l l e l o c h e m i c s were added at known concentrations. A number of p l a n t a l l e l o c h e m i c s with p o s s i b l e allomone f u n c t i o n s were t e s t e d over a 10,000-fold range o f concentrations f o r e f f e c t s on l a r v a l s u r v i v a l , growth, pupation r a t e , and pupal weight. I f i n c o r p o r a t i o n of a compound i n t o the d i e t r e s u l t e d i n a s t a t i s t i c a l l y s i g n i f i c a n t c o r r e l a t i o n between i t s d i e t a r y concentration and one of the above parameters, then the n u t r i t i o n a l i n d i c e s were determined f o r that compound a t 3.75 χ 10 M. A more d e t a i l e d d e s c r i p t i o n of the methods used f o r these experiments are described elsewhere (9). The experimental r e s u l t s (Table I) (summarized from Reese and Beck (9-12) p l u s some m a t e r i a l being prepared f o r p u b l i c a t i o n ) show a number o f things. p-Benzoquinone ( F i g . 1) reduced the amount o f i n g e s t i o n and a s s i m i l a t i o n . The reduced a s s i m i l a t i o n appeared t o be compensated f o r by an increase i n ECD. The i n t e r a c t i o n of these two parameters r e s u l t e d i n an o v e r a l l ECI



136

Table I.

E f f e c t s o f some a l l e l o c h e m i c s on black cutworm s u r v i v a l , weight at 10 days, pupation at 28 days, pupal weight, pupation at 35 days, i n g e s t i o n , dry weight gain, AD, ECD, and ECI. + indicates s i g n i f i c a n t c o r r e l a t i o n between compound concentration and parameter measured f o r s u r v i v a l , weight, 28-day pupat i o n , and 35-day pupation. + i n d i c a t e s s i g n i f i c a n t d i f f e r e n c e between experimental and c o n t r o l i n s e c t s f o r i n g e s t i o n , dry weight gain, AD, ECD, and ECI. Survival


PESTS

10Day Wt.

p-Benzoquinone Dur oqui none

+

28Day Wt.

Pupal Wt.

+

+

+

+

+

+

+

+

35Day Pup.

+

Ingestion

Dry AD ECD Wt. Gain

+

+

+

+

+

+

+

+

+

Hydroquinone Catechol L-Dopa

+

+

+

+

+

Dopamine Chlorogenic Acid Resorcinol

+

Phloroglucinol +

+

G a l l i c Acid

+

+ +

% +

+

p-Coumaric A c i d

+ +

+

Cinnamic A c i d

+

+

Ferulic Acid Benzyl A l c o h o l Pyrogallol

+ +

+

+

+

+

Orcinol


ECI

Plant Biochemicals

REESE

P-BENZOQUINONE

and Insect

HYDROQUINONE

Growth

ρ - C O U M A R I C ACID OH

CINNAMIC A C I D

V Ο

DUROQUINONE

BENZYL A L C O H O L

FERULIC

ACID

Ο


H

3

c A c H

3

PHLOROGLUCINOL

CATECHOL OH

J\0H

V

GALLIC

CHLOROGENIC

ACID

ACID H C I ^ \ 0 H

J\PH

V

Y

C O O H

^CH-COO

y h

o

Figure 1.

\çoo \ _ / ° h

Plant allelochemics bioassay ed with black cutworm larvae (duroquinone is probably not found in phnts)



138

HOST P L A N T

RESISTANCE T O PESTS

that was s i m i l a r t o the c o n t r o l l a r v a e . The reduced form o f pbenzoquinone, hydroquinone, showed l i t t l e b i o l o g i c a l a c t i v i t y . Catechol i n h i b i t e d ECD, r e s u l t i n g i n a low ECI, while the amount ingested was not a f f e c t e d . L-Dopa a l s o exerted no e f f e c t on i n g e s t i o n , but i n h i b i t e d both AD and ECD. I n t e r e s t i n g l y , the s t r u c t u r e s o f both c a t e c h o l and L-dopa i n c l u d e hydroxyl groups l o c a t e d on adjacent carbon atoms. No compound that we have t e s t e d to data that i n c l u d e d t h i s s t r u c t u r e has had any e f f e c t on i n g e s t i o n . A l s o , compounds having t h i s s t r u c t u r e may a c t as c h e l a t i n g agents, thus b i n d i n g some e s s e n t i a l mineral or t r a c e element. L-Dopa might a l s o be a c t i n g as some type o f competitor with t y r o s i n e o r phenylalanine. L-Dopa without the carboxyl group (dopamine) had no d e t e c t a b l e b i o l o g i c a l a c t i v i t y . Thus, having hydroxyl groups on adjacent carbon atoms c e r t a i n l y d i d not guarantee a c t i v i t y . R e s o r c i n o l had no apparent e f f e c t on the n u t r i t i o n a l i n d i c e s ; the data suggest the reduced growth was mainly due to a reduced r a t e o f i n g e s t i o n . P h l o r o g l u c i n o l (with one more hydroxyl group than r e s o r c i n o l ) ( F i g . 1) i n h i b i t e d growth mainly through a r e d u c t i o n o f ECD and thus a reduced ECI. O r c i n o l , d i f f e r i n g from p h l o r o g l u c i n o l i n having a methyl group s u b s t i t u t e d f o r a hydroxyl group ( F i g . 1), showed no b i o l o g i c a l a c t i v i t y . Although cinnamic a c i d i n h i b i t e d pupation i n the p r e l i m i n a r y t e s t s , i t had no e f f e c t on the n u t r i t i o n a l i n d i c e s . p-Coumaric a c i d , s t r u c t u r a l l y s i m i l a r t o cinnamic a c i d , a f f e c t e d only pupal weight and ECD. F e r u l i c a c i d showed no apparent a c t i v i t y . Benzyl a l c o h o l i n h i b i t e d pupation, and i n h i b i t e d ECD s l i g h t l y . P y r o g a l l o l i n h i b i t e d growth, apparently through an accumulation o f s t a t i s t i c a l l y i n s i g n i f i c a n t e f f e c t s on v a r i o u s , parameters such as i n g e s t i o n and ECI. The a d d i t i o n o f a carboxyl group t o get g a l l i c a c i d lessened the a c t i v i t y , at l e a s t i n terms o f growth. Duroquinone was i n c l u d e d i n these experiments because o f i t s s t r u c t u r a l s i m i l a r i t i e s to the p l a n t compound, p-benzoquinone, although I am not aware o f duroquinone having been reported to occur i n p l a n t t i s s u e s . Duroquinone i n c r e a s e d AD but s t r o n g l y i n h i b i t e d other parameters. Growth was e x c e p t i o n a l l y slow i n the presence of duroquinone. In r e l a t i o n to the chronic e f f e c t s hypothesis, i t i s i n t e r e s t i n g t o note t h a t o f 37 compounds t e s t e d (not a l l shown i n Table I due t o incomplete n u t r i t i o n a l index d a t a ) , only 10 compounds reduce s u r v i v a l ( t o x i c i t y ) , whereas 25 compounds reduced growth, pupation, o r pupal weight. Thus, the chronic e f f e c t s hypothesis i s supported. I t h i n k t h a t n u t r i t i o n a l index data are extremely u s e f u l i n s t a r t i n g t o i d e n t i f y p h y s i o l o g i c a l processes i n f l u e n c e d by p l a n t a l l e l o c h e m i c s , p a r t i c u l a r l y i f the chemicals are t e s t e d s i n g l y under d e f i n e d experimental c o n d i t i o n s . Unfortunately, t h i s type o f data does not e l u c i d a t e s p e c i f i c biochemical modes o f a c t i o n . A l s o , i t does not e l i m i n a t e the p o s s i b i l i t y of a d d i t i o n a l o r nonn u t r i t i o n a l e f f e c t s . However, n u t r i t i o n a l index techniques w i l l s u r e l y help us make progress i n these d i r e c t i o n s . In a d d i t i o n ,


9.

REESE

Plant Biochemicals

and

Insect

Growth

139

these techniques have already shown t h a t growth can be i n h i b i t e d by p l a n t a l l e l o c h e m i c s i n d i f f e r e n t ways. A l s o , they c l e a r l y demonstrate t h a t the n u t r i t i o n a l physiology ( " d i e t e t i c s ) of i n s e c t s can be i n f l u e n c e d by n o n - n u t r i t i o n a l substances.


I n t e r r e l a t i o n s h i p s o f N u t r i t i o n a l Indices and D i e t a r y Moisture Levels Recently, I examined the i n t e r r e l a t i o n s h i p s between various growth-related parameters and the e f f e c t s of d i e t a r y moisture l e v e l s on these parameters. Although water i s o f obvious importance to animal n u t r i t i o n , l i t t l e q u a n t i t a t i v e work has been done on how water intake a f f e c t s the dynamics of growth. Storedproducts i n s e c t s have remarkable a b i l i t i e s to conserve water (44), while a number of phytophagous i n s e c t s s u f f e r d e l e t e r i o u s e f f e c t s i f d i e t a r y moisture i s not r e l a t i v e l y high (38, 41, 45). House (46) found t h a t as the n u t r i e n t s o f food were d i l u t e d by water, C e l e r i o euphorbiae larvae tended to eat more. House d i d not f i n d s t a t i s t i c a l l y s i g n i f i c a n t d i f f e r e n c e s i n the e f f i c i e n c i e s o f conversion o f ingested food between the c o n t r o l and experimental i n s e c t s over the range o f moisure l e v e l s he t e s t e d . However, I c a l c u l a t e d the c o r r e l a t i o n c o e f f i c i e n t between percentage of n u t r i e n t s ( i n g r e d i e n t s other than agar, c e l l u l o s e f l o u r , water, and f o l i a g e e x t r a c t ) and the mean e f f i c i e n c y of conversion of ingested food i n House's experiment, and I found i t to be negative and h i g h l y s i g n i f i c a n t . Soo Hoo and Fraenkei (39, 40) suggested t h a t water content o f the d i e t was important to e f f i ciency of conversion i n Prodenia e r i d a n i a ; d i l u t i o n o f the d i e t caused an apparent increase i n e f f i c i e n c y . Feeny (_3) found t h a t e f f i c i e n c y of conversion o f a s s i m i l a t e d food decreased with decreasing moisture o f the r e s p e c t i v e food p l a n t s o f various lepiodopterous l a r v a e . Hoekstra and Beenakkers (47) suggested t h a t p a r t o f the d i f f e r e n c e i n e f f i c i e n c i e s of conversion f o r Locusta m i g r a t o r i a were due to d i f f e r e n c e s i n moisture content o f the d i f f e r e n t species of p l a n t s they were fed. S c r i b e r (48) r e c e n t l y i n v e s t i g a t e d the e f f e c t s of v a r i e d moisture l e v e l s i n host leaves on Hyalophora c e c r o p i a l a r v a e . He, too, found t h a t lower moisture l e v e l s decrease the e f f i c i e n c y o f conversion of food i n t o biomass, as w e l l as the e f f i c i e n c y o f conversion o f the c a l o r i c content and the e f f i c i e n c y o f conversion o f the n i t r o genous content of the food. The f o l l o w i n g abbreviations w i l l be used i n t h i s s e c t i o n : IFrWtL — DWG DWE FWE DML uWF

— — — — —

i n i t i a l f r e s h weight o f larvae (weight o f larvae at beginning o f experiment) dry weight gain dry weight eaten f r e s h weight eaten percent dry matter of larvae at end o f experiment dry weight o f feces


H O S T P L A N T RESISTANCE

140


DMF DMD

— —

T O PESTS

percent dry matter o f feces at end o f experiment percent dry matter o f d i e t at beginning o f e x p e r i ment

A stock c u l t u r e o f the b l a c k cutworm was maintained using the methods of Reese e t a l (43) as modified by Reese and Beck (j)) . C o r r e l a t i o n c o e f f i c i e n t s were c a l c u l a t e d between a l l p o s s i b l e combinations o f p a i r s o f growth-related parameters from the cont r o l s o f p u b l i s h e d and unpublished a l l e l o c h e m i c experiments (ji12). The mean values from each o f 18 experiments were used i n t h i s p o r t i o n o f the study. In each o f these 18 experiments, there were 20 l a r v a e i n each group and the experiments were performed from the 10th through the 20th day o f l a r v a l l i t e . In a second s e r i e s o f experiments, the d i e t a r y moisture l e v e l s were v a r i e d . There were 20 larvae i n each c o n t r o l group and 20 l a r v a e i n each experimental group. The experimental p e r i o d was again from the 10th through the 20th day o f l a r v a l l i f e . Remaining d e t a i l s o f the n u t r i t i o n a l index techniques employed have been d i s c u s s e d e a r l i e r i n t h i s paper. I n t e r r e l a t i o n s h i p s o f I n d i c e s . IFrWtL v a r i e d from e x p e r i ment t o experiment, apparently due t o s l i g h t v a r i a t i o n s i n r e a r i n g c o n d i t i o n s and v a r i a t i o n s w i t h i n the p o p u l a t i o n ; the importance o f running p r e c i s e c o n t r o l s along with experiments i s apparent. In such a s i t u a t i o n , i t would be reasonable t o expect t h a t l a r v a e t h a t s t a r t e d the experiment a t a h e a v i e r weight would have a h i g h e r DWE, DWG, and DWF during the experiment. T h i s i s c l e a r l y shown i n Table I I . Likewise, i t would be reasonable t o assume that DWG (other f a c t o r s being e q u a l ) , should be d i r e c t l y p r o p o r t i o n a l to DWE. T h i s too proved to be the case (Table I I and F i g . 2) and confirmed the work of Kogan and Cope (42) who used Pseudoplusia i n c l u d e n s feeding on soybeans. Kogan and Cope presented the l i n e a r r e g r e s s i o n c h a r a c t e r i s t i c s f o r the r e l a t i o n s h i p they found between DWE and DWG. I used these c h a r a c t e r i s t i c s to c a l c u l a t e DWG values i n our cutworm experiments ( F i g . 2 ) . The DWG v a l u e s c a l c u l a t e d from the Kogan and Cope r e g r e s s i o n were higher than those f o r the b l a c k cutworm. T h i s i n d i c a t e s t h a t P_. includens was more e f f i c i e n t a t converting soybeans i n t o i n s e c t t i s s u e than the b l a c k cutworm was a t converting the a r t i f i c i a l d i e t used i n my experiments. Since IFrWtL and DWG were p o s i t i v e l y c o r r e l a t e d , and since IFrWtL and DWF were p o s i t i v e l y c o r r e l a t e d , i t i s reasonable to suspect t h a t a r e l a t i o n s h i p might e x i s t between DWG and DWF. T h i s was the case, as shown i n Table I I . Dr. Beck and I have s t a t e d (9) that i t may not be necessary to use s p e c i f i c i n s t a r t s f o r n u t r i t i o n a l index experiments, espec a l l y when c o n t r o l s are always run. I t was t h e r e f o r e i n t e r e s t i n g t h a t there was no s i g n i f i c a n t c o r r e l a t i o n between IFrWtL and any of the three n u t r i t i o n a l i n d i c e s (AD, ECD, and E C I ) ; i n f a c t , the c o r r e l a t i o n s were e x c e p t i o n a l l y low (Table I I ) . In other words, as long as the experiments were s t a r t e d with 10 day o l d l a r v a e ,



ECD

AD

DMF

DWF

DWG

DML

DWE

IFrWtL

Table I I .

+0.866 ***

DWE

* ** ***

+0.611 **

+0.541 *

DML

+0.703

+0.912 ***

+0.795 ***

DWG

+0.892 ***

+0.652

+0.958 ***

+0.822 ***

DWF

•

+0.004 -0.291

+0.126 -0.828 ***

-0.142

+0.749 ***

+0.001

+0.166

-0.311

+0.331

+0.236

-0.072

-0.093

ECI

-0.344

+0.209

+0.206

-0.065

-0.076

ECD

-0.106

-0.151

-0.037

-0.001

AD

-0.397

+0.179

-0.453

-0.409

DMF

< 0.05 l e v e l .

(17 degrees o f freedom i n each case) between p a i r s o f n u t r i -

indicates s t a t i s t i c a l significance at Ρ s i g n i f i c a n c e a t Ρ < 0.01 l e v e l . s i g n i f i c a n c e a t Ρ < 0.0005 l e v e l .

Coefficients of correlation t i o n a l index parameters.


^

;s-

s? 3

| ο

•—I

a

g

2

S*

g3

*α §

g w w


HOST P L A N T RESISTANCE

D W E (g) Figure 2. Refotionship between DWG and DWE in black cutworm larvae. Kogan and Cope 1974 line refers to line calculated from data in Ref. 42.


T O PESTS


9.

REESE

Plant Biochemicals

and

Insect

Growth

143

d i f f e r e n c e s i n weight (and i n i n s t a r s ) w i t h i n the range s t u d i e d , had no apparent r e l a t i o n s h i p to AD, ECD, o r ECI. A s i m i l a r l a c k of r e l a t i o n s h i p s i s suggested by Kogan and Cope (42). They found ECI t o be roughly s i m i l a r from day 6 through day 12, when c a l c u l a t e d i n terms o f f r e s h weights. I t was more e r a t i c when c a l c u l a t e d i n terms o f dry weights, but there was s t i l l l i t t l e apparent trend up o r down, as the i n s e c t grew. A l l o f these l i n e s o f evidence suggest t h a t e f f i c i e n c y changes l i t t l e d u r i n g the l o g phase o f growth. T h i s f u r t h e r strengthens our proposal (9) t h a t i t i s during t h i s l o g phase that i t i s a p p r o p r i a t e to conduct experiments on growth-reducing d i e t a r y f a c t o r s . AD i n c r e a s e d as ECD decreased (Table I I ) . In other words, as more food was a s s i m i l a t e d , a s m a l l e r p r o p o r t i o n of the a s s i m i l a t e d food was converted i n t o i n s e c t biomass. Since ECI depends d i r e c t l y upon a s s i m i l a t i o n and the e f f i c i e n c y with which a s s i m i l a t e d food i s converted, t h i s compensating mechanism had the e f f e c t o f decreasing the v a r i a b i l i t y of ECI. ECI had a range of o n l y 7.6 percentage p o i n t s , while ECD and AD had ranges o f 39.5 and 19.0 percentage p o i n t s , r e s p e c t i v e l y , over the 18 e x p e r i ments. DML and DMF I n t e r r e l a t i o n s h i p s . Larvae that s t a r t e d the experimental p e r i o d at a h e a v i e r weight tended to end the e x p e r i ment a t a higher DML (Table I I ) . I t would appear from t h i s t h a t black cutworm larvae may have some a b i l i t y to conserve moisture. T h i s i s s u b s t a n t i a t e d by the f a c t that DML f o r c o n t r o l s was u s u a l l y around 16%, whereas DMD a t the beginning o f experiments was u s u a l l y c l o s e to 21%. A l s o , i t f i t s w e l l with the f a c t t h a t many i n s e c t s i n c r e a s e i n percent dry matter as they i n c r e a s e i n age (49, 50). Given that b l a c k cutworm l a r v a e can apparently conserve moisture when feeding on a d i e t o f 21% DMD, how much a b i l i t y do they have to maintain a constant moisture l e v e l when fed d i e t s o f v a r y i n g moisture l e v e l s ? T h i s and other questions prompted us to vary the d i e t a r y moisture l e v e l . The r e s u l t s o f these experiments w i l l be d i s c u s s e d i n a subsequent s e c t i o n . Considering the r e l a t i o n s h i p between IFrWtL and DML, and c o n s i d e r i n g the r e l a t i o n s h i p s between IFrWtL and DWE, DWG, and DWF, i t was not s u r p r i s i n g to f i n d p o s i t i v e c o r r e l a t i o n s between DML and DWE, DWG, and DWF (Table II) . DMF, however, was appar e n t l y independent o f any other parameter, i n c l u d i n g DML. This was somewhat unexpected, since i f there were a r e g u l a t o r y mechanism f o r DML, one would assume t h a t i t would operate by absorbing v a r i o u s amounts o f water from the hindgut. I n t e r e s t i n g l y , DML and DMF were both very p r e c i s e parameters i n any given experiment. Standard e r r o r o f the mean values were almost always l e s s than 1% and were u s u a l l y l e s s than 0.5%. The range o f DMF values over the 18 experiments was 15.1 percentage p o i n t s , while the range o f DML values was only 3.4 percentage p o i n t s . Compared t o the wide i n d i v i d u a l v a r i a t i o n o f such things as l a r v a l weight, t h i s v a r i a t i o n i n DML seemed e x c e p t i o n a l l y


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small. We have a l s o noted t h a t i n c e r t a i n o f our a l l e l o c h e m i c experiments, DMF and DML seemed to be among the most s e n s i t i v e parameters. In some experiments i n which parameters ( i . g . DWG) were not s i g n i f i c a n t l y d i f f e r e n t from the c o n t r o l s , DMF or DML showed s i g n f i c a n t d i f f e r e n c e s . A l s o , such small v a r i a t i o n over many experiments i m p l i e s t h a t black cutworm larvae have the a b i l i t y to regulate DML. To i n v e s t i g a t e t h i s p o s s i b i l i t y and to t r y to answer questions discussed e a r l i e r , a s e r i e s o f e x p e r i ments were performed i n which the experimental i n s e c t s were fed on d i e t s o f varying moisture content. E f f e c t s of Various D i e t a r y Moisture L e v e l s . FWE was not a f f e c t e d by d i l u t i o n with water ( F i g . 3). U n l i k e C e l e r i o euphorbiae larvae (46), black cutworm larvae seem to have l i t t l e a b i l i t y t o compensate f o r a d i l u t i o n of the d i e t . D i e t s d r i e r than the c o n t r o l s were consumed much more p o o r l y ( F i g . 3). With FWE remaining e s s e n t i a l l y constant f o r the d i l u t e d d i e t s , the a c t u a l DWE n e c e s s a r i l y decreased with i n c r e a s i n g moisture from the c o n t r o l l e v e l ( F i g . 4 ) . With d i e t s d r i e r than the c o n t r o l s , DWE a l s o decreased. Thus, the optimal moisture content i n terms o f DWE was a d i e t s l i g h t l y d r i e r than the c o n t r o l s . DWF followed a s i m i l a r p a t t e r n . As DMD increased, ECD decreased over the e n t i r e range of concentrations t e s t e d ( F i g . 5 ) . Note t h a t the optimal DMD i n terms o f ECD i s not at a l l c l o s e to the c o n t r o l (100%) l e v e l . These data confirmed what Feeny (3), S c r i b e r (48), and House (46) found with other lepidopterous l a r v a e . I f e e l t h i s i s a s i g n i f i cant f i n d i n g s i n c e Feeny (_3) and S c r i b e r (48) used d i f f e r e n t p l a n t s (among which water content happened to v a r y ) , or the water content o f a p l a n t was changed a r t i f i c i a l l y . In e i t h e r case, many other biochemical f a c t o r s may a l s o have been d i f f e r e n t . In my experiments, a standardized d i e t was used, t h e r e f o r e , I was confident when I changed the d i e t a r y moisture l e v e l , that t h i s was the only t h i n g t h a t was v a r i e d . In a d d i t i o n , I was able to study moisture l e v e l s above the optimal growth l e v e l , as w e l l as those below t h i s l e v e l . As might be expected from the r e l a t i o n s h i p between ECD and ECI, ECI was a l s o i n v e r s e l y p r o p r t i o n a l to DMD ( F i g . 6 ) . As discussed above, ECD and AD were i n v e r s e l y p r o p o r t i o n a l to each other; t h i s apparently had a s t a b i l i z i n g e f f e c t on ECI. In the experiments i n which d i e t a r y moisture was v a r i e d , ECD was i n v e r s e l y c o r r e l a t e d with DMD, so i f AD and ECD were r e l a t e d to each other, one would expect DMD and AD to be c o r r e l a t e d . This was not the case. Thus, over a winde range o f d i e t a r y moisture l e v e l s , the compensating mechanism between ECD and AD apparently does not operate a t a detectable l e v e l . A l s o , the f a c t that AD was not a f f e c t e d by d i e t a r y moisture was i n i t s e l f somewhat s u r p r i s i n g , s i n c e one would think t h a t the moisture content o f food would have some e f f e c t on how r e a d i l y i t passes across the gut w a l l .


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DMD (% of Control) Figure 3.

Effect of DMD on FWE laroae

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Growth depends upon a combination o f how much an organism eats and how e f f i c i e n t l y t h a t m a t e r i a l i s converted i n t o t i s s u e . T h i s i s c l e a r l y demonstrated and q u a n t i f i e d by our data. Figs. 4 and 6 show the r e l a t i o n s h i p between DMD and DWE, and between DMD and ECI, r e s p e c t i v e l y . The r e l a t i o n s h i p of DMD and DWG ( F i g . 7) can be viewed as a r e s u l t a n t o f F i g s . 4 and 6. On low DMD d i e t s , the l a r v a e ate l e s s , but converted i t more e f f i c i e n t l y , g i v i n g net DWG values t h a t were somewhat lower than the c o n t r o l s . Had the i n f l u e n c e o f a higher ECI and a b i l i t y o f the l a r v a e to regulate moisure l e v e l s to some extent (discussed i n a l a t e r section) not been f a c t o r s , the curve f o r the moist d i e t s i n F i g . 7 might have followed the dashed l i n e more c l o s e l y . This dashed l i n e was c a l c u l a t e d on the b a s i s of the three p o i n t s between 80% and 100% DMD. For d i e t s d r i e r than the c o n t r o l s , both ECI and DWE dropped sharply, and combined to give g r e a t l y reduced growth. Note t h a t although the optimal DWE was a p o i n t above the c o n t r o l s (higher DMD than c o n t r o l s ) , t h a t the optimal DWG was q u i t e c l o s e to the c o n t r o l s , due to the i n f l u e n c e s of ECI. The d i e t developed e m p i r i c a l l y by Reese e t a l (43) was thus shown to be very c l o s e to the optimum i n terms o f moisture l e v e l . These experiments were o r i g i n a l l y conducted f o r the purpose of i n v e s t i g a t i n g the a b i l i t y o f black cutworm larvae to deal p h y s i o l o g i c a l l y with various d i e t a r y moisture l e v e l s . As has been shown, d i e t a r y moisture has s t r i k i n g e f f e c t s on growth and e f f i c i e n c y of conversion, as w e l l as r e l a t e d parameters. I t a l s o had r a t h e r p r e c i s e e f f e c t s on DMF, as shown i n F i g . 8. Fecal percent dry matter i n terms o f percent of the c o n t r o l s , was e s s e n t i a l l y the same as d i e t a r y dry matter ( F i g . 8). The dashed l i n e i n F i g . 8 i l l u s t r a t e s the s i t u a t i o n i f DMF were e x a c t l y equal to DMD, i n terms o f percent of the c o n t r o l s . The larvae seemed t o have l i t t l e a b i l i t y to regulate dry matter compared to many stored-products i n s e c t s . Nevertheless, evidence f o r a c e r t a i n amount o f water conserving a b i l i t y i s presented i n F i g . 9. Note the p o s i t i o n o f the curve i n r e l a t i o n to the dashed l i n e , which shows the s i t u a t i o n i f DML were e x a c t l y equal to DMD, i n terms o f percent of the c o n t r o l s . The f a c t that the observed data f a l l s above the dashed l i n e f o r low dry matter d i e t s , but lags behind the dashed l i n e f o r d r i e r d i e t s , i n d i c a t e d a degree of r e g u l a t o r y a b i l i t y . A l s o , when viewed i n terms of a c t u a l DML and DMD values (rather than i n terms of percent o f c o n t r o l s ) , DML was always s l i g h t l y lower than DMD, demonstrating some a b i l i t y to conserve water. The a b i l i t y i s not very great, though, as i n d i cated by the f a c t that as DMD increased, DML a l s o increased over the range o f d i e t a r y moisture l e v e l s t e s t e d . Apparently the mechanism was not p u t t i n g excess moisture back i n t o the gut tract. I f t h i s were happening, the feces from larvae on low dry matter d i e t s should have been much lower i n dry matter. From these data the mechanism seems to be a water conservation mechanism i n the sense of maintaining DML lower than DMD, but not a r e g u l a t o r y mechanism i n the sense of maintaining a constant DML over a range o f DMD values.

American Chemicaf Society Library 1155 16th St. N. W. Hedin; Host Plant Resistance to Pests Washington, D. C. 20036 ACS Symposium Series; American Chemical Society: Washington, DC, 1977.



Figure 7.

Effect of DMD on DWG in black cutworm larvae. Dashed line based on three points closest to it.

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% Dry Matter of Diet (% of Control) Figure 8. Effect of DMD on DMF in black cut worm larvae. Dashed line represents situation if DMF equaled DMD, in terms of percent of control.


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Growth

Figure 9. Effect of DMD on DML in black cutworm larvae. Dashed line represents situation if DML equaled DMD, in terms of percent of control.


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Moisture l e v e l i s c e r t a i n l y an important f a c t o r i n p l a n t i n s e c t i n t e r a c t i o n s . Black cutworm l a r v a e , although h i g h l y polyphagous, are undoubtedly capable of s u c c e s s f u l l y l i v i n g only on p l a n t s with f a i r l y high moisture l e v e l s . Outbreaks of t h i s p e s t are a s s o c i a t e d with f l o o d s and unusually wet years. T h e i r a b i l i t y to r e g u l a t e t h e i r moisture l e v e l i s l i m i t e d . Moisture l e v e l may be a u s e f u l f a c t o r i n host p l a n t r e s i s t a n c e . I f t h i s proves to be the case, i t would c e r t a i n l y s i m p l i f y some o f the problems encountered i n biochemically-based r e s i s t a n c e , i n which we must be concerned with p o s s i b l e e f f e c t s of these biochemicals on l i v e s t o c k and man. I b e l i e v e t h i s area of research warrants f u r t h e r study.


Concluding Remarks T h i s summary o f i n t e r a c t i o n s between p l a n t biochemicals and i n s e c t d i e t e t i c s i s c e r t a i n l y not complete or comprehensive. I t does p o i n t out some o f the c u r r e n t areas o f research. More important, I hope that i t may stimulate more work i n areas where our knowledge i s s t i l l l a c k i n g depth. We know l i t t l e about the e f f e c t s o f a l l e l o c h e m i c s on the a s s i m i l a t i o n and conversion processes i n the gut and body t i s s u e s o f the i n s e c t . Nor do we have a very broad understanding o f the e f f e c t s o f a l l e l o c h e m i c s on amino a c i d u t i l i z a t i o n i n p r o t e i n s y n t h e s i s and other synthet i c pathways i n the developmental and reproductive physiology of the i n s e c t . The n u t r i t i o n a l index techniques discussed i n t h i s paper o f f e r a beginning, but only a beginning. Nutritional i n d i c e s can serve as i n d i c a t o r s i d e n t i f y i n g some o f the aspects t h a t should be s t u d i e d i n d e t a i l with biochemical techniques. Acknowledgement T h i s research was supported by the College o f A g r i c u l t u r a l and L i f e Sciences, U n i v e r s i t y o f Wisconsin and by a research grant (PCM 74-24001) t o Dr. Stanley D. Beck from the N a t i o n a l Science Foundation. I would l i k e to thank Dr. Dale M. N o r r i s f o r h e l p f u l suggestions f o r improving t h i s manuscript. I a l s o thank H o l l y Beermann f o r t e c h n i c a l a s s i s t a n c e . Literature Cited

1. Beck, S. D., Reese, J. C., Recent Adv. Phytochem, (1976), 10, 4192. 2. Whittaker, R. H., pp. 43-70, "Chemical Ecology", Sondheimer, E., Simeone, J. B., Eds., Academic Press, New York, 1970. 3. Feeny, P. P., pp. 3-19, "Coevolution of Animals and Plants". Gilbert, L. E., Raven, P. H., Eds., University of Texas Press, Austin, 1975. 4. Feeny, P. P., Recent Adv. Phytochem., (1976), 10, 1-40. 5. Rhoades, D. F., Cates, R. G., Recent Adv. Phytochem, (1976), 10, 168-213.



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6. Fraenkel, G. S., Science, (1959), 129, 1466-1470. 7. Hedin, P. Α., Maxwell, F. G., Jenkins, J. Ν., pp. 494-527, "Proceedings of the Summer Institute on Biological Control of Plant Insects and Diseases", Maxwell, F. G., Harris, F. Α., Eds., University Press of Mississippi, Jackson, 1974. 8. Kogan, Μ., pp. 103-146, "Introduction to Insect Pest Manage ment", Metcalf, R. L., Luckmann, W., Eds., John Wiley and Sons, New York, 1975. 9. Reese, J. C., Beck, S. D., Ann. Entomol. Soc. Am., (1976a), 69, 59-67. 10. Reese, J. C., Beck, S. D., Ann. Entomol. Soc. Am., (1976b), 69, 68-72. 11. Reese, J. C., Beck, S. D., Ann. Entomol. Soc. Am., (1976c), 69, 999-1003. 12. Reese, J. C., Beck, S. D., pp. 217-221, "The Host-Plant in Relation to Insect Behaviour and Reproduction", Jermy, T., Ed., Plenum Press, New York, 1976. 13. Kogan, M., pp. 211-227, "Proc. XV Internat. Congress Ent.", Packer, J. S., D. White, Eds., 1977. 14. Beck, S. D., Ann. Rev. Entomol., (1965), 10, 207-232. 15. Beck, S. D., pp. 1-6, "Insect and Mite Nutrition", Rodriguez, J. G., Ed., North Holland Publishing Co., Amsterdam, 1972. 16. Beck, S. D., pp. 290-311, "Proceedings of the Summer Insti tute on Biological Control of Plant Insects and Diseases", Maxwell, F. G., Harris, F. Α., Eds., University Press of Mississippi, Jackson, 1974. 17. Beck, S. D., Schoonhoven, L. Μ., "Breeding Plants Resistant to Insects", Maxwell, F. G., Jennings, P. R., Eds., John Wiley and Sons, New York, (In Press). 18. Erickson, J. Μ., Feeny, P. P., Ecology, (1974), 55: 103-111. 19. Shaver, T. N., Lukefahr, M. J., Garcia, J. Α., J. Econ. Entomol., (1970), 63: 1544-1546. 20. Elliger, C. Α., Zinkel, D. F., Chan, B. G., Waiss, A. C., Jr., Experientia, (1976), 32: 1364-1365. 21. Hatfield, Ε. E., pp. 171-179, "Effect of Processing on the Nutritional Value of Feeds", National Acad. Sci., Washing ton, D. C., 1973. 22. Cheeke, P. R., Nutrition Reports Internat., (1972), 5: 159-170. 23. Stephenson, E. L., pp. 67-71, "Effect of Processing on the Nutritional Value of Feeds", National Acad. Sci., Washington, D. C., 1973. 24. Liener, I. E., J. Food. Sci., (1976), 41: 1076-1081. 25. Rosenthal, G. Α., Janzen, D. H., Dahlman, D. L., Science (1976), 192: 256-258. 26. Vandersant, E. S., Chremos, J. H., Ann. Entomol. Soc. Am., (1971), 64: 480-485. 27. Isogai, Α., Chang, C., Murakoshi, S., Suzuki, Α., Tamura, S., J. Agr. Chem. Soc. Japan, (1973a), 47: 443-447. 28. Isogai, Α., Murakoshi, S., Suzuki, Α., Tamura, S., J. Agr. Chem. Soc. Japan, (1973b), 47: 449-453.



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29. Dahlman, D. L., Rosenthal, G. Α., Comp. Biochem. Physiol., (1975), 51A: 33-36. 30. Hegdekar, D. Μ., J. Econ. Entomol., (1970), 63: 1950-1956. 31. Feeny, P. P., J. Insect Physiol., (1968), 14: 805-817. 32. Feeny, P. P., Ecology, (1970), 51: 565-581. 33. Ryan, C. Α., Ann. Rev. Plant Physiol., (1973), 24: 173-196. 34. Ryan, C. Α., Green, T. R., Recent, Adv. Phytochem., (1974), 8: 123-140. 35. Birk, Y., Applebaum, S. W., Enzymologia, (1960), 22: 318326. 36. Su, H. C. F., Speirs, R. D., Mahany, P. G., J. Georgia Entomol. Soc., (1974), 9: 86-87. 37. Green, T. R., Ryan, C. Α., Science, (1972), 175: 776-777. 38. Waldbauer, G. P., Adv. Insect Physiol., (1968), 5: 229288. 39. Soo Hoo, C. F., Fraenkel, G., J. Insect Physiol., (1966a), 12: 693-709. 40. Soo Hoo, C. F., Fraenkel, G., J. Insect Physiol., (1966b), 12: 711-730. 41. Waldbauer, G. P., Entomol. Exp. Appl., (1964),7: 253-269. 42. Kogan, Μ., Cope, D., Ann. Entomol. Soc. Am., (1974), 67: 66-72. 43. Reese, J. C., English, L. M., Yonke, T. R., Fairchild, M. L., J. Econ. Entomol., (1972), 65: 1047-1050. 44. Fraenkel, G., Blewett, Μ., Bull. Entomol. Res., (1944), 35: 127-139. 45. Waldbauer, G. P., Entomol. Exp. Appl., (1962), 5: 147-158. 46. House, H. L., Canad. Entomol., (1965), 97: 62-68. 47. Hoekstra, Α., Beenakkers, A. M. T., Entomol. Exp. Appl., (1976), 19: 130-138. 48. Scriber, J. Μ., Oecologia (Berl.), (1977), 28: 269-287. 49. Wigglesworth, V. B., "The Principles of Insect Physiology", 741 pp., Methuen and Co. Ltd., London, 1965. 50. Beck, S. D., Hanec, W., J. Insect Physiol., (1960), 4: 304-418.


The Effects of Plant Biochemicals on Insect Growth and Nutritional

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