A Challenge for Controlled Release Pesticide Technology

3 A Challenge for Controlled Release Pesticide Technology PHILIP C. KEARNEY

Downloaded by PURDUE UNIVERSITY on August 5, 2013 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0053.ch003

Agricultural Environmental Quality Institute, U.S. Department of Agriculture, Beltsville, Md. 20705

C o n t r o l l e d r e l e a s e p e s t i c i d e technology holds great promise f o r improving the efficacy o f some existing p e s t i c i d e s and reduci n g the environmental problems a s s o c i a t e d w i t h others. Previous c o n t r o l l e d r e l e a s e d technology has been concerned w i t h f i n d i n g the c o r r e c t m a t r i x f o r combining the pesticide in a form t h a t will alter its availability t o the t a r g e t organism. C o n t r o l l e d - r e l e a s e f o r m u l a t i o n s , which improve p e s t i c i d e performance are now under i n v e s t i g a t i o n . Some o f the g r e a t e s t challenges t o the new field o f technology, however, may be in altering the environmental beh a v i o r of pesticides. Several environmental parameters, i n c l u d ing volatility, photodecomposition, l e a c h i n g , m i c r o b i a l metabolism, b i n d i n g , and accumulation, determine the p e r s i s t e n c e , distribution, and indirectly t h e efficacy of pesticides. Laboratory and field techniques have been developed, which allow f o r the measurement o f some o f these environmental parameters affecting pesticides. These techniques o f f e r o p p o r t u n i t i e s o f studying t h e effects o f formulation on p e r s i s t e n c e and movement o f p e s t i c i d e s . The purpose o f this paper is t o challenge scientists engaged in c o n t r o l l e d - r e l e a s e technology t o consider some o f t h e broader o p p o r t u n i t i e s o f f e r e d f o r improving p e s t i c i d e efficacy and s a f e t y . The ability t o manipulate t h e c o n c e n t r a t i o n o f the t o x i n presented t o the pest and t h e environment may make some o l d e r compounds more attractive commercially and may overcome some problems a s s o c i a t e d w i t h potentially u s e f u l new compounds t h a t otherwise might not be r e g i s t e r e d . The f o l l o w i n g d i s c u s s i o n will center p r i m a r i l y on the organic h e r b i c i d e s , s i n c e t h e author's e x p e r t i s e is more e x t e n s i v e in this branch o f pesticide chemistry. Persistence The o p p o r t u n i t y o f a l t e r i n g the p e r s i s t e n c e o f c e r t a i n herb i c i d e s seems t o be one o f the primary advantages o f c o n t r o l l e d r e l e a s e technology. There i s a group o f extremely usef u l herbicides that unfor30

In Controlled Release Pesticides; Scher, H.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

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Figure 1. Persistence of seven herbicides in soil. The length of each bar represents the time required for 90% loss of the herbicide from the site of application.



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C O N T R O L L E D R E L E A S E PESTICIDES

t u n a t e l y do not p e r s i s t long enough under a c t u a l f i e l d usage t o a f f o r d f u l l - s e a s o n weed c o n t r o l . Some o f these l e a s t - p e r s i s t e n t compounds are shown i n Figure 1, where the length o f the bar rep resents the time f o r 90% l o s s . From an environmental standpoint, these commpounds have been s t u d i e d e x t e n s i v e l y and seem to o f f e r no major problems. The r o l e t h a t c o n t r o l l e d - r e l e a s e technology can b r i n g t o im proved weed c o n t r o l can be represented i n a diagram r e c e n t l y sug gested by Dawson (1976) ( F i g 2 ) . One o f the f a c t o r s governing h e r b i c i d e s e l e c t i v i t y i s the maximum amount o f h e r b i c i d e t o l e r a t e d by the crop p l a n t and the minimum amount that c o n t r o l s the weed. Herbicides are g e n e r a l l y a p p l i e d at some economic r a t e between these two l i m i t s . I f the h e r b i c i d e degrades according to a f i r s t order r e a c t i o n r a t e , then an excess amount must be a p p l i e d t o achieve a reasonably long c o n t r o l p e r i o d . U n f o r t u n a t e l y , many of the short-term h e r b i c i d e s are d i s s i p a t e d before d e s i r e d c o n t r o l l e v e l i s achieved. In t h i s i d e a l i z e d scheme ( F i g . 2 ) , the c o n t r o l led r e l e a s e (CR) formation provides j u s t enough h e r b i c i d e f o r weed c o n t r o l at any one time, and the p e r i o d of a c t i v i t y i s s i g n i f i c a n t l y extended. Figure 2 i s s i m i l a r i n concept to one proposed by Cowan (1973) f o r responses from a s i n g l e i n j e c t i o n of a drug. Microencapsulation o f chlorpropham, one of the s h o r t - l i v e d h e r b i c i d e s ( F i g 1 ) , extended h e r b i c i d a l e f f e c t i v e n e s s f o r a s i g n i f i c a n t l y longer p e r i o d o f time than the commercial f o r m u l a t i o n (Gentner and Danielson, 1976). A 4.5 kg/ha a p p l i c a t i o n o f micro encapsulated chlorpropham at concentrations o f 17, 22, and 26 per cent were h e r b i c i d a l l y e f f e c t i v e f o r longer than 66 days, w h i l e a s i m i l a r a p p l i c a t i o n o f the commercial f o r m u l a t i o n was completely i n a c t i v e a f t e r 41 days, and i t s e f f i c a c y was questionable a f t e r 31 days. V o l a t i l i z a t i o n i s one o f the mechanisms by which c h l o r p r o pham i s l o s t from the s i t e o f a p p l i c a t i o n . Turner et a l . (1977) compared the vapor losses o f chlorpropham i n a microencapsulated f o r m u l a t i o n w i t h those i n an e m u l s i f i a b l e concentrate under f i e l d c o n d i t i o n s . In the f i r s t 4 hours a f t e r a p p l i c a t i o n , 86 g/ha of the . e m u l s i f i a b l e and 5.3 g/ha microencapsulated chlorpropham were v o l a t i l i z e d from the t a r g e t area. S o i l a n a l y s i s confirmed that a f t e r 7 days, 26% of the a p p l i e d e m u l s i f i a b l e concentrate and 51% of the microencapsulated chlorpropham remained i n the t a r g e t area. A major b e n e f i t that c o n t r o l l e d r e l e a s e technology can b r i n g t o weed c o n t r o l i s t o expand h e r b i c i d e s e l e c t i v i t y t o a d d i t i o n a l crops. As i l l u s t r a t e d i n Figure 3, based on a suggestion by Dawson (1976), the amount o f h e r b i c i d e r e q u i r e d t o c o n t r o l weeds i n crop A may i n j u r e crop B. U n f o r t u n a t e l y , the amount o f h e r b i cide t o l e r a t e d by crop Β i s too low t o p r o v i d e a s u f f i c i e n t p e r i o d of weed c o n t r o l . The c o n t r o l l e d - r e l e a s e f o r m u l a t i o n provides a continuous amount of h e r b i c i d e at a l e v e l s u f f i c i e n t to c o n t r o l weeds but not i n j u r e e i t h e r crop. Several experimental techniques are now a v a i l a b l e f o r measur ing the e f f e c t o f c o n t r o l l e d - r e l e a s e formulations on the mechanism


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Figure 2. Degradation of formulated and controlled release (CR) herbicide as a function of time in a single-crop system

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Figure 3. Degradation of formulated and controlled release (CR) herbicide as a function of time in a two-crop system



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and r a t e o f h e r b i c i d e l o s s . Turner and G l o t f e l t y (1977) measured v o l a t i l i t y l o s s e s under f i e l d c o n d i t i o n s by many polyurethane foam plugs. A simple m o d i f i c a t i o n o f the polyurethane foam p l u g technique has been adopted by Kearney and Kontson (1976) to measure both v o l a t i l i z a t i o n and metabolic metabolism under l a b o r a t o r y cond i t i o n s . The development o f the agroecosystem by B e a l l e_t a l . (1976) o f f e r s another, more e l a b o r a t e , technique f o r a n a l y z i n g v a r i o u s environmental components t h a t e f f e c t a p e s t i c i d e molecule i n a c l o s e d system. Agroecosysterns are inexpensive to c o n s t r u c t and operate, and o f f e r many advantages over conventional growth chambers f o r balance s t u d i e s on p e s t i c i d e s . A l l o f these systems o f f e r s c i e n t i s t s engaged i n c o n t r o l l e d - r e l e a s e technology s e v e r a l methods f o r measuring v o l a t i l i t y , degradation, and p e r s i s t e n c e under both l a b o r a t o r y and f i e l d c o n d i t i o n s . The most c h a l l e n g i n g concept i n a l t e r i n g h e r b i c i d e p e r s i s tence has been to shorten the e f f e c t i v e l i f e o f some of the more s t a b l e compounds. To date, there i s l i m i t e d experimental evidence that some compounds can be rendered l e s s p e r s i s t e n t . E a r l y work w i t h the phenoxy h e r b i c i d e s (Audus, 1960) showed that s o i l induced to metabolize MCPA would a l s o metabolize s e v e r a l phenoxyacetic a c i d s s u p p l i e d t o i t f a s t e r than would uninduced s o i l . This suggested that s o i l m i c r o b i a l populations would be induced t o metabol i z e a p a r t i c u l a r c l a s s o f h e r b i c i d e s , and more s p e c i f i c a l l y a l i n k a g e i n the molecule common t o members of t h a t c l a s s . In f i e l d s t u d i e s , MCPA was found t o degrade s i g n i f i c a n t l y f a s t e r i n s o i l s p r e v i o u s l y r e c e i v i n g f i v e annual a p p l i c a t i o n s o f the h e r b i c i d e than i n untreated s o i l (Fryer and K i r k l a n d , 1970). Therefore, i t may be f e a s i b l e t o a l t e r the s o i l m i c r o b i a l p o p u l a t i o n near a s o i l a p p l i e d h e r b i c i d e t o a c c e l e r a t e i t s metabolism. Loos and Kearney (1977) have attempted t o a l t e r the s o i l mic r o b i a l environment by p r o v i d i n g degradable substrates which s t r u c t u r a l l y resemble a t r a z i n e . Previous research on a t r a z i n e degradat i o n r e v e a l s that there are two major types o f cleavage: chemical replacement o f the c h l o r i n e i n the 2 - p o s i t i o n w i t h a hydroxyl group and m i c r o b i a l d e a l k y l a t i o n o f the e t h y l group (Esser ejt a l . , 1976). In the experiments o f Loos and Kearney (1977), s e v e r a l compounds c o n t a i n i n g carbon n i t r o g e n bonds were a p p l i e d to s o i l s t o a c c e l e r ate d e a l k y l a t i o n . These compounds i n c l u d e d ammelide, ammeline, cyanuric a c i d , N-ethylammelide, N^ethylammeline, urea, b i u r e t , and quanidine. To date, these experiments have o n l y met w i t h l i m i t e d success. Whether c o n t r o l l e d - r e l e a s e technology could provide a v e h i c l e f o r combining the h e r b i c i d e and a c c e l e r a t o r compound remains t o be t e s t e d . The goal o f being able t o p r e d i c t and modify the time of e f f e c t i v e n e s s o f p e r s i s t e n t h e r b i c i d e s i s an e x c i t i n g challenge that may yet be p o s s i b l e t o achieve. Movement As discussed i n the previous s e c t i o n , microencapsulation r e tarded the v o l a t i l i z a t i o n o f CIPC and, thereby, extended i t s e f f e c -



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t i v e n e s s under f i e l d c o n d i t i o n s . Another form o f movement that can reduce a h e r b i c i d e ' s e f f e c t i v e n e s s i s v e r t i c a l l e a c h i n g i n t o the s o i l p r o f i l e . For many preemergence h e r b i c i d e s , the compound must remain i n the germination zone o f the weed s e e d l i n g t o be e f f e c t i v e . M e t r i b u z i n i s a promising h e r b i c i d e f o r weed c o n t r o l i n soybeans and s e v e r a l other crops. The compound i s somewhat water s o l u b l e and, under c e r t a i n c o n d i t i o n s , extensive v e r t i c a l movement has been observed. I f t h i s movement can be c o n t r o l l e d , then much b e t t e r weed c o n t r o l i s obtained. Recently, Savage and McCormick (1977) combined m e t r i b u z i n i n a polymer system, which r e s t r i c t e d l e a c h i n g and s i g n i f i c a n t l y improved weed c o n t r o l . Several simple and s o p h i s t i c a t e d techniques are a v a i l a b l e t o r measuring l e a c h i n g o f h e r b i c i d e s i n s o i l s . H e l l i n g (1971) devised a s o i l t h i n - l a y e r system, which allows f o r a r a p i d , simple method o f measuring l e a c h i n g . H e r b i c i d e m o b i l i t y can be detected by u s i n g 14C-labeled compounds, f o l l o w e d by autoradiography on t h i c k e r p l a t e s ; thus, nonlabeled p e s t i c i d e s have been v i s u a l i z e d u s i n g b i o assay organisms. Savage and McCormick (1977) used s o i l t h i n - l a y e r m o b i l i t y s t u d i e s t o demonstrate the slow r e l e a s e o f m e t r i b u z i n . More complex column procedures are a l s o a v a i l a b l e f o r determining the l e a c h a b i l i t y o f p a r t i c u l a r h e r b i c i d e s . New

Challenges While the most promising challenges f o r c o n t r o l l e d r e l e a s e technology i s a l t e r i n g p e s t i c i d e p e r s i s t e n c e and movement, there may be other environmental parameters t h a t can be m o d i f i e d or a l t e r e d by f o r m u l a t i o n . P e s t i c i d e b i n d i n g t o p h y s i c a l and b i o l o g i c a l surfaces may be i n f l u e n c e d by f o r m u l a t i o n . The f u l l s i g n i f i c a n c e o f b i n d i n g i s not completely understood at t h i s time, although Kearney (1976) has discussed two aspects of p e s t i c i d e b i n d i n g to soils. Bound residues may be b e n e f i c i a l as a decontamination mechanism f o r i n a c t i v a t i n g p e s t i c i d e s i n s o i l s or u n d e s i r a b l e as potent i a l storage s i t e s f o r subsequent r e l e a s e o f the p e s t i c i d e s back i n t o the environment. When the f u l l i m p l i c a t i o n o f the b e n e f i c i a l or adverse e f f e c t s o f p e s t i c i d e b i n d i n g are understood, c o n t r o l l e d r e l e a s e technology may provide f o r m u l a t i o n s that enhance or deter b i n d i n g . The c a r r i e r could p r e f e r e n t i a l l y i n t e r a c t w i t h the b i n d ing s u r f a c e to modify the nature o f attachment o f the p e s t i c i d e . Several methods are a v a i l a b l e f o r measuring bound residues (Kaufman ot a l . , 1976) and these may provide i n f o r m a t i o n on the e f f e c t o f v a r i o u s f o r m u l a t i o n s on the extent and s t r e n g t h o f b i n d i n g . As c o n t r o l l e d r e l e a s e p e s t i c i d e technology continues t o expand i n the f i e l d o f p e s t i c i d e chemistry, new o p p o r t u n i t i e s w i l l a r i s e f o r s o l v i n g some o f the environmental problems a s s o c i a t e d w i t h expanded p e s t i c i d e usage. One o f the g r e a t e s t challenges t o improving c o n t r o l l e d r e l e a s e technology i s t o develop and u t i l i z e reasonable model systems f o r measuring v o l a t i l i t y , photodecomposition, metabolism, and other parameters. These simple systems o f f e r the most reasonable approach t o r a p i d l y a c h i e v i n g new advances i n cont r o l l e d - r e l e a s e technology.


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6. 7. 8. 9.

10.

11. 12. 13. 14.

Andus, L. J. in H e r b i c i d e s and the Soil (Ε. Κ. Woodford and G. R. Sager, eds.), B l a c k w e l l , Oxford pp 1-19 (1960). B e a l l , M. L., Jr., Nash, R. G., Kearney, P. C., Proc. Conf. Environ. Modeling S i m u l a t i o n EPA 600/9-76-016 (1976). Cowan, D. R., I n t r o d u c t i o n t o C o n t r o l l e d Release o f Biolog ically A c t i v e Agents, E d i t e d by A. C. Tanquary 2nd; R. E. Lacy, Plenum Press pp 1-13 (1973). Dawson, J. Η., Personal Communication (1976). Esser, H. O., Dupuis, G., E b e r t , Ε., Vogel, C., Marco, G. J., s - T r i a z i n e s in H e r b i c i d e s : Chemistry, Degradation and Mode o f A c t i o n (P. C. Kearney and D. D. Kaufman, eds.) V o l . 1, pp 129-208, 2 ed., Marcel Dekker, Inc., New York (1975). F r y e r , J. D., K i r k l a n d , Κ., Weed Res. 10, 133 (1970). Gentner, W. Α., Danielson, L. L., Proc. 1976 C o n t r o l l e d Re lease P e s t i c . Symp. 7, 27, Akron, Ohio (1976). H e l l i n g , C. S., Soil Sci. Soc. Am. Proc. 35, 737 (1971). Kaufman, D. D., Still, G. G., Paulson, G. D., Bandai, S. Κ., Bound and Conjugated P e s t i c i d e Residues. ACS Symp. Ser. 29, 396 pp. (1976). Kearney, P. C., Summary o f Soil Bound Residue D i s c u s s i o n Ses s i o n in Bound and Conjugated P e s t i c i d e Residues (D. D. Kauf man, G. G. Still, G. D. Paulson and S. K. Bandal, eds.). ACS Symp. Ser. 29, 378-382 (1976). Kearney, P. C., Kontson, Α., J. A g r i c . Food Chem. 24, 424 (1976). Savage, Κ. Ε., McCormick, C. L., Proc. Southern Weed Sci. Soc., D a l l a s , Texas (1977). Turner, B. C., Glotfelty, D. Ε., A n a l . Chem. 49, 7 (1977). Turner, B. C., Glotfelty, D. Ε., T a y l o r , A. W. and Watson, D. R., 173rd N a t l . Mtg. Amer. Chem. Soc. (Abstr.) #8, New Orleans, La. (1977).


A Challenge for Controlled Release Pesticide Technology

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