Thiophenes and Acetylenes: Phototoxic Agents to ... - ACS Publications

C h a p t e r 18

Thiophenes and Acetylenes: Phototoxic Agents to Herbivorous and Blood-Feeding Insects 1

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J. T. Arnason , B. J . R. Philogène , P. Morand , J . C. Scaiano , N. Werstiuk , and J. Lam Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 10, 2016 | http://pubs.acs.org Publication Date: May 7, 1987 | doi: 10.1021/bk-1987-0339.ch018

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Ottawa-Carleton Institute for Graduate Studies and Research in Biology and Chemistry, Ottawa K1N 6N5, Canada Division of Chemistry, National Research Council, Ottawa K1A 0R6, Canada Department of Chemistry, McMaster University, Hamilton, Ontario, Canada Department of Organic Chemistry, University of Aarhus, Aarhus, Denmark

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Thiophenes and acetylenes of the plant family Asteraceae have been found to be highly phototoxic to insects. Effects on herbivorous insects include formation of necrotic lesions, growth reduction and mortality. Mosquito larvae can be controlled at concentrations as low as a few parts per billion. E f f i c i e n t synthetic methods have been developed to produce naturally occurring compounds and derivatives for laboratory and f i e l d trials. A high temperature dilute acid technique has produced t r i t i a t e d phototoxins for pharmacokinetic studies i n i n s e c t s . In fundamental studies on the photos e n s i t i z a t i o n mechanism, laser flash photolysis has been used to determine t r i p l e t lifetimes of the s e n s i t i z e r , rates of energy transfer to O and electron transfer to acceptor, or from donor molecules. The available information suggests that this group of phototoxins has excellent potential for development as i n s e c t i c i d e s . 2

The d i s c o v e r y t h a t many d i v e r s e secondary m e t a b o l i t e s from d i f f e r e n t p l a n t f a m i l i e s a r e p h o t o s e n s i t i z e r s (1,2) suggests n o t o n l y t h a t p h o t o t o x i c i t y has a r i s e n i n d e p e n d e n t l y many times i n e v o l u t i o n as a defense mechanism but t h a t i t may have s i g n i f i c a n t advantages i n d i s c o u r a g i n g p l a n t p e s t s . S t u d i e s o f these naturally occurring systems o f defense p r o v i d e , i n a d d i t i o n , new models f o r t h e development o f p e s t c o n t r o l a g e n t s . The p r e s e n t r e p o r t concerns a l a r g e group o f p h o t o t o x i c compounds, t h e t h i o p h e n e s and b i o s y n t h e t i c a l l y r e l a t e d p o l y a c e t y l e n e s o f t h e A s t e r a c e a e , t h e i r r o l e as a l l e l o c h e r a i c a l s t o i n s e c t p e s t s , and t h e i r p o s s i b l e e x p l o i t a t i o n as i n s e c t i c i d e s . The l i g h t - m e d i a t e d t o x i c i t y t o organisms o t h e r than i n s e c t s by substances from t h i s p l a n t f a m i l y was e s t a b l i s h e d by Gommers' group i n t h e N e t h e r l a n d s (3) and Towers group i n Canada ( 4 ) . S u b s e q u e n t l y , i t was demonstrated t h a t 9 out of 14 compounds t e s t e d were p h o t o t o x i c t o mosquito l a r v a e a t a c o n c e n t r a t i o n o f 0.5 ppm under sources o f 1

0097-6156/87/0339-0255$06.00/0 © 1987 American Chemical Society

Heitz and Downum; Light-Activated Pesticides ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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LIGHT-ACTIVATED PESTICIDES near UV i r r a d i a t i o n ( 5 ) . The a c t i v i t y was even g r e a t e r i n s u n l i g h t and two compounds, a l p h a - t e r t h i e n y l (a-T) ( I ) a n d a f u r a n o a c e t y l e n e ( I I ) had e x c e p t i o n a l i n s e c t i c i d a l a c t i v i t y w i t h LC 'S of 19 and 79 ppb r e s p e c t i v e l y ( 6 ) . T h i s u n u s u a l l y h i g h a c t i v i t y l e d t o the p a t e n t i n g of t h e s e substances as i n s e c t i c i d a l agents ( 7 ) and t o the i n v e s t i g a t i o n of the p o t e n t i a l of the compounds as mosquito l a r v i c l d e s .

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 10, 2016 | http://pubs.acs.org Publication Date: May 7, 1987 | doi: 10.1021/bk-1987-0339.ch018

Herbivorous i n s e c t s W h i l e we may h y p o t h e s i z e t h a t p h o t o t o x i c c h e m i c a l s i n the A s t e r a c e a e have a r i s e n i n the course of e v o l u t i o n through s e l e c t i v e p r e s s u r e s e x e r t e d by h e r b i v o r o u s i n s e c t s or p l a n t pathogens, the study of p h o t o s e n s i t l z a t l o n of i n s e c t h e r b i v o r e s has been l i m i t e d because of the l a r g e amount of the c h e m i c a l s r e q u i r e d f o r c a r e f u l i n v e s t i g a t i o n of the e f f e c t of these s u b s t a n c e s on i n s e c t development. Downum e t a l (8) and Champagne e t a l . (9) have demonstrated p h o t o t o x i c e f f e c t s i n Manduca s e x t a and Euxoa m e s s o r l a induced by i n j e s t e d or t o p i c a l l y a p p l i e d a-T. L a t e r Champagne e t a l . (10) examined the e f f e c t of seven a c e t y l e n e s and t h i o p h e n e s on t h r e e i n s e c t s p e c i e s . S e v e r a l ( I , I I I , I V ) but not a l l of these compounds were p h o t o t o x i c t o M. s e x t a and/or E. m e s s o r l a . However, O s t r i n i a n u b i l a l i s tended t o a v o i d the e f f e c t s of p h o t o s e n s i t i z a t i o n by b u r r o w i n g i n t o d i e t or s p i n n i n g s i l k , b e h a v i o r s which may be a d a p t a t i o n s t o a v o i d p h o t o s e n s i t l z a t l o n by an I n s e c t which i s known t o feed on p h o t o t o x i c A s t e r a c e a e . S p e c i f i c a l l y these t h r e e s t u d i e s i n d i c a t e d t h a t a b s o r p t i o n of l i g h t by the p h o t o t o x i c c h e m i c a l s may i n d u c e m o r t a l i t y , l e n g t h e n l a r v a l development t i m e , and reduce f e e d i n g and growth of s e n s i t i v e i n s e c t s . The most acute e f f e c t s observed were p u p a l d e f o r m i t i e s and n e c r o t i c l e s i o n s i n the c u t i c l e t h a t p r e v e n t e c d y s i s . The e f f e c t s of p h o t o s e n s i t l z a t l o n by a c e t y l e n e s and thiophenes of the A s t e r a c e a e on i n s e c t h e r b i v o r e s i s comparable t o Berenbaum s d e s c r i p t i o n of the e f f e c t s of furanocoumarins of the Apiaceae on unadapted i n s e c t s ( 1 1 ) . I t I s not s u r p r i s i n g t h a t both these f a m i l i e s share an adapted i n s e c t fauna t h a t appears to t o l e r a t e or a v o i d the p h o t o s e n s i t i z i n g chemicals. P h o t o t o x i c a c e t y l e n e s and thiophenes presumably p r o v i d e enhanced p r o t e c t i o n of the p l a n t by v i r t u e of t h e i r i n v o l v e m e n t i n h i g h energy p h o t o c h e m i c a l p r o c e s s e s and the c a t a l y t i c n a t u r e of s i n g l e t oxygen g e n e r a t i o n which they mediate. Thus, p l a n t s may m i n i m i z e t h e i r m e t a b o l i c investment i n c h e m i c a l defenses by a r e l a t i v e l y f r e e commodity, l i g h t , which I s always a v a i l a b l e i n s u i t a b l e p l a n t h a b i t a t s . These compounds are found i n a v a r i e t y of above ground l o c a t i o n s . F o r example, p h e n y l h e p t a t r i y n e (PHT) ( I I I ) i s found i n h i g h c o n c e n t r a t i o n s i n t h e c u t i c l e of l e a v e s of B i d e n s p i l o s a ( 1 ) , w h i l e thiophenes are found i n h i g h c o n c e n t r a t i o n s i n prominent m a r g i n a l l e a f glands of P o r o p h y l l u m spp (Arnason, u n p u b l i s h e d ) . Many members of the s u b t r i b e P e c t i n i d a e are r i c h i n t h i o p h e n e s i n t h e i r above ground parts (38). I t s h o u l d be emphasized, however, t h a t w i t h o u t p h o t o s e n s i t i z i n g r a d i a t i o n , a c e t y l e n e s and thiophenes s t i l l p o s s e s s many of the i n s e c t d e t e r r e n t e f f e c t s observed w i t h o t h e r 1



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Thiophenes and Acetylenes

n o n - p h o t o s e n s i t i z i n g p l a n t secondary m e t a b o l i t e s e.g. f e e d i n g d e t e r r e n c e , growth r e d u c t i o n , and reduced n u t r i e n t u t i l i z a t i o n (8,9,10,12). The presence of the compounds i n m o d e r a t e l y h i g h c o n c e n t r a t i o n s i n the r o o t s of many A s t e r a c e a e (1) s t i l l suggests a d e f e n s i v e r o l e , a l t h o u g h more e x p e n s i v e m e t a b o l i c a l l y . D i f f e r e n t i a t i o n between l i g h t and dark t o x i c i t y f o r h e r b i v o r o u s i n s e c t s i s not as marked i n microorganisms or mosquito l a r v a e , p o s s i b l y because of the l a c k of l i g h t p e n e t r a t i o n i n t o l a r g e r organisms. Other r e p o r t s of t h e i n s e c t i c i d a l a c t i v i t y of a c e t y l e n e s and t h i o p h e n e s e x i s t i n t h e l i t e r a t u r e (e.g. 12, 14, and 1 5 ) , but l i t t l e a t t e n t i o n was p l a c e d on the p o s s i b l e r o l e of p h o t o s e n s i t l z a t l o n i n these i n v e s t i g a t i o n s . Recent s t u d i e s have attempted t o e x p l a i n why some i n s e c t s p e c i e s a r e more s e n s i t i v e t o a-T than o t h e r s . F o r example, l a t e i n s t a r MU_ s e x t a and P i e r i s rapae a r e v e r y s e n s i t i v e t o a-T but 0. n u b i l a l i s and H e l i o t h l s v l r e s c e n s a r e more t o l e r a n t ( T a b l e 1 ) .

Table 1 Contact P h o t o t o x i c i t y o f q - t e r t h l e n y l to l a s t I n s t a r lavae

Manduca s e x t a P i e r i s rapae H e l i o t h l s vlrescens Ostrinla nubilalis

10 15 474 698

Note: L a r v a e i n t h e i r l a s t i n s t a r were weighed and t r e a t e d w i t h a - t e r t h i e n y l d i s s o l v e d i n acetone a t the f o l l o w i n g r a t e s of a p p l i c a t i o n 0, 1, 3, 30, 100, 300, 1000, ug/g l a r v a e . Insects were i r r a d i a t e d under b l a c k l l g h t b l u e lamps f o r 12 hours a t 2w/m^ and L C v a l u e s c a l c u l a t e d from a p r o b i t a n a l y s i s of the m o r t a l i t y data. 5 Q

I n o r d e r t o e x p l a i n these d i f f e r e n c e s , t r i t i a t e d p h o t o t o x i n s were produced i n h i g h y i e l d (80-100%) by a h i g h temperature d i l u t e a c i d t e c h n i q u e developed by W e r s t i u k (16) f o r m e t a b o l i c s t u d i e s . The procedure was o p t i m i z e d by p r e l i m i n a r y s t u d i e s u s i n g D 0 as the i s o t o p e source b e f o r e f i n a l i n c o r p o r a t i o n of t r i t i u m from HT0. The n a t u r e of the e l e c t r o p h i l i e exchange of a r o m a t i c p r o t o n s p e r m i t s p r e d i c t i o n of t h e r e l a t i v e r a t e s of exchange which has been c o n f i r m e d by NMR s p e c t r o s c o p y . Both H a-T and H-Me-a-T have been produced by t h i s p r o c e d u r e . P h a r m a c o k i n e t i c s t u d i e s (17) have shown t h a t , a f t e r a t o p i c a l a p p l i c a t i o n of t h e l a b e l t o t h e t e s t i n s e c t s , t h e h a l f time f o r c l e a r a n c e was v e r y slow f o r s e n s i t i v e M. s e x t a (48 h r ) , but was much more r a p i d f o r t o l e r a n t (). n u b i l a l i s (6 h r ) and H. v l r e s c e n s (20 h r ) . I n a d d i t i o n , a-T a d m i n i s t e r e d I n t h e d i e t was found t o c r o s s t h e gut and e n t e r the hemolymph t o a g r e a t e r e x t e n t i n M. s e x t a than i n t h e two r e s i s t a n t s p e c i e s . At l e a s t one major 2

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m e t a b o l i t e has been d e t e c t e d i n f e c e s of these i n s e c t s but i t s c h e m i c a l i d e n t i t y i s not y e t known. A p p a r e n t l y , e f f i c i e n t metabolism and c l e a r a n c e of a-T i s one mechanism by which some h e r b i v o r o u s i n s e c t s can d e a l w i t h t h i s p h o t o t o x i c a l l e l o c h e m i c a l . B i o l o g i c a l e f f e c t s at the t h i r d t r o p h i c l e v e l have a l s o been demonstrated w i t h a-T. I n c o r p o r a t i o n of 100 |ig/g of a-T i n t o d i e t s of 0. n u b i l a l i s s i g n i f i c a n t l y reduced the r a t e of p a r a s i t i z a t i o n , s u r v i v a l to p u p a t i o n and a d u l t emergence of the hymenopterous p a r a s i t o i d , Dladegma t e r e b r a n s r e l a t i v e to c o n t r o l s ( 1 8 ) . D e t e c t i o n of a-T i n the a d u l t p a r a s i t o i d suggested t h a t p h o t o s e n s i t l z a t l o n of the p a r a s i t o i d may be p o s s i b l e a l t h o u g h I t has not yet been d i r e c t l y demonstrated. Mosquito l a r v a e The e v a l u a t i o n of a-T or o t h e r m o l e c u l e s as commercial l a r v i c i d e s has been hampered by l a c k of an e f f i c i e n t s y n t h e s i s . E x i s t i n g s y n t h e s e s were e i t h e r h i g h l y I n e f f i c i e n t or were m u l t i s t e p p r o c e d u r e s . A p p l i c a t i o n of a c a t a l y z e d c o u p l i n g r e a c t i o n , f i r s t r e p o r t e d by Tomao et_ a l . (19) on a m l l l i m o l e s c a l e , has made i t p o s s i b l e to produce a-T on 50 g s c a l e u s i n g r e a d i l y a v a i l a b l e 2-bromothlophene and 2,5-dlbromothlophene as s t a r t i n g m a t e r i a l s . The p r o c e s s i s a one pot Grignard-Wurtz r e a c t i o n I n v o l v i n g a N i c a t a l y s t which d r a s t i c a l l y reduces the f o r m a t i o n of b y p r o d u c t s , f o l l o w e d by a s i m p l e p u r i f i c a t i o n step to g i v e 99% pure m a t e r i a l ( 2 0 ) . A r e c e n t improvement i s the e l i m i n a t i o n of d i e t h y l e t h e r , an i n d u s t r i a l l y u n d e s i r a b l e s o l v e n t , f o r which a p r o c e s s p a t e n t a p p l i c a t i o n (21) has been f i l e d . F i e l d t r i a l s have been u n d e r t a k e n at a deciduous f o r e s t s i t e f o r 3 y e a r s and at a b o r e a l s i t e f o r 2 y e a r s . Spray a p p l i c a t i o n t e s t s of a-T f o r m u l a t e d i n e t h a n o l to n a t u r a l snow melt p o o l s t h a t are Aedes spp. b r e e d i n g s i t e s , r e v e a l e d t h a t e f f e c t i v e c o n t r o l c o u l d be a c h i e v e d at an a p p l i c a t i o n r a t e as low as 10 g a c t i v e i n g r e d i e n t / h a w i t h i n 7 days p o s t a p p l i c a t i o n ( 2 2 ) . More r a p i d (1-2 days p o s t a p p l i c a t i o n ) and r e l i a b l e c o n t r o l i s a c h i e v e d a t lOOg a . i . / h a . T h i s i s comparable to the e f f i c a c y of c u r r e n t l y used organophosphates (e.g. Temephos and P i r i m i p h o s M e t h y l ) but l e s s than t h a t of some p y r e t h r o i d s . Recent work has focussed on the development of a s u i t a b l e c o n c e n t r a t e and an o i l - b a s e d s p r e a d i n g f o r m u l a t i o n t h a t are c o n v e n i e n t to use and are as e f f e c t i v e or b e t t e r than the o r i g i n a l e t h a n o l f o r m u l a t i o n . A p o t e n t i a l l i m i t a t i o n f o r p h o t o t o x i c c o n t r o l agents i s d i m i n i s h e d l i g h t . We have observed reduced e f f i c a c y i n t u r b i d p o o l s as compared to c l e a r p o o l s (Table 2) or under heavy o v e r c a s t as compared to c l e a r , sunny c o n d i t i o n s . However, w i t h i n a s h o r t t i m e , t h e r e i s a comparable t o x i c i t y at the reduced l i g h t l e v e l s u g g e s t i n g t h a t the l i g h t requirement i s s a t u r a t e d f a i r l y rapidly.



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E v a l u a t i o n of e n v i r o n m e n t a l s a f e t y of a-T w i t h r e s p e c t t o n o n - t a r g e t organisms r e q u i r e s more p r e c i s e t o x i c i t y d a t a . This has been a c h i e v e d by l a b o r a t o r y t r i a l s or s i m u l a t e d pond t r i a l s at the f i e l d s i t e w i t h f i e l d c o l l e c t e d mosquitos and n o n - t a r g e t organisms h e l d i n b i o a s s a y cages and p l a c e d i n p l a s t i c wading p o o l s f i l l e d w i t h pond w a t e r . T a r g e t and non t a r g e t data from t h i s t r i a l are shown i n T a b l e 3. A p p l i c a t i o n of 0.10 kg/ha of a-T can e f f e c t i v e l y c o n t r o l mosquito l a r v a e w i t h m i n i m a l e f f e c t s toward damsel and c a d d i s f l y l a r v a e , t r o u t , or s n a i l s . These n o n - t a r g e t r e s u l t s w i t h damsel and c a d d i s f l y are b e t t e r than c u r r e n t l y used p y r e t h r o i d s . However Daphnia and midge are a d v e r s e l y a f f e c t e d at t h i s r a t e of a p p l i c a t i o n by a-T. Table 2 L a r v i c i d a l E f f i c a c y o f q-T Treatment

as a f u n c t i o n o f l i g h t and

time

Days p o s t - a p p l i c a t i o n

EC

g Q

(g/ha) Turbid pool*

1 2

4365 132

Clear pool

1 2

104 60

* F i n e sediment was d i s t u r b e d i n p o o l s t o c r e a t e a s i t u a t i o n where l i g h t p e n e t r a t i o n of water was reduced. EC f o r mosquito l a r v i c i d e a c t i v i t y was determined as d e s c r i b e d i n ( 2 0 ) . Q

Table 3 T a r g e t and Non-Target Data f o r A l p h a T e r t h i e n y l EC (kg7ha) 5 Q

L C

5n (ppb)

reference

T a r g e t organism mosquito l a r v a e

0.046

15-30

6,22

N o n - t a r g e t organisms damsel f l y l a r v a e 0.38 caddis f l y larvae 1.32 midge l a r v a e 0.178 daphnia l a r v a e 0.044 trout fingerlings 10 snail 10 tadpole 18-111 f a t h e a d minnows 50-650 Note E C v a l u e s are based on s u r f a c e area a p p l i c a t i o n s v a l u e s are c a l c u l a t e d on a c o n c e n t r a t i o n b a s i s . 5 0

22 22 22 22 22 22 23 24 and


LC

5 0

260

LIGHT-ACTIVATED PESTICIDES Kagan (23-24 and t h i s volume) has r a i s e d e n v i r o n m e n t a l concerns w i t h r e s p e c t to use of a-T as a l a r v i c i d e because of i t s t o x i c i t y to t a d p o l e s and f a t h e a d minnows when f o r m u l a t e d i n d i m e t h y l s u l f o x i d e (DMSO). While the L C v a l u e s f o r these organisms are h i g h e r than f o r mosquito l a r v a e ( T a b l e 3) and t h e use of DMSO as a f o r m u l a t i n g agent may be q u e s t i o n e d , t h e r e i s c l e a r l y a need f o r more n o n - t a r g e t d a t a . E v a l u a t i o n s of the t o x i c o l o g y of a-T show t h a t i t i s n o n - t o x i c to mice at 300 mg/kg by o r a l or dermal r o u t e s of a d m i n i s t r a t i o n s e v e r a l days a f t e r a d m i n i s t r a t i o n ( 2 0 ) , but t h a t i t i s t o x i c to r a t s at 1000 mg/kg i n 24 h r . P h y s i o l o g i c a l e f f e c t s at h i g h doses i n c l u d e CNS d e p r e s s i o n and h y p o t e n s i v e a c t i v i t i e s Te.g. decreased motor a c t i v i t y and l o s s of r i g h t i n g r e f l e x ( 2 5 ) ] . A c e t y l e n e s and thiophenes are non-mutagenic ( 2 6 ) , but can p h o t o s e n s i t i z e human s k i n at h i g h doses ( 1 ) . The l a t t e r was not found to be a problem under d i l u t i o n s used f o r l a r v i c i d i n g .


5 Q

Analogues and d e r i v a t i v e s o f

q-T

A number of i n v e s t i g a t o r have s y n t h e s i z e d q-T d e r i v a t i v e s and analogues, u s u a l l y by a m u l t i s t e p procedure i n v o l v i n g v a r i o u s c y c l i z a t i o n r e a c t i o n s (see r e f 20). S e v e r a l 1,3-butadiene and thiophene d e r i v a t i v e s prepared i n one of these procedures have been r e p o r t e d to be p h o t o t o x i c t o mosquito l a r v a e on a q u a l i t a t i v e b a s i s ( 2 7 ) . R e c e n t l y , the c a t a l y z e d G r i g n a r d c o u p l i n g r e a c t i o n was used to produce a s e r i e s of compounds (28) s t r u c t u r a l l y r e l a t e d to q-T. LC v a l u e s between 15 and 1000 ppb were r e p o r t e d f o r these compounds. Only one compound, a monomethyl d e r i v a t i v e of a-T ( V ) , was more t o x i c than the parent compound ( T ) . C o n s i d e r a b l e success i n the p e s t i c i d e i n d u s t r y has been a c h i e v e d by the p r o d u c t i o n of designed analogues of n a t u r a l p r o d u c t s such as p y r e t h r i n s and j u v e n i l e hormones. We have i n v e s t i g a t e d the analogues and d e r i v a t i v e s of a-T i n r e l a t i o n to t h e i r p h o t o c h e m i c a l and p h y s i c a l p r o p e r t i e s ( 2 9 ) . L a s e r f l a s h p h o t o l y s i s has demonstrated t h a t most of the compounds have l o n g 5 0

Structures I - V


18.

ARNASON ET

AL.


261

t r i p l e t l i f e t i m e s , and t h a t t h e s e t r i p l e t s are e f f i c i e n t l y quenched by 0 to produce s i n g l e t 0 w i t h a quantum y i e l d between 0.4 and 1.0; the quantam y i e l d i s 0.86 f o r a-T i t s e l f which i s one of the most e f f i c i e n t s i n g l e t 0 g e n e r a t o r s i n the s e r i e s t e s t e d . O c t a n o l - w a t e r p a r t i t i o n c o e f f i c e n t s (P) have been e s t i m a t e d to range from l o g P v a l u e s of 4 t o 7. M u l t i p l e r e g r e s s i o n of the d a t a suggested t h a t t o x i c i t y was at l e a s t p a r t i a l l y p r e d i c t a b l e on the b a s i s of p h o t o c h e m i s t r y and p a r t i t i o n c o e f f i c i e n t s . A d e s i g n model based on t h i s and o t h e r i n f o r m a t i o n s h o u l d l e a d to the p r o d u c t i o n of more e f f i c i e n t p h o t o t o x i n s i n the future. 2


2

Mechanisms o f A c t i o n o f q-T, i t s s y n t h e t i c analogues d e r i v a t i v e s , and n a t u r a l l y o c c u r l h g a c e t y l e n e s

and

Assignement of mechanisms of a c t i o n f o r these p h o t o t o x i n s i s c o n t r o v e r s i a l . At the m o l e c u l a r l e v e l , q-T was o r i g i n a l l y r e p o r t e d to have no 0 r e q u i r e m e n t f o r p h o t o t o x i c a c t i o n ( 3 0 ) . Other r e p o r t s have now c l e a r l y e s t a b l i s h e d s i n g l e t 0 generation as the i n v i v o and i n v i t r o mode of a c t i o n of q-T ( 1 , 2 ) . S u b s e q u e n t l y , c o n f l i c t i n g i n t e r p r e t a t i o n s have emerged s u g g e s t i n g t h a t p o l y a c e t y l e n e s such as p h e n y l h e p t a t r i y n e (PHT) might a c t by a w h o l l y photodynamic mechanism (31) or by competing photodynamic and f r e e r a d i c a l mechanisms ( 3 2 ) . The r e c e n t a p p l i c a t i o n of l a s e r f l a s h p h o t o l y s i s has r e s o l v e d some of the m e c h a n i s t i c c o n t r o v e r s i e s and p r o v i d e s new i n s i g h t i n t o the p h o t o c h e m i s t r y of these m o l e c u l e s (33-35). With b o t h t h i o p h e n e s and a c e t y l e n e s , l a s e r e x c i t a t i o n r e s u l t s i n the f o r m a t i o n of s t r o n g t r i p l e t s i g n a l s w i t h long l i f e t i m e s (28 us f o r PHT i n MeOH, 30 or 57 us f o r q-T i n MeOH or EtOH r e s p e c t i v e l y ) . The t r i p l e t s are e f f i c i e n t l y quenched by 0 , and by methyl v i o l o g e n but not by amines. W i t h PHT, quenching by 0 and e l e c t r o n t r a n s f e r to m e t h y l v i o l o g e n occur w i t h comparable r a t e c o n s t a n t s ( 3 4 ) , but the back r e a c t i o n of the e l e c t r o n t r a n s f e r (presumably from PHTj" to m e t h y l v i o l o g e n ) was l e s s e f f i c i e n t (50-60%) ( 3 5 ) . R e c e n t l y , d e t e c t i o n of a f r e e r a d i c a l s i g n a l by ESR s p e c t r o s c o p y , i n liposomes c o n t a i n i n g PHT, has been a c h i e v e d a f t e r i r r a d i a t i o n w i t h UV-A ( 3 6 ) . Thus d i r e c t and i n d i r e c t e v i d e n c e suggests t h a t PHT may act by g e n e r a t i o n of f r e e r a d i c a l s or s i n g l e t oxygen. At the b i o c h e m i c a l l e v e l , Kagan (27) has r e p o r t e d I n a c t i v a t i o n of the a c e t y l c h o l i n e s t e r a s e of mosquito l a r v a e under c o n d i t i o n s t h a t induce severe i n s e c t m o r t a l i t y . A more immediate e f f e c t may be the d e s t r u c t i o n of a n a l g i l l membranes of l a r v a e which can be d i r e c t l y observed by l i g h t m i c r o s c o p y . Loss of membrane i n t e g r i t y r e s u l t s i n the r e l e a s e of e l e c t r o l y t e s i n t o w a t e r , and t h i s i s g r e a t l y enhanced under p h o t o s e n s i t i z i n g c o n d i t i o n s (Table 4) at s u b l e t h a l c o n d i t i o n s . W i t h h e r b i v o r o u s i n s e c t s , t h e r e i s e v i d e n c e of i n t e r f e r e n c e w i t h raelanization and s c l e r a t l z a t i o n of pupae (8) and damage to the gut (10) of l a r v a e , i n a d d i t i o n to the d r a m a t i c c u t l c u l a r l e s i o n s . 2

2

2

2



262

Table 4 Halide detected

I n démineraitzed Water a f t e r 2 hours (meq/L. ±S.D.)


-gT

+gT

-UV

0.080 ±0.012

0.075 ±0.007

+UV

0.105 ±0.004

0.237 ±0.0002

E i g h t y Aedes a t r o p a l p u s f o u r t h i n s t a r l a r v a e were t r e a t e d w i t h c o m b i n a t i o n s o f 100 ppb a-T and near TJV (4w/m^) i 8 ml d e m i n e r a l l z e d w a t e r . H a l i d e leakage was d e t e c t e d w i t h a B u c h l e r Chloridometer. No m o r t a l i t y was observed i n t h e 2 h. t r e a t m e n t . n

Conclusion R e s i s t a n c e and c r o s s - r e s i s t a n c e a r e major problems w i t h c u r r e n t l y used i n s e c t i c i d e s ( 3 7 ) . The development o f new p e s t c o n t r o l agents i s , f o r t h i s r e a s o n , e s s e n t i a l . With a-T, c r o s s r e s i s t a n c e t o organophosphates does not appear t o be a problem ( 2 4 ) . R e s i s t a n c e t o p h o t o t o x i n s may be slow t o develop i n a q u a t i c l a r v a e because of the r a p i d and n o v e l mode o f a c t i o n o f these s u b s t r a c t s . F i n a l commercial development o f thiophenes o r a c e t y l e n e s w i l l depend, however, on many f a c t o r s i n c l u d i n g p r o p r i e t a r y r i g h t s , market s i z e , and s u i t a b i l i t y f o r r e g i s t r a t i o n . T h e i r demonstrated mode o f a c t i o n and e f f i c a c y and the p r e l i m i n a r y d a t a on t o x i c o l o g y and n o n - t a r g e t e f f e c t s suggests t h a t these s u b s t a n c e s have an e x c e l l e n t p o t e n t i a l f o r t a k i n g t h e i r p l a c e a l o n g s i d e of o t h e r p h o t o t o x i c i n s e c t i c i d e s , h e r b i c i d e s and cheraotherapeutic agents (1,2) i n t h e new p h o t o t o x i n t e c h n o l o g y t h a t i s d e v e l o p i n g i n industry. Acknowledgment T h i s work was supported by t h e Department of N a t i o n a l Defence (Canada) and by the N a t u r a l S c i e n c e s and E n g i n e e r i n g Research C o u n c i l o f Canada. The c o n t r i b u t i o n o f many s t u d e n t s and t e c h n i c i a n s t o t h i s work i s g r a t e f u l l y acknowledged. L i t e r a t u r e Cited 1. 2. 3. 4.

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RECEIVED March10,1987


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