Insecticides of Plant Origin - American Chemical Society

Downloaded by KTH ROYAL INST OF TECHNOLOGY on September 16, 2015 | http://pubs.acs.org Publication Date: February 23, 1989 | doi: 10.1021/bk-1989-0387.ch007

Chapter 7

Antipest Secondary Metabolites from African Plants Ahmed Hassanali and Wilber Lwande International Centre of Insect Physiology and Ecology, P.O. Box 30772, Nairobi, Kenya

African plants used in folk medicine and in traditional pest management practices continue to yield interesting and potentially useful research leads. The leaves of the wild shrub Ocimum suave and the flower buds (cloves) of Eugenia aromatica are traditionally used as effective stored grain protectants. Eugenol, a common constituent of the two, is repellant to the maize weevil, Sitophilus zeamais. The medicinal plants, Spilanthes mauritiana and Plumbago zeylanica contain insecticidal isobutyl amides and the naphthoquinone, plumbagin, respectively and may have potential in small-scale mosquito control programmes. Hildecarpin, a pterocarpan from Tephrosia hildebrandtii is an antifeedant against the legume pod borer, Maruca testulalis, and the rotenoids tephrosin and rotenone are very potent antifeedants against a number of lepidopteran larvae. Comparison of the antifeedant activities of citrus limonoids and some of their modifications against the spotted stalk borer, Chilo partellus suggest that the units associated with antifeedant activity are spread out in the limonoid skeleton. P l a n t p r o d u c t s have p l a y e d an i m p o r t a n t r o l e i n t r a d i t i o n a l m e d i c i n e and i n p r o t e c t i o n a g a i n s t p e s t s i n A f r i c a . Chemical i n v e s t i g a t i o n s of the p l a n t s i n v o l v e d have o f t e n l e d t o t h e i s o l a t i o n of s t r u c t u r a l l y and/or b i o l o g i c a l l y i n t e r e s t i n g compounds. Our c u r r e n t concerns a t ICIPE i n c l u d e (a) p l a n t s which may be used i n crude form f o r s m a l l - s c a l e p r o t e c t i o n o f s t o r e d g r a i n ; (b) i n s e c t i c i d a l p l a n t s which may have p o t e n t i a l i n s m a l l - s c a l e mosquito c o n t r o l ; (c) f l a v o n o i d s from Tephrosia P e r s . s p e c i e s (Leguminosae) f o r p o s s i b l e use i n crop p r o t e c t i o n ; and (d) t e t r a n o r t r i t e r p e n e s from l o c a l M e l i a c e a e , Rutaceae and Simaroubaceae s p e c i e s and t h e i r s y n t h e t i c m o d i f i c a t i o n s i n the hope of i d e n t i f y i n g new s o u r c e s o f p o t e n t insect-active limonoids. The p r e s e n t o v e r v i e w h i g h l i g h t s major accomplishments t o d a t e i n t h e s e s t u d i e s .

c

0097-6156/89/0387-0078$06.00/0 1989 American Chemical Society

In Insecticides of Plant Origin; Arnason, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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HASSANALI & LWANDE

Antipest Secondary MetabolitesfromAfrica


P l a n t s used i n P r o t e c t i n g S t o r e d

Products

P l a n t m a t e r i a l s and m i n e r a l s have f o r l o n g been i n use as t r a d i t i o n a l p r o t e c t a n t s of s t o r e d p r o d u c t s ( 1 ) . I n E a s t e r n A f r i c a , communities i n d i f f e r e n t l o c a t i o n s appear t o have e v o l v e d t h e i r own g r a i n p r o t e c t i o n s t r a t e g i e s based on t h e s p e c i f i c s i t u a t i o n s i n t h e i r areas and t h e f l o r a a v a i l a b l e around them. Unfortunately, t h e s e p r a c t i c e s remain l a r g e l y undocumented and t h e s c i e n t i f i c r a t i o n a l e f o r t h e i r c o n t i n u e d use u n t i l r e c e n t times has remained, by and l a r g e , u n i n v e s t i g a t e d . At I C I P E , N a i r o b i , Kenya, we have s t a r t e d a modest s u r v e y of the p l a n t s used i n t h e p r o t e c t i o n of s t o r e d g r a i n s , a s i m p l e e v a l u a t i o n of t h e i r e f f i c a c i e s and c h e m i c a l i n v e s t i g a t i o n s of t h e i r secondary components w i t h t h e o b j e c t i v e of i d e n t i f y i n g t h e a c t i v e compounds. Our e v a l u a t i o n assay c o n s i s t s of undamaged maize seeds (10g) i n v i a l s ( 3 " χ 3/4") t o which a r e i n t r o d u c e d maize w e e v i l s , Sitophilus zeamais (10 males and 10 females) and powdered t e s t m a t e r i a l ( 1 , 2.5 and 5 % ) . These a r e then compared v i s u a l l y w i t h c o n t r o l s f o r 8 weeks. I n our f i r s t round of e v a l u a t i o n , two p l a n t m a t e r i a l s appeared t o show p r o t e c t i v e p r o p e r t i e s : t h e l e a v e s o f t h e w i l d shrub Ocimum suave ( L a b i a t a e ) and t h e f l o w e r buds ( c l o v e s ) of Eugenia aromatica (Zangibaraceae). The two a r e used m e d i c i n a l l y f o r stomach t r o u b l e s and coughs, and as i n s e c t r e p e l l a n t s ( p a r t i c u l a r l y a g a i n s t mosquitoes) and g r a i n p r o t e c t a n t s ( 2 , 3 ) . Gas chromatographic and mass s p e c t r o m e t r i c e x a m i n a t i o n of t h e e s s e n t i a l o i l of 0. suave ( F i g . 1) showed t h a t i n a d d i t i o n t o mono and s e s q u i t e r p e n e s i t c o n t a i n e d eugenol ( I ) which has been known t o be t h e p r i n c i p a l v o l a t i l e component of t h e o i l of c l o v e s ( 4 ) . A gas chromatogram of o i l of c l o v e s of E. aromatica i s shown i n F i g . 2 f o r comparison. A l i t e r a t u r e s e a r c h f o r b i o l o g i c a l a c t i v i t i e s of e u g e n o l , however, r e v e a l e d t h a t t h e p h e n y l p r o p a n o i d phenol had n o t been p r e v i o u s l y r e p o r t e d as an allomone of any i n s e c t . On t h e c o n t r a r y , eugenol has been r e p o r t e d t o be a component of an a t t r a c t a n t b l e n d f o r t h e Japanese b e e t l e , P o p i l l i a japonica ( 5 ) , and the pure compound has been demonstrated t o be an a t t r a c t a n t f o r t h e h o u s e f l y ( 6 ) . Our o b s e r v a t i o n s w i t h 5. zeamais w e e v i l s , however, s u g g e s t e d t h a t t h e s e might be r e p e l l e d by t h i s p h e n o l i c secondary m e t a b o l i t e , and i t became i m p e r a t i v e t o demonstrate t h i s i n a q u a n t i t a t i v e l y u n e q u i v o c a l f a s h i o n . F o r t h i s purpose, a number of Y-shaped o l f a c t o m e t e r d e s i g n s were c o n s t r u c t e d and t e s t e d . F i g . 3 i l l u s t r a t e s t h e one which gave t h e best r e s u l t s . Comparison of t h e b e h a v i o u r of t h e maize w e e v i l s toward d i f f e r e n t doses of eugenol and DEET (Ν,Ν-diethyltoluamide), a p o t e n t and w e l l s t u d i e d s y n t h e t i c i n s e c t r e p e l l a n t , c l e a r l y demonstrated t h e r e p e l l a n t p r o p e r t y of eugenol toward t h i s i n s e c t ( F i g . 4 ) . Eugenol was found t o be s i g n i f i c a n t l y more r e p e l l a n t than i t s isomer i s o e u g e n o l ( I I ) . S u r p r i s i n g l y , o l f a c t o m e t r i c t e s t s w i t h t h e o i l of 0. suave and t h e o i l minus eugenol suggested t h a t both t h e t e r p e n o i d and t h e p h e n o l i c f r a c t i o n s c o n t r i b u t e d t o t h e r e p e l l a n t a c t i v i t y of t h e e s s e n t i a l oil. The r e l a t i v e importance of the t e r p e n o i d components w i l l become c l e a r once t h e i r i d e n t i f i c a t i o n and assays on a u t h e n t i c samples a r e completed. I n view of i t s m i l d e r (and some say more


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INSECTICIDES OF PLANT ORIGIN

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F i g u r e 1. Gas chromatogram o f Ocimum suave o i l . Column: 15m c r o s s l i n k e d m e t h y l s i l i c o n e c a p i l l a r y column; temperature programme: 50°C (5 min) t o 270°C a t 50/min.



HASSANALI & LWANDE

Antipest Secondary Metabolites from Africa

F i g u r e 2. Gas chromatogram of o i l of c l o v e s . Column: 15m c r o s s l i n k e d m e t h y l s i l i c o n e c a p i l l a r y column; temperature programme: 50°C (5 min) t o 270°C a t 50/min.




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p l e a s a n t ) odour than c l o v e s and the f a c t t h a t i t i s found t h r i v i n g under v a r y i n g growth c o n d i t i o n s , 0. suave (and o t h e r Ocimum s p e c i e s ) i s now our major focus f o r e v a l u a t i o n as a s m a l l - s c a l e p r o t e c t a n t of s t o r e d p r o d u c t s i n r u r a l homes.


I n s e c t i c i d a l P l a n t s w i t h P o t e n t i a l f o r Mosquito C o n t r o l The b r e e d i n g h a b i t s of some of the more important mosquito s p e c i e s i n t r o p i c a l A f r i c a may p r o v i d e a means f o r s m a l l - s c a l e c o n t r o l s t r a t e g i e s based on i n s e c t i c i d a l p l a n t s grown by r u r a l communities t h e m s e l v e s . Anopheles gambia, the most important v e c t o r of m a l a r i a , Culex quinquefasciatus, the v e c t o r of B a n c r o f t i a n f i l a r i a s i s and Aedes aegypti, the v e c t o r of y e l l o w f e v e r , a l l breed i n s m a l l c o l l e c t i o n s of water such as temporary r a i n p u d d l e s , man-made c o n t a i n e r s , d r a i n s , and so on (7) where the p o s s i b i l i t y e x i s t s of c o n s i d e r a b l y r e d u c i n g the m u l t i p l i c a t i o n of the mosquitoes by p e r i o d i c treatment w i t h m a t e r i a l s d e r i v e d from such p l a n t s . Two p l a n t s i n v e s t i g a t e d r e c e n t l y at the ICIPE, and s p e c i e s r e l a t e d t o these p l a n t s , may p r o v i d e c a n d i d a t e s f o r such an approach t o mosquito c o n t r o l . The f i r s t i s Spilanthes mauritiana (Compositae), a m e d i c i n a l p l a n t used t r a d i t i o n a l l y f o r mouth i n f e c t i o n s , stomach-ache, d i a r r h o e a and t o o t h - a c h e ( 8 ) . A m e t h a n o l i c e x t r a c t of the wet v e g e t a t i v e a e r i a l p a r t s of the p l a n t gave, a f t e r r e p e a t e d chromatographic s e p a r a t i o n s , a l a r v i c i d e which was i d e n t i f i e d as dodeca-(E,E,E,Z)-2,4,8,10-tetraene N - i s o b u t y l a m i d e ( I I I ) ( 9 ) . The amide caused 100% m o r t a l i t y of t h i r d i n s t a r l a r v a e of Aedes aegypti a t 10- mg/ml ( 9 ) . The c h l o r o f o r m e x t r a c t of the f l o w e r heads of the p l a n t gave e i g h t a d d i t i o n a l amides (IV-XI) which are shown i n F i g . 5 ( 1 0 ) . The i n s e c t i c i d a l p r o p e r t i e s of s u b s t i t u t e d amides of u n s a t u r a t e d f a t t y a c i d s have been r e c o g n i z e d f o r many y e a r s (11), and r e c e n t l y they have been a s u b j e c t f o r more systematic s t r u c t u r e - a c t i v i t y s t u d i e s (12). Although t h e i r i n s t a b i l i t y has been a major o b s t a c l e a g a i n s t c o m m e r c i a l i z a t i o n , t h i s a t t r i b u t e may be of s p e c i a l advantage f o r s m a l l - s c a l e use w i t h i n r u r a l human h a b i t a t s . The second m e d i c i n a l p l a n t we have examined f o r mosquito l a r v i c i d a l components i s Plumbago zeylanica (Plumbiginaceae) used e x t e r n a l l y f o r s k i n d i s o r d e r s and i n t e r n a l l y f o r hookworm (13). The major a c t i v e l a r v i c i d a l compound was found t o be plumbagin ( X I I ) which was o b t a i n e d pure from the r o o t s of the p l a n t i n 0.15% y i e l d . Plumbagin has p r e v i o u s l y been shown t o be an a n t i f e e d a n t f o r the A f r i c a n armyworm Spodoptera exempta (14), and an e c d y s i s i n h i b i t o r f o r a number of l e p i d o p t e r a n l a r v a e (15). Naphthoquinones as a group have been the s u b j e c t of i n t e n s i v e m e c h a n i s t i c s t u d i e s as f e e d i n g d e t e r r e n t s of i n s e c t s (16-19). However, t h e r e has been no p r e v i o u s r e p o r t s of t h e i r t o x i c e f f e c t s a g a i n s t mosquito l a r v a e . We have measured the l a r v i c i d a l e f f e c t s of a number of a v a i l a b l e s t r u c t u r a l v a r i a n t s of plumbagin i n c l u d i n g j u g l o n e ( X I I I ) , 2-methyl1,4-naphthoquinone ( X I V ) , 1,4-naphthoquinone (XV),.2,3-epoxy-l,4naphthoquinone ( X V I ) , 2-hydroxy-l,4-naphthoquinone ( X V I I ) , 1,4benzoquinone (XVIII) and 1,2-naphthoquinone ( X I X ) . T a b l e I g i v e s LC30 v a l u e s of the q u i n o n e s . The h i g h a c t i v i t y of the parent 1,4naphthoquinone (XV) r e l a t i v e t o the 1,2-analogue (XIX) and 1,4benzoquinone ( X V I I I ) , shows t h a t the a c t i v i t y i s l a r g e l y a s s o c i a t e d w i t h the 1,4-naphthoquinone n u c l e u s . 5




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Table I .

LCso f o r v a r i o u s quinones a g a i n s t i n s t a r Aedes aegypti l a r v a e *

3rd

L C s o ( H M/20ml) Compound 0.60 + 0. 05 Plumbagin 0.70 + 0. ,05 Juglone 1.15 + 0. .04 2-Methyl-l,4-naphthoquinone 1.82 + 0. ,05 1,4-*Naphthoquinone 2.20 + 0..05 2,3-Epoxy-l,4-naphthoquinone 13.1 + 0..2 2-Hydroxy-l,4-naphthoquinone 16 1,4-Benzoquinone + 1 17 1,2-Naphthocruinone + 1 * R e s u l t s o b t a i n e d from 9 r e p l i c a t e s of 20 l a r v a e at each of 5-6 c o n c e n t r a t i o n s i n the range 0-4 pM/20ml f o r the top f o u r compounds, and 0-16 μΜ/20πι1 f o r the bottom f o u r compounds.

S u b s t i t u t i o n of the 2 - p o s i t i o n of t h i s n u c l e u s , however, appears t o l e a d t o reduced a c t i v i t y w i t h the OH s u b s t i t u e n t c a u s i n g a s i g n i f i c a n t l y l a r g e drop. Whether t h i s drop i s a s s o c i a t e d w i t h a l l the s u b s t i t u e n t s i n t h i s p o s i t i o n i r r e s p e c t i v e of t h e i r e l e c t r o n i c e f f e c t s , or s p e c i f i c a l l y , w i t h e l e c t r o n - d o n a t i n g +M groups w i l l become c l e a r when o t h e r s u b s t i t u t e d naphthoquinones are assayed. Naphthoquinones o c c u r w i d e l y i n t r o p i c a l p l a n t s (20) and assays of a broader range of n a t u r a l l y - o c c u r r i n g s t r u c t u r a l v a r i a n t s may l e a d t o the i d e n t i f i c a t i o n of r e a d i l y grown p l a n t s s u i t a b l e f o r s m a l l - s c a l e mosquito c o n t r o l programmes. S t u d i e s on Some Tephrosia

Species

Tephrosia P e r s . i s a l a r g e genus of p e r e n n i a l and woody herbs (over 300) t h a t are d i s t r i b u t e d i n the t r o p i c a l and s u b - t r o p i c a l r e g i o n s of the w o r l d (21-22). E x t r a c t s of a number of Tephrosia species have been used m e d i c i n a l l y t o t r e a t a range of a i l m e n t s , i n p e s t c o n t r o l as i n s e c t i c i d e s and r a t p o i s o n s , and as f i s h p o i s o n s (22,23,24). The i n s e c t i c i d a l and t o x i c e f f e c t s of e x t r a c t s of some Tephrosia s p e c i e s have been shown t o be due t o the presence of r o t e n o i d s (24,25,26). However, the m a j o r i t y of Tephrosia species t h a t have been i n v e s t i g a t e d c o n t a i n n o n - i n s e c t i c i d a l p r e n y l a t e d f l a v o n o i d s (27) which have not been screened f o r o t h e r b i o l o g i c a l activities. I n E a s t e r n and Southern A f r i c a , Γ. vogelii Hook f . , T. densiflora Hook f . and T. Candida (Roxb.) DC have been c u l t i v a t e d , i n a l i m i t e d way, by peasant farmers f o r use i n c r o p p r o t e c t i o n (21,24) but, s u r p r i s i n g l y , these p r a c t i c e s have not developed i n t o a s i g n i f i c a n t commercial a c t i v i t y . Our i n t e r e s t i n Tephrosia spp. i s twofold: (a) t o s c r e e n f o r r o t e n o i d - b e a r i n g s p e c i e s w i t h the purpose of g e n e r a t i n g a sound i n f o r m a t i o n base f o r t h e i r more w i d e s p r e a d use and f o r the growth of an a g r o c h e m i c a l i n d u s t r y ; and (b) t o i s o l a t e and c h a r a c t e r i z e n o v e l compounds t h a t may be p r e s e n t and t o assay t h e s e f o r a n t i - i n s e c t e f f e c t s . S p e c i e s i n v e s t i g a t e d t o d a t e i n c l u d e T. hildebrandtii Vatke and



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T. elata D e f l e r s (28,31). A number of f l a v o n o i d s i n c l u d i n g some s t r u c t u r a l l y n o v e l compounds have been i s o l a t e d and c h a r a c t e r i z e d . The a n t i - i n s e c t a c t i v i t i e s of two s e t s of compounds a r e of s p e c i a l i n t e r e s t . F i r s t , the p t e r o c a r p a n h i l d e c a r p i n (XX) showed r e l a t i v e l y h i g h a n t i f e e d a n t a c t i v i t y t o the l a r v a e of the legume pod-borer Maruca testulalis (Table I I ) , an i m p o r t a n t pest which l i m i t s p r o d u c t i o n of the cowpea c r o p i n the t r o p i c s ( 3 2 ) . I t b e a r s c l o s e s t r u c t u r a l resemblance t o t h e p t e r o c a r p a n p h y t o a l e x i n s , m e d i c a r p i n ( X X I ) , p h a s e o l l i n (XXII) and p h e s e o l l i d i n ( X X I I I ) , i s o l a t e d from cowpea p l a n t s when i n f e c t e d w i t h m i c r o organisms (33,35). I f t h e s e p h y t o a l e x i n s a l s o demostrate h i g h a n t i f e e d a n t a c t i v i t i e s a g a i n s t M. testulalis, then they might c o n s t i t u t e an i n t e r e s t i n g b a s i s f o r r e s i s t a n c e i n d u c t i o n i n t h e p l a n t a g a i n s t t h i s i n s e c t (28,36). Secondly, r o t e n o i d s l i k e t e p h r o s i n (XXIV) and rotenone (XXV) a r e v e r y p o t e n t a n t i f e e d a n t s a g a i n s t l e p i d o p t e r a n A f r i c a n p e s t s such as Spodoptera exempta, Eldana saccharina and M. testulalis (Table I I I ) , when assayed i n c h o i c e b i o a s s a y s u s i n g d i s c s d e r i v e d from h o s t l e a v e s ( 3 1 ) . Indeed, i n our assays the a c t i v i t i e s of t e p h r o s i n a g a i n s t 5. exempta and E. saccharina, and t h a t of rotenone a g a i n s t t h e l a t t e r a r e unmatched even by a z a d i r a c h t i n and o t h e r l i m o n o i d s . R o t e n o i d s have p r e v i o u s l y been e v a l u a t e d l a r g e l y as i n s e c t t o x i c a n t s and doses used have been those r e q u i r e d t o cause h i g h l e v e l s of m o r t a l i t y on t a r g e t p e s t s . However, i n view of the p r e s e n t f i n d i n g s , a r e - e v a l u a t i o n of d i f f e r e n t members of t h i s group of a n t i - i n s e c t n a t u r a l p r o d u c t s and the manner of t h e i r use i s c l e a r l y w a r r a n t e d .

Table I I .

Dose ( μα/disc) '100

The Feeding I n h i b i t o r y a c t i v i t y of H i l d e c a r p i n a g a i n s t M. testulalis Larvae

Batch Number* 1 2 3

Control (C) 11.86 29.19 61.58

Treated (T) 1.97 2.07 11.69

Deterrence** (D) % 83.4) 92.9) 85.8 + 6 81.0)

51.8) ) 51.9 + 0 52.0) 25.41 52.91 5 * For each b a t c h , 20 l a r v a e of the t e s t i n s e c t were used. Two l a r v a e were a l l o w e d t o f e e d on a p a i r of t r e a t e d d i s c (T) and c o n t r o l d i s c ( C ) . The r e s u l t of each b a t c h i s based on a summation of 10 such f e e d i n g a s s a y s . * * D e f i n e d as (1 - T/C) χ 100. (Reproduced w i t h p e r m i s s i o n from R e f . 28. C o p y r i g h t 1985 Pergamon P r e s s ) . 10

4

28.27

13.62


87


88 INSECTICIDES OF PLANT ORIGIN


7.

HASSANALI & LWANDE


Table I I I .

Compound μσ/disc Tephrosin 100 10 1


A n t i f e e d a n t a c t i v i t i e s of some r o t e n o i d s a g a i n s t some A f r i c a n i n s e c t p e s t s Spodoptera exempt a

Eldana saccharina

* 89 + 8(5) 89 + 5(5)

94 + 86 + 72 +

Maruca testulalis

6(3) 5(3) 7(3)

Rotenone 97 + 2 ( 4 ) 89 + 2(4) 58 + 1 8 ( 3 ) * 100 86 + 8 ( 4 ) 92 + 5(3) 82 + 7 ( 3 ) 10 45 + 11(3) 81 + 11(3) 52 + 4 ( 3 ) 1 * Evidence of t o x i c a t i o n , l i t h a r g y , r e g u r g i t a t i o n and much reduced d i s c r i m i n a t i o n between t r e a t e d and u n t r e a t e d d i s c s . (Reproduced w i t h p e r m i s s i o n from Ref. 31. C o p y r i g h t 1987 ICIPE S c i e n c e Press).

Limonoids

and t h e i r Analogues

Recent i s o l a t i o n of n o v e l a n t i - i n s e c t t e t r a n o r t r i t e r p e n e s b e l o n g i n g t o t h e genera o t h e r than Melia, such as Trichilia (37.) *d Turreae (Rajab, M. e t a l . , P h y t o c h e m i s t r y , i n p r e s s ) and t o f a m i l i e s o t h e r than M e l i a c e a e such as Simaroubaceae (38,39,40), a t t e s t t o t h e e x i s t e n c e of r i c h , y e t u n d i s c o v e r e d sources of t h i s c l a s s o f compounds i n A f r i c a n p l a n t s b e l o n g i n g t o s e v e r a l f a m i l i e s and genera. The major o b j e c t i v e s of our r e s e a r c h on l i m o n o i d s a r e t o i d e n t i f y limonoid-bearing plants that t h r i v e i n d i f f e r e n t A f r i c a n environments and t o e l u c i d a t e the s t r u c t u r a l requirements f o r t h e a n t i f e e d a n t and g r o w t h - d i s r u p t i v e e f f e c t s by s t r u c t u r e - a c t i v i t y s t u d i e s o f the n a t u r a l l i m o n o i d s and t h e i r s y n t h e t i c m o d i f i c a t i o n s against the A f r i c a n crop p e s t s . The p r e s e n t work, which has i n v o l v e d t h e study of a n t i f e e d a n t a c t i v i t i e s of c i t r u s l i m o n o i d s and t h e i r s y n t h e t i c m o d i f i c a t i o n s , has been c a r r i e d o u t i n c o l l a b o r a t i o n w i t h P r o f e s s o r M i c h a e l B e n t l e y and h i s co-workers a t t h e U n i v e r i s t y o f Maine, U.S.A. Two d i f f e r e n t i n s e c t s were chosen f o r t h e s t u d y , Colorado p o t a t o b e e t l e , Leptinotarsa decemlineata and t h e stem-borer, Chilo partellus* R e s u l t s on L. decemlineata have been r e p o r t e d elsewhere ( B e n t l e y , M.D. et. a l . , Entomol. Exp. A p p l . , i n p r e s s ) and here we s h a l l h i g h l i g h t our f i n d i n g s on C. partellus. The l i m o n o i d s t e s t e d i n c l u d e l i m o n i n (XXVI) d e o x y e p i l i m o n o l ( X X V I I ) , e p i l i m o n y l a c e t a t e ( X X V I I I ) , obacunone (XXIX), n o m i l i n (XXX), l i m o n i n d i o s p h e n o l (XXXI), d e o x y l i m o n i n ( X X X I I ) , e p i l i m o n o l ( X X X I I I ) , d e o x y t e t r a h y d r o l i m o n i n (XXXIV) and h e x a h y d r o l i m o n i n i c a c i d (XXXV). Each o f these compounds was assayed a t t h r e e doses a g a i n s t f i f t h i n s t a r C. p a r t e l l u s l a r v a e i n a c h o i c e arena of t r e a t e d and u n t r e a t e d d i s c s of maize l e a f , s i m i l a r t o t h a t r e p o r t e d e a r l i e r (41.). The r e s u l t s a r e summarised i n F i g . 6. The most noteworthy f e a t u r e of these r e s u l t s i s t h a t none of t h e s t r u c t u r a l a l t e r a t i o n s r e p r e s e n t e d by t h e d i f f e r e n t l i m o n o i d s l e a d t o a t o t a l l o s s o f antifeedant a c t i v i t y . This i s i n contrast t o r e s u l t s obtained f o r al






7.

HASSANALI & LWANDE

Antipest Secondary Metabolites from Africa

F i g u r e 6. A n t i f e e d a n t a c t i v i t y of m o d i f i e d l i m o n o i d s a g a i n s t Chilo partellus larvae.



92

the C o l o r a d o p o t a t o b e e t l e where t h e presence o f t h e u n s a t u r a t e d f u r a n moiety appears t o be c r u c i a l f o r s i g n i f i c a n t a n t i f e e d a n t a c t i v i t y of the limonoid. Nevertheless, a point of s i m i l a r i t y between t h e two i n s e c t s i s t h a t t h e f u r a n group i s a l s o an i m p o r t a n t requirement f o r h i g h a n t i f e e d a n t a c t i v i t y a g a i n s t C. partellus. Thus d e o x y t e t r a h y d r o l i m o n i n (XXXIV) i s s i g n i f i c a n t l y l e s s a c t i v e a t 20 χ 1 0 - m o l e / d i s c than d e o x y l i m o n i n ( X X X I I ) , and h e x a h y d r o l i m o n i n i c a c i d (XXXV) i s t h e l e a s t a c t i v e i n t h e s e r i e s a t a l l t h e t h r e e doses t e s t e d . S e v e r a l o t h e r d i f f e r e n c e s between t h e f e e d i n g responses o f the two i n s e c t s emerge from t h e two s e t s of d a t a . Significantly h i g h e r a c t i v i t i e s o f obacunone (XXIX) r e l a t i v e t o those o f l i m o n i n (XXVI) a t t h e two lower doses, and n o m i l i n (XXX) a t a l l t h e t h r e e doses, c l e a r l y demonstrate t h e r o l e of r i n g A i n i n t e r a c t i o n w i t h the r e c e p t o r m o l e c u l e i n C. partellus. I n assays w i t h t h e C o l o r a d o p o t a t o b e e t l e , however, m o d i f i c a t i o n o f r i n g A o f l i m o n i n produced no s i g n i f i c a n t change i n t h e a n t i f e e d a n t a c t i v i t y o f t h e r e s u l t i n g compound s u g g e s t i n g no s i g n i f i c a n t r o l e f o r t h i s end o f t h e molecule. The epoxy group of r i n g D i s i m p o r t a n t f o r h i g h a c t i v i t y a g a i n s t t h e C o l o r a d o p o t a t o b e e t l e . On t h e o t h e r hand, i t s removal produces no s i g n i f i c a n t change i n a c t i v i t y a g a i n s t C. partellus. A summation of t h e d a t a on t h e C o l o r a d o p o t a t o b e e t l e suggests t h a t the s t r u c t u r a l u n i t s a s s o c i a t e d w i t h a n t i f e e d a n t a c t i v i t y a r e l a r g e l y l o c a l i s e d i n r i n g D and t h e f u r a n group. I n C. p a r t e l l u s these appear t o be spread o u t i n t h e l i m o n o i d s k e l e t o n , an i n f e r e n c e which was a l s o a r r i v e d a t i n an e a r l i e r s t u d y w i t h two o t h e r l e p i d o p t e r a n A f r i c a n p e s t s E. saccharina and M. testulalis (42).


8

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Golab, P., Webley, D.J. The use of Plants and Minerals as Traditional Protectants of Stored Products, Tropical Products Institute: London, Report G 138, 1980. 2. Kokwaro, J.O. Medicinal Plants of East Africa; East African Literature Bureau: Nairobi, 1976; p111. 3. Williams R.O. The Useful and Ornamental Plants in Zanzibar and Pemba; St. Ann's Press: Timperley, Altrincham, 1949; p383. 4. The Merck Index, Merck & Co., Inc., 1983, 10th ed. 5. Ladd, T.L. J. Econ. Entomol. 1980, 73, 718-720. 6. Sharma, R.N., Saxena, K.N. J . Med. Entomol. 1974, 11, 617621. 7. G i l l e t , J.D. Common African Mosquitoes; Heinemann: London, 1972; p26,68 and 102. 8. Kokwaro, J.O. Medicinal Plants of East Africa; East African Literature Bureau: Nairobi, 1976; p71. 9. Jondiko, I.J.O. Phytochem. 1986, 25, 2289-2290. 10. Jondiko, I.J.O.; Achieng' G., Pattenden, G. Fourteenth Annual Report, ICIPE, Nairobi, 1986, p84. 11. Jacobson, M. In Naturally Occurring Insecticides; Jacobson, M.; Crosby D.G., Eds., Dekker, New York, 1971; pl39-176. 12. E l l i o t , M. In Recent Advances in the Chemistry of Insect Control; Janes, N.F. Ed.; Royal Society of Chemistry, London, 1985; p73-102. 13. Kokwaro, J.O. Medicinal Plants of East Africa; East African Literature Bureau: Nairobi, 1976; pl77.



7. HASSANALI & LWANDE Antipest Secondary Metabolites from Africa 93

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40. Hassanali, Α.; Bentley, M.D.; Slawin, A.M.Z.; Williams, D.J.; Shepherd, R.N.; Chapya, A. Phytochemistry 1987, 26, 573-575. 41. Hassanali, A; Bentley, M.D.; Ole Sitayo, E.N.; Njoroge, P.E.W.; Yatagai, M. Insect Sci. Applic. 1986, 7, 495-499. 42. Hassanali, Α.; Bentley, M.D. In Natural Pesticides from the Neem Tree and other Plants: Schmutterer, H., Ascher, K.R.S.; Eds.; Proc. 3rd Int. Neem Conf.; Gesellschaft fur Technische Zusammenarbeit (GTZ): Eschborn, 1987; p683. RECEIVED

November 2,1988


Insecticides of Plant Origin - American Chemical Society

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