1 Principles of Controlled Release Pesticides D. H. LEWIS and D. R. COWSAR
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Southern Research Institute, Birmingham,Ala.35205
T h e r a p i d l y g r o w i n g d e m a n d f o r food a n d energy presents t h e scientific c o m m u n i t y w i t h a major c h a l l e n g e . T h e need f o r m o r e e f f i c i e n t operations i n both industrial a n d a g r i c u l t u r a l production is paramount. M e t h o d s and processes t h a t a f f o r d higher y i e l d s and b e t t e r q u a l i t y , r e q u i r e less t i m e a n d m o n e y , a n d do n o t pose a t h r e a t t o t h e e n v i r o n m e n t a r e b e i n g sought a t a n u n p a r a l l e l e d p a c e w h i c h undoubtedly w i l l i n t e n s i f y f u r t h e r i n t h e l a s t q u a r t e r o f t h i s c e n t u r y . T h e c o n t r o l o f pests, a b a t t l e fought f o r g e n e r a t i o n s , i s u n q u e s t i o n a b l y a crucial t a s k if our goals f o r t h e f u t u r e c o n c e r n i n g food and e n e r g y a r e t o b e a c h i e v e d . R e c e n t e s t i m a t e s suggest t h a t losses due t o i n e f f i c i e n t p e s t - c o n t r o l t e c h n i q u e s a m o u n t t o b e t w e e n $10 and 30 billion e a c h y e a r (1). Losses o f cotton alone t o i n s e c t s e x c e e d $500 million p e r a n n u m . (2) In India, rodents a r e r e p o r t e d (3) t o destroy a fourth of t h e harvested grain crop annually. The astounding losses due t o pests a r e n o t limited t o food supplies a l o n e . T h e f o u l i n g o f s h i p hulls c o s t s t h e U . S . N a v y a n a d d i t i o n a l $150 m i l l i o n / y e a r i n f u e l (4), $15 m i l l i o n / y e a r i n labor a n d m a t e r i a l s f o r r e p a i n t i n g ships w i t h a n t i f o u l a n t s , and a p p r o x i m a t e l y $200 million e a c h y e a r f o r r e p l a c e m e n t o f b i o l o g i c a l l y d e t e r i o r a t e d m a r i n e p i l i n g s (5). T h e i m p o r t a n c e o f p e s t i c i d e s is e v i d e n c e d by t h e e s t i m a t e d sale o f $2.5 billion o f p e s t i c i d e s e a c h y e a r i n t h e U . S . alone (6). A l t h o u g h t h e reality o f i n c r e a s i n g danger t o m a n from persistent pesticides is recognized, the frightening fact remains that if terrestrial h e r b i c i d e s alone w e r e b a n n e d , s t a r v a t i o n w o u l d b e c o m e m o r e p r e v a l e n t i n t h e w o r l d p o p u l a t i o n i n short o r d e r . Historically, s c i e n t i s t s h a v e d e a l t w i t h t h e p r o b l e m o f pest c o n t r o l by designing new, .more potent a g e n t s . H o w e v e r , use o f these agents t o produce t h e d e s i r e d b i o l o g i c a l response is o f t e n i n e f f i c i e n t , p r i m a r i l y because o f i n a b i l i t i e s t o d e l i v e r t h e agents t o t h e i r t a r g e t s a t t h e p r e c i s e t i m e and i n t h e o p t i m u m q u a n t i t i e s r e q u i r e d . E n o r m o u s a m o u n t s o f funds are r e q u i r e d f o r t h e d e v e l o p m e n t o f a n e w b i o c i d e . R e c e n t e s t i m a t e s p l a c e t h e d e v e l o p m e n t costs f o r a new p e s t i c i d e a t about $8 million (6). R e c o g n i z i n g t h e c o s t a n d limitations i n t h e design o f n e w p e s t i c i d e s , s c i e n t i s t s began t o t u r n i n t h e 1960s t o a n alternative a p p r o a c h , t h a t o f i m p r o v i n g t h e d e l i v e r y o f t h e a g e n t s , b o t h n e w e r agents and o l d . In today's t e r m i n o l o g y , a 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 o r d e l i v e r y s y s t e m i s d e f i n e d as a c o m b i n a t i o n o f b i o l o g i c a l l y a c t i v e agent and e x -
1
In Controlled Release Pesticides; Scher, H.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.
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CONTROLLED
RELEASE
PESTICIDES
c i p i e n t , usually a p o l y m e r , a r r a n g e d t o a l l o w d e l i v e r y of t h e agent t o the t a r g e t at c o n t r o l l e d r a t e s o v e r a s p e c i f i e d p e r i o d . The r a p i d e m e r g e n c e of c o n t r o l l e d r e l e a s e as an e s t a b l i s h e d s c i e n t i f i c f i e l d is e v i d e n c e d by the g r o w i n g n u m b e r of r e l a t e d p u b l i c a t i o n s a p p e a r i n g i n t h e l i t e r a t u r e and the i n c r e a s i n g n u m b e r of s y m p o s i a on t h e subject e a c h y e a r . A s r e c e n t as 1973, for e x a m p l e , only one s y m p o s i u m was d e v o t e d t o c o n t r o l l e d r e l e a s e , w h i l e d u r i n g 1976 s e v e r a l major m e e t i n g s addressed t h e subject i n d e p t h . F e w areas of r e s e a r c h have a c t i v a t e d t h e i n t e r e s t and a t t e n t i o n of s u c h a m u l t i d i s c i p l i n a r y group of p r o f e s s i o n a l s . A t t e n d e e s at w o r k s h o p s , c o n f e r e n c e s , and s y m p o s i a on c o n t r o l l e d r e l e a s e t y p i c a l l y i n c l u d e b i o l o g i s t s , chemists, engineers, pharmacologists, agronomists, entomologists, vete r i n a r i a n s , d e n t i s t s , and p h y s i c i a n s . It is now a w e l l r e c o g n i z e d f a c t t h a t t h e t e c h n o l o g y or c o n t r o l l e d r e l e a s e c a n c o n t r i b u t e p o s i t i v e l y t o man's f i g h t against disease and hunger. C o n t r o l l e d r e l e a s e as a p p l i e d t o a g r i c u l t u r a l c h e m i c a l s is fast b e c o m i n g the subject of i n t e n s i v e r e s e a r c h . Although slow-release f e r t i l i z e r s w e r e k n o w n at l e a s t 30 years ago, m o s t of the s i g n i f i c a n t a d v a n c e s have c o m e i n t h e l a s t 10 y e a r s . O b v i o u s l y , i n a f i e l d e x p a n d i n g so r a p i d l y , a c o m p l e t e r e v i e w of t h e l i t e r a t u r e c a n n o t be g i v e n h e r e . H o w e v e r , a n u m b e r of e x c e l l e n t p u b l i c a t i o n s are a v a i l a b l e w h i c h o f f e r m o r e d e t a i l e d t r e a t m e n t s on f u n d a m e n t a l s of c o n t r o l l e d r e l e a s e and discussions of 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 f o r m u l a t i o n s (7-12). C a r d a r e l l i (1), a pioneer i n t h e f i e l d , has a u t h o r e d an e x c e l l e n t c o m p r e h e n s i v e r e v i e w of 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 f o r m u l a t i o n s . T h i s c h a p t e r is i n t e n d e d to i n t r o d u c e the n e w c o m e r t o the f i e l d , and for t h e e x p e r i e n c e d r e s e a r c h e r , t o m a k e t h e papers t h a t f o l l o w m o r e c o h e s i v e . F i r s t , a b r i e f d i s c u s s i o n of t h e s h o r t c o m i n g s of c o n v e n t i o n a l methods of d e l i v e r y w i l l be g i v e n , t h e n an o v e r v i e w of 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 , and f i n a l l y a f e w of t h e c u r r e n t a p p l i c a t i o n s of c o n t r o l l e d r e l e a s e . A d v a n t a g e s of C o n t r o l l e d R e l e a s e The p r i n c i p a l a d v a n t a g e of 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 is t h a t t h e y a l l o w m u c h less p e s t i c i d e t o be used for the s a m e p e r i o d of a c t i v i t y . M o r e o v e r , w h e n t h e n o r m a l h a l f - l i f e of a potent p e s t i c i d e is s h o r t , the r e l e a s e f o r m u l a t i o n s are e s p e c i a l l y advantageous i n c o m p a r i s o n t o c o n v e n t i o n a l methods of a p p l i c a t i o n . T o f u l l y u n d e r s t a n d t h i s b e n e f i t , one m u s t f i r s t u n d e r s t a n d the m a g n i t u d e of t h e e n v i r o n m e n t a l f o r c e s t h a t a c t t o r e m o v e excesses of p e s t i c i d e s f r o m t h e i r s i t e s of a p p l i c a t i o n . When a p p l i e d by c o n v e n t i o n a l m e t h o d s , p e s t i c i d e s are i n v a r i a b l y subject t o l e a c h i n g , e v a p o r a t i o n , and d e g r a d a t i o n ( p h o t o l y t i c , h y d r o l y t i c , and m i c r o b i a l ) , a l l of w h i c h r e m o v e t h e a c t i v e m a t e r i a l s f r o m t h e i r t a r g e t b e f o r e they c a n p e r f o r m t h e i r f u n c t i o n . In m o s t c a s e s , t h e r a t e of r e m o v a l f o l l o w s f i r s t - o r d e r k i n e t i c s , i . e . , t h e r a t e of r e m o v a l a t any t i m e is d i r e c t l y p r o p o r t i o n a l t o the a m o u n t (or c o n c e n t r a t i o n of t h e p e s t i c i d e present i n t h e e n v i r o n m e n t at t h a t t i m e . A m a t h e m a t i c a l e x p r e s s i o n of t h e f i r s t - o r d e r r a t e l a w is g i v e n by E q u a t i o n 1, (1)
In Controlled Release Pesticides; Scher, H.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.
1.
LEWIS
AND
COWSAR
Principles
of Controlled
Release
Pesticides
3
w h e r e dM/dt is t h e r a t e of r e m o v a l , k is t h e r a t e c o n s t a n t , and M is t h e a m o u n t of p e s t i c i d e present at any f i m e t . The i n t e g r a t e d s o l u t i o n t o E q u a t i o n 1 is M 1 η (
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(2)
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where M The r a t e expressed order rate
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is t h e a m o u n t present at t = 0; M is thus t h e a m o u n t a p p l i e d . of r e m o v a l of a p e s t i c i d e f r o m t h e e n v i r o n m e n t is o f t e n as t h e agent's h a l f - l i f e , ty. The h a l f - l i f e is r e l a t e d t o the f i r s t c o n s t a n t f o r r e m o v a l , k , as f o l l o w s : œ
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or,
k
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(3)
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M
If M is the m i n i m u m e f f e c t i v e l e v e l of p e s t i c i d e and M is t h e a m o u n t
