Characterization and Exploitation of Instability of Spider Mite

Chapter 5

Characterization and Exploitation of Instability of Spider Mite Resistance to Acaricides

Downloaded by STONY BROOK UNIV SUNY on October 5, 2017 | http://pubs.acs.org Publication Date: February 23, 1990 | doi: 10.1021/bk-1990-0421.ch005

T. J. Dennehy, J. P. Nyrop, and T. E. Martinson Department of Entomology, New York State Agricultural Experiment Station, Cornell University, Geneva, NY 14456 Resistance of spider mites to some important acaricides has been observed to increase in response to selection and to decline in the absence of selection. Management of such resistances could be based on development of methods for monitoring resistance and characterization of the processes responsible for declining resistance frequencies in the absence of selection. Studies of the dynamics of resistance to dicofol in two ecologically dissimilar acarine pests of New York apple, Panonychus ulmi and Tetranychus urticae, are described. Methods were developed which allow rapid estimation of the frequency of resistant mites in field populations. Studies conducted in 1987 related the susceptibility of field populations to acaricide use. Laboratory studies conducted in 1987-88 characterized the rate of decline of resistant genotypes in caged populations and derived selection coefficients to estimate the relative fitness of resistant individuals. Strategies for management of dicofol resistance will be based on information from dynamics of resistance and cross-resistance, and ultimately will prescribe temporal rotations of dicofol and alternate chemicals in order to keep the frequency of dicofol resistance below levels which impair acaricide efficacy. S p i d e r m i t e s have d e m o n s t r a t e d t h e a b i l i t y t o become r e s i s t a n t t o e s s e n t i a l l y a l l categories of organic a c a r i c i d e s . In concert with t h e t r e n d toward fewer new a c a r i c i d e s e n t e r i n g t h e m a r k e t p l a c e each year, the successive l o s s o f e f f e c t i v e n e s s o f a c a r i c i d e s t o r e s i s t a n c e has r e s u l t e d i n a d e a r t h o f e f f e c t i v e a c a r i c i d e s f o r many a g r i c u l t u r a l commodities. This problem i s e s p e c i a l l y acute f o r t r e e f r u i t p r o d u c t i o n i n t h e n o r t h e a s t e r n U n i t e d S t a t e s , where r e s i s t a n c e has been documented, o r i s s u s p e c t e d t o e x i s t , t o e v e r y r e g i s t e r e d a c a r i c i d e except petroleum o i l . An e x c e l l e n t o v e r v i e w o f t h e h i s t o r y o f p e s t i c i d e r e s i s t a n c e i n t h e T e t r a n y c h i d a e i s p r o v i d e d by Cranham and H e l l e (1) S p i d e r m i t e r e s i s t e n c e t o organophosphates has been w i d e s p r e a d f o r many y e a r s ( 2 ) . O r g a n o t i n r e s i s t a n c e became an i s s u e i n t h e e a r l y 1 9 8 0 s ,

0097-6156/90/0421-0077$06.00/0 © 1990 American Chemical Society

Green et al.; Managing Resistance to Agrochemicals ACS Symposium Series; American Chemical Society: Washington, DC, 1990.


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MANAGING RESISTANCE TO AGROCHEMICALS

(2u. A) and r e s i s t a n c e t o t h e i m p o r t a n t o r g a n o s u l f u r a c a r i c i d e , p r o p a r g i t e , was documented soon t h e r e a f t e r ( 5 - 8 ) . R e s i s t a n c e o f s p i d e r m i t e s t o f o r m e t a n a t e h y d r o c h l o r i d e , a carbamate a c a r i c i d e , has l i m i t e d t h e u s e f u l n e s s o f t h i s m a t e r i a l i n some m a j o r a g r i c u l t u r a l systems (3*. .2.) . R e s i s t a n c e t o t h e c h l o r i n a t e d h y d r o c a r b o n a c a r i c i d e , d i c o f o l , was f i r s t documented n e a r l y t h i r t y y e a r s ago (lfl_), and c o n t i n u e s t o be a p r o b l e m i n some a g r i c u l t u r a l systems (11-13). The a c a r i c i d a l p y r e t h r o i d s r e p r e s e n t r e l a t i v e l y recent products f o r a g r i c u l t u r a l c o n t r o l of acarines, yet, recent r e p o r t s o f f i e l d f a i l u r e (T. A r c h e r , Texas A&M U n i v . , p e r s o n a l communication), and l a b o r a t o r y s t u d i e s ( u n p u b l i s h e d r e s u l t s from our l a b o r a t o r i e s ) i n d i c a t e t h a t r e s i s t a n c e problems w i t h t h i s c l a s s o f a c a r i c i d e s are l i k e l y . D i s c o n c e r t i n g r e p o r t s from A u s t r a l i a (14.) have d e s c r i b e d r e c e n t f i e l d f a i l u r e s o f t h e new o v i c i d a l a c a r i c i d e s , c l o f e n t i z i n e and h e x y t h i a z o x . Though t h e s e o v i c i d e s a r e j u s t g a i n i n g r e g i s t r a t i o n i n t h e U.S., t h e s p e c t e r o f r e s i s t a n c e i s s u f f i c i e n t l y t h r e a t e n i n g t h a t manufacturers are p r u d e n t l y f o r m u l a t i n g ' r e s i s t a n c e management' s t r a t e g i e s f o r t h e s e m a t e r i a l s from t h e i n c e p t i o n o f t h e i r r e g i s t r a t i o n s i n t h e U.S.. S p i d e r M i t e / D i c o f o l System f o r Studying Resistance Dynamics I t a p p e a r s t h a t s p i d e r m i t e resistance, o r , more p r e c i s e l y , s p i d e r m i t e s which p o s s e s s h e r i t a b l e , s u b s t a n t i a l r e d u c t i o n s i n s u s c e p t i b i l i t y , occur to e s s e n t i a l l y a l l of the a c a r i c i d e s widely u s e d i n a g r i c u l t u r e , and p o s s i b l y f o r some newer c l a s s e s o f a c a r i c i d a l compounds t h a t have not been w i d e l y employed t o - d a t e . T h e r e f o r e , f o r most a c a r i c i d e s i t i s not a q u e s t i o n o f whether r e s i s t a n c e e x i s t s p e r se, r a t h e r i t i s o f g r e a t e s t r e l e v a n c e t o determine i f problematic frequencies of r e s i s t a n t mites occur at s p e c i f i c l o c a t i o n s and, t h r o u g h r e s e a r c h , d e t e r m i n e what management e f f o r t s w i l l keep f r e q u e n c i e s o f r e s i s t a n t p e s t s below t h o s e which impair f i e l d performance of a c a r i c i d e s . C l e a r l y , we cannot manage (or study) what we cannot measure reliably. Yet, t h e development o f r e l i a b l e methods f o r e s t i m a t i o n of r e s i s t a n c e f r e q u e n c i e s i n s p i d e r m i t e p o p u l a t i o n s has p r o v e n t o be a major o b s t a c l e w i t h a number o f i m p o r t a n t a c a r i c i d e s . D e t e c t i o n o f r e s i s t a n c e t o t h e o r g a n o t i n compounds, c y h e x a t i n and f e n b u t a t i n o x i d e , and t o t h e o r g a n o s u l f u r , p r o p a r g i t e , has p r o v e n t o be q u i t e d i f f i c u l t because t h e s e m a t e r i a l s a r e r e p e l l a n t , and r e l a t i v e l y slow a c t i n g , and d i f f e r e n c e s between r e s i s t a n t and s u s c e p t i b l e f i e l d populations are r e l a t i v e l y small ( p r o p a r g i t e : [ 1 5 . ] ; c y h e x a t i n : [2+ 1£] / f e n b u t a t i n o x i d e : [11]) . Overlap i n c o n c e n t r a t i o n - r e s p o n s e o f r e s i s t a n t and s u s c e p t i b l e p o p u l a t i o n s t o t h e s e a c a r i c i d e s e s s e n t i a l l y p r e c l u d e s p r a c t i c a l use o f s i n g l e d i s c r i m i n a t i n g c o n c e n t r a t i o n s f o r monitoring f r e q u e n c i e s of r e s i s t a n t i n d i v i d u a l s and o t h e r w i s e g r e a t l y c o m p l i c a t e s m o n i t o r i n g of r e s i s t a n c e . N e v e r t h e l e s s , v a l u a b l e and arduous d o c u m e n t a t i o n o f r e s i s t a n c e t o t h e s e m a t e r i a l s has been done ( e . g . , i u . 18 H ) . D i c o f o l r e s i s t a n c e i n s p i d e r m i t e s , though o f l e s s e r p r a c t i c a l i m p o r t a n c e i n some a g r i c u l t u r a l systems t h a n r e s i s t a n c e t o t h e o r g a n o t i n and o r g a n o s u l f u r a c a r i c i d e s , p r o v i d e s a s y s t e m c o n d u c i v e t o t h e s t u d y o f r e s i s t a n c e dynamics. R e s i s t a n c e i s g e n e r a l l y m a n i f e s t as a > 1 0 0 0 - f o l d d i f f e r e n c e i n s u s c e p t i b i l i t y o f r e s i s t a n t v e r s u s s u s c e p t i b l e i n d i v i d u a l s t o d i c o f o l r e s i d u e s (12. 2SL) . D i c o f o l i s r e l a t i v e l y f a s t a c t i n g and, t h e r e f o r e , amenable t o use i n 24 h, ' r a p i d b i o a s s a y s ' (21), whereby p l a s t i c p e t r i d i s h e s a r e t r e a t e d w i t h a d i s c r i m i n a t i n g c o n c e n t r a t i o n o f d i c o f o l and u s e d t o estimate the p r o p o r t i o n of r e s i s t a n t i n d i v i d u a l s i n p o p u l a t i o n s . f


5. DENNEHY ET AL.

79 Instability of Spider Mite Resistance to Acaricides

The e a s e and r e l i a b i l i t y o f m o n i t o r i n g d i c o f o l r e s i s t a n c e makes t h i s r e s i s t a n c e e s p e c i a l l y conducive f o r experimentation that involves t r a c k i n g changes i n r e s i s t a n c e gene f r e q u e n c y i n p o p u l a t i o n s .


A s s e m b l i n g a Management Program The f a c t t h a t r e s i s t a n c e has become a l i m i t i n g f a c t o r f o r many s p i d e r m i t e management programs t h r o u g h o u t t h e c o u n t r y i s e v i d e n c e d by t h e c u r r e n t commonplace use o f t h e t e r m r e s i s t a n c e management i n p e s t management. U n f o r t u n a t e l y , t h e use o f t h i s t e r m i s , i n l a r g e p a r t , h y p e r b o l e , s i n c e o n l y a l i m i t e d number o f s i t u a t i o n s e x i s t where t h e p r e r e q u i s i t e groundwork has been d e v e l o p e d t o a l l o w e v a l u a t i o n , much l e s s i m p l e m e n t a t i o n , o f s p e c i f i c s t r a t e g i e s t h a t impact r e s i s t a n c e development. In our own s i t u a t i o n , up t o now we have managed v e r y l i t t l e w i t h r e g a r d t o r e s i s t a n c e t o d i c o f o l i n s p i d e r m i t e p o p u l a t i o n s i n New York a p p l e o r c h a r d s . However, i n an attempt t o e x p l o i t t h e h y p o t h e s i z e d i n s t a b i l i t y o f s p i d e r m i t e r e s i s t a n c e t o d i c o f o l , we have begun c o m p i l i n g t h e b a s i c b u i l d i n g b l o c k s o f a r e s i s t a n c e management program. R e s i s t a n t phenotypes have been c h a r a c t e r i z e d , and t h e f r e q u e n c y o f r e s i s t a n c e i n f i e l d p o p u l a t i o n s has been documented (12.) . The i n h e r i t a n c e o f d i c o f o l r e s i s t a n c e has been d e s c r i b e d f o r two r e s i s t a n t p o p u l a t i o n s f r o m New York a p p l e o r c h a r d s (22) . A p r a c t i c a l method f o r r a p i d d e t e c t i o n o f d i c o f o l r e s i s t a n c e has been d e v e l o p e d and e x t e n s i v e l y u s e d i n f i e l d t r i a l s t o d e t e r m i n e t h e f r e q u e n c y o f r e s i s t a n c e a t which e f f i c a c y o f d i c o f o l t r e a t m e n t s becomes i n a d e q u a t e ( c r i t i c a l f r e q u e n c y ) . Crossr e s i s t a n c e s t u d i e s ( c u r r e n t l y underway) w i l l show which a l t e r n a t e a c a r i c i d e s a r e most s u i t a b l e f o r use a g a i n s t d i c o f o l - r e s i s t a n t populations.

S t a b i l i t y of Spider Mite Resistance

to Acaricides

S t a b i l i t y of r e s i s t a n c e to a c a r i c i d e s i n s p i d e r mite populations has been a l o n g - s t a n d i n g p o i n t o f d i s c u s s i o n i n t h e l i t e r a t u r e . Here we use t h e t e r m stability of resistance t o d e s c r i b e changes i n s u s c e p t i b i l i t y o f p o p u l a t i o n s which o c c u r i n t h e absence o f s e l e c t i o n . Stable r e s i s t a n c e s a r e t h o s e i n which t h e s u s c e p t i b i l i t y o f p o p u l a t i o n s does not change i n t h e absence o f s e l e c t i o n . For example, organophosphate r e s i s t a n c e i n s p i d e r m i t e s has been observed t o p e r s i s t at r e l a t i v e l y high f r e q u e n c i e s i n f i e l d p o p u l a t i o n s t h r o u g h o u t t h e w o r l d , and has been o b s e r v e d t o p e r s i s t f o r l o n g p e r i o d s o f time i n l a b o r a t o r y c u l t u r e s t h a t have not been t r e a t e d with a c a r i c i d e s . W i t h i n the context of t h i s paper, instability of resistance r e f e r s t o t h e f i e l d phenomenon o f d e c l i n i n g r e s i s t a n c e f r e q u e n c i e s i n t h e absence o f s e l e c t i o n . W i t h unstable r e s i s t a n c e s , the s u s c e p t i b i l i t y of p o p u l a t i o n s i n c r e a s e s i n t h e absence o f s e l e c t i o n . Whether due t o i m m i g r a t i o n o f s u s c e p t i b l e i n d i v i d u a l s from refuges or other e c o l o g i c a l or g e n e t i c processes s u c h as a s s o r t a t i v e m a t i n g o r r e d u c e d f i t n e s s o f r e s i s t a n t o r g a n i s m s , u n s t a b l e r e s i s t a n c e s o f f e r t h e o p p o r t u n i t y t o manage r e s i s t a n c e s by u s i n g r o t a t i o n s o f m a t e r i a l s which do not d e m o n s t r a t e p o s i t i v e c r o s s - r e s i s t a n c e t o one a n o t h e r . Though s u b j e c t i v e l y d e f i n e d , and o f t e n not i n t e n s i v e l y i n v e s t i g a t e d w i t h t h e n e c e s s a r y f i e l d and l a b o r a t o r y s t u d i e s , t h e phenomenon o f i n s t a b i l i t y has been n o t e d f o r TEPP r e s i s t a n c e (22.) i t h e o r g a n o t i n a c a r i c i d e s (24-26), p r o p a r g i t e (&), d i c o f o l (see b e l o w ) , and f o r m e t a n a t e h y d r o c h l o r i d e . Noteworthy c o n t r a d i c t i o n s r e g a r d i n g r e s i s t a n c e s t a b i l i t y e x i s t . W e l t y e t a l . (22) found r e s i s t a n c e t o c y h e x a t i n i n Panonychus ulmi t o be s t a b l e i n two f i e l d


80


p o p u l a t i o n s e x p o s e d f o r two y e a r s t o t h r e e d i f f e r e n t l e v e l s o f c y h e x a t i n s e l e c t i o n ( i n c l u d i n g an u n t r e a t e d g r o u p ) . S i m i l a r l y , Hoy (12) r e p o r t e d t h a t greenhouse p o p u l a t i o n s o f Tetranychus pacificus h e l d f o r n i n e months w i t h o u t s e l e c t i o n w i t h c y h e x a t i n r e t a i n e d t h e i r original resistance levels. D i f f e r e n c e s i n species of s p i d e r mite t e s t e d , s e l e c t i o n pressure i n the surrounding a g r i c u l t u r a l areas, and d i f f e r e n c e s i n magnitude and s u s c e p t i b i l i t y o f immigrant m i t e s may a c c o u n t f o r some o f t h e c o n f l i c t i n g r e s u l t s . D i c o f o l has l o n g been t h e s u b j e c t o f d i s c u s s i o n s o f r e s i s t a n c e stability. T h i s a c a r i c i d e has been u s e d f o r o v e r 25 y e a r s and c o n t i n u e s t o be e f f e c t i v e i n c o n t r o l l i n g s p i d e r m i t e s i n many commodities t h r o u g h o u t t h e w o r l d . Inoue (28 29), c i t e d f i e l d and l a b o r a t o r y e v i d e n c e from Japan t h a t r e s i s t a n c e o f Panonychus citri t o d i c o f o l i n c r e a s e d f o l l o w i n g s e l e c t i o n , but d e c r e a s e d r e l a t i v e l y q u i c k l y a f t e r r e l a x a t i o n o f s e l e c t i o n . Inoue emphasized t h e v a l u e o f s t u d y i n g dynamics o f d i c o f o l r e s i s t a n c e and c i t e d two o t h e r f i e l d s t u d i e s which d e t a i l i n s t a b i l i t y o f d i c o f o l r e s i s t a n c e i n Japanese c i t r u s g r o v e s (2&) . D i t t r i c h (20.) r e f e r r e d to the comparatively slow development o f r e s i s t a n c e t o d i c o f o l , d e s p i t e i t s worldwide use, and c i t e d a 1965 s u r v e y by G h o b r i a l e t a l . (21) of suscept i b i l i t y o f s p i d e r m i t e s c o l l e c t e d from Egypt i n which t h e p r e s e n c e o f d i c o f o l r e s i s t a n c e i n T. arabicus and T. cucurbitacearum was f o u n d i n o n l y 2 o f 10 l o c a t i o n s . S i m i l a r m o n i t o r i n g c o n d u c t e d i n 1968-70 showed g r e a t l y i n c r e a s e d f r e q u e n c i e s o f d i c o f o l - r e s i s t a n t m i t e s (32.) . M o n i t o r i n g o f r e s i s t a n c e i n A u s t r a l i a by Unwin (23.) d e m o n s t r a t e d d e c l i n i n g r e s i s t a n c e t o d i c o f o l i n T. urticae, and showed t h a t r e s i s t a n t phenotypes were p r e s e n t a t o n l y 2 o f 11 locations tested. S o v i e t workers have a l s o r e p o r t e d u n s t a b l e r e s i s t a n c e t o d i c o f o l (34 35).


f

r

D u t c h workers f o u n d t h a t r e s i s t a n c e t o d i c o f o l , p a r a t h i o n , and t e t r a d i f o n was s t i l l p r e s e n t i n most g l a s s h o u s e p o p u l a t i o n s o f T. urticae d e s p i t e t h e f a c t t h e s e m a t e r i a l s had not been u s e d i n t h e g l a s s h o u s e s f o r 7 y e a r s (36 37). However, s p i d e r m i t e p o p u l a t i o n s c o l l e c t e d from w i l d p l a n t s a t v a r y i n g d i s t a n c e s from t h e g l a s s h o u s e s were r e s i s t a n t t o p a r a t h i o n and t e t r a d i f o n , but not t o d i c o f o l , even though a d j a c e n t g l a s s h o u s e s h a r b o r e d t h e m u l t i p l y - r e s i s t a n t s p i d e r mites. Monitoring of spider mite s u s c e p t i b i l i t y to d i c o f o l i n C a l i f o r n i a c o t t o n d e m o n s t r a t e d t h a t , a f t e r o v e r 25 y e a r s o f use, problems w i t h d i c o f o l r e s i s t a n c e were l i m i t e d t o c a . 10% o f San Joaquin V a l l e y cotton f i e l d s (M). S i m i l a r l y , P r e e and Wagner (31) r e p o r t e d t h a t d i c o f o l r e s i s t a n c e i n P. ulmi was d e t e c t e d i n o n l y 2 o f 45 N i a g a r a P e n i n s u l a , O n t a r i o , o r c h a r d s i n 1984, whereas H e m e (13) p r e v i o u s l y f o u n d r e s i s t a n c e i n o v e r 50% o f 35 o r c h a r d s s u r v e y e d f r o m t h e same a r e a . Dennehy e t a l . (12) described resistance to d i c o f o l i n T. u r t i c a e and P. ulmi c o l l e c t e d i n 1985 f r o m New York apple orchards. Of 10 a p p l e o r c h a r d s from which T. urticae was c o l l e c t e d , only 3 possessed detectable l e v e l s of r e s i s t a n t i n d i v i d uals. F r e q u e n c i e s o f d i c o f o l r e s i s t a n c e i n P. ulmi p o p u l a t i o n s v a r i e d f r o m 0 t o >70% and were, on t h e a v e r a g e , much h i g h e r t h a n o b s e r v e d w i t h T. u r t i c a e . In t h i s p a p e r we d e t a i l i n v e s t i g a t i o n s c o n d u c t e d i n 1987-88 o f t h e dynamics o f r e s i s t a n c e t o d i c o f o l i n p o p u l a t i o n s o f T. urticae and P. ulmi. F i r s t , we r e l a t e t h e s u s c e p t i b i l i t y o f f i e l d p o p u l a t i o n s o f b o t h s p i d e r m i t e s p e c i e s c o l l e c t e d f r o m o v e r 30 d i f f e r e n t New York a p p l e o r c h a r d s t o t h e use o f d i c o f o l w i t h i n t h o s e o r c h a r d s . Next, we d e s c r i b e t h e changes i n s u s c e p t i b i l i t y t o d i c o f o l o f caged, h e t e r o g e n e o u s p o p u l a t i o n s o f T. urticae m a i n t a i n e d f o r 1015 g e n e r a t i o n s i n t h e absence o f s e l e c t i o n w i t h d i c o f o l . Selection c o e f f i c i e n t s e s t i m a t i n g t h e r e l a t i v e f i t n e s s (under t h e e x p e r i m e n t f


5. DENNEHY ETAL.

Instability of Spider Mite Resistance to Acaricides 81

conditions) of d i c o f o l - r e s i s t a n t versus derived.


S u s c e p t i b i l i t y of F i e l d Populations

- s u s c e p t i b l e T. urticae

are

versus Treatment H i s t o r y

M a t e r i a l s and Methods. Records o f a c a r i c i d e u s e i n 50 W e s t e r n New York a p p l e o r c h a r d s ( c a . 1-10 ha i n s i z e ) , from 1982-87, were obt a i n e d f r o m t h e C o r n e l l U n i v e r s i t y A p p l e IPM Program. T o t a l s o f 0-3 a p p l i c a t i o n s of the a c a r i c i d e s petroleum o i l , d i c o f o l , cyhexatin, and p r o p a r g i t e were a p p l i e d a n n u a l l y t o each o r c h a r d . No d i c o f o l t r e a t m e n t s were made i n any o f t h e s e o r c h a r d s i n e i t h e r 1986 o r 1987. E s t i m a t e s o f s u s c e p t i b i l i t y t o d i c o f o l were d e r i v e d f o r a t o t a l o f 30 o f t h e s e o r c h a r d f o r Panonychus ulmi and from 15 o r c h a r d p o p u l a t i o n s o f Tetranychusurticae. S p i d e r m i t e s t o be b i o a s s a y e d were c o l l e c t e d f r o m m i d - J u l y t o mid-September, 1987. C o l l e c t i o n s were made by c l i p p i n g 100 t e r m i n a l ends, c a . 30 cm i n l e n g t h , f r o m apple branches w i t h i n a n i n e - t r e e sampling u n i t i n each orchard. The n i n e - t r e e s a m p l i n g u n i t was composed o f a c e n t r a l t r e e and t h e eight trees comprising a rectangle surrounding the c e n t r a l t r e e . A p p l e t e r m i n a l s were sampled from a h e i g h t o f 0.5-2.0 m above t h e orchard f l o o r . Twenty t e r m i n a l s were c o l l e c t e d f r o m t h e 360 d e g r e e c i r c u m f e r e n c e o f t h e c e n t r a l t r e e , and 80 t e r m i n a l s were c o l l e c t e d f r o m t h e 8 t r e e s s u r r o u n d i n g t h e c e n t r a l t r e e . These 80 t e r m i n a l s c o m p r i s e d t h e sum o f t e n t e r m i n a l s p e r t r e e , t a k e n f r o m t h e 180 degree a r c o f t h e adjacent t r e e s o r i e n t e d c l o s e s t t o the c e n t r a l tree. The 100 t e r m i n a l s c o l l e c t e d a t e a c h o r c h a r d were p l a c e d i n 23 l a r g e p l a s t i c bags and were p r o m p t l y t r a n s p o r t e d t o t h e laboratory. B i o a s s a y s o f m i t e s were c o n d u c t e d w i t h i n 48 h r o f s a m p l i n g orchards. Though p o p u l a t i o n s o f b o t h T. u r t i c a e and P. u l m i were sometimes c o l l e c t e d from a s i n g l e sample o f t e r m i n a l s f r o m an o r c h a r d , b i o a s s a y s o f t h e two s p e c i e s were done s e p a r a t e l y . Mites were removed from t h e i n f e s t e d f o l i a g e w i t h f i n e b r u s h e s . To e n s u r e t h a t m i t e s t o be b i o a s s a y e d were s e l e c t e d f r o m t h r o u g h o u t t h e 100 t e m i n a l samples, a f t e r one m i t e was removed from any t e r m i n a l , no a d d i t i o n a l m i t e s were removed from t h a t t e r m i n a l u n t i l a l l o t h e r t e r m i n a l s had been i n s p e c t e d s i m i l a r l y . A p p r o x i m a t e l y 120 h e a l t h y a d u l t f e m a l e m i t e s o f each s p e c i e s p r e s e n t were b i o a s s a y e d f r o m each orchard. The r a p i d b i o a s s a y methodology d e s c r i b e d by Dennehy e t a l . (2) was u s e d t o e s t i m a t e t h e f r e q u e n c y o f r e s i s t a n t a n i m a l s i n orchard populations. T h i s method e n t a i l e d p l a c i n g s p i d e r m i t e s i n s m a l l (50 x 9 mm; F a l c o n 1006) p e t r i d i s h e s t h a t were p r e t r e a t e d w i t h a d i s c r i m i n a t i n g c o n c e n t r a t i o n o f d i c o f o l ( K e l t h a n e 4 EC [ e m u l s i f i a b l e c o n c e n t r a t e ] , Rohm and Haas Co.) i n 95% e t h a n o l and a l l o w e d t o d r y . C o n t r o l p e t r i d i s h e s were t r e a t e d w i t h 95% e t h a n o l and a l l o w e d t o d r y . The o n l y s i g n i f i c a n t d e p a r t u r e f r o m t h e methodology employed p r e v i o u s l y by Dennehy e t a l . (2) was t h a t a d i s c r i m i n a t i n g c o n c e n t r a t i o n o f 320 |lg/g s o l u t i o n was u s e d i n New York r a t h e r t h a n t h e 56.2 |i.g/g u s e d i n C a l i f o r n i a . The h i g h e r c o n c e n t r a t i o n was f o u n d t o be a p p r o p r i a t e f o r use i n r a p i d b i o a s s a y s o f d i c o f o l - r e s i s t a n t T. u r t i c a e and P. u l m i f r o m New York a p p l e o r c h a r d s ( u n p u b l i s h e d r e s u l t s from o u r l a b o r a t o r i e s ) . Twenty s p i d e r m i t e s were p l a c e d i n each p e t r i d i s h u s i n g a f i n e b r u s h . One c o n t r o l and f o u r o r f i v e t r e a t e d p e t r i d i s h e s were s e t up f o r e a c h l o c a t i o n and s p e c i e s b i o a s s a y e d . A s s a y s were h e l d a t room t e m p e r a t u r e , and m o r t a l i t y was a s s e s s e d a f t e r 24 h . To a s s e s s m o r t a l i t y , p e t r i d i s h e s were each t a p p e d 20 t i m e s on t h e i r edge on t h e c o u n t e r t o p , opened, and t h e m i t e s i n t h e t o p and b o t t o m h a l v e s


82



o f t h e d i s h e s viewed under t h e m i c r o s c o p e . Subjects e x h i b i t i n g r e p e t i t i v e movement o f any appendages were r e c o r d e d as a l i v e . Mean and s t a n d a r d e r r o r v a l u e s f o r c o r r e c t e d m o r t a l i t y (A&) were computed f o r a l l o r c h a r d p o p u l a t i o n s . Dicofol selection p r e s s u r e was e s t i m a t e d by summing t h e number o f d i c o f o l t r e a t m e n t s made a t e a c h o r c h a r d d u r i n g t h e p e r i o d 1982-85. No d i c o f o l was u s e d i n 1986-87 a t any o f t h e o r c h a r d s sampled. A d d i t i o n a l l y , a w e i g h t e d sum method was e v a l u a t e d f o r e s t i m a t i n g d i c o f o l s e l e c t i o n p r e s s u r e . The d i c o f o l t r e a t m e n t s made i n 1985, 1984, 1983, and 1982 were e a c h m u l t i p l i e d by t h e w e i g h t i n g f a c t o r s o f 1.0, 0.75, 0.50, and 0.25, r e s p e c t i v e l y , and t h e n t h e s e w e i g h t e d v a l u e s were summed f o r t h e p e r i o d o f 1982-85 f o r each l o c a t i o n . Mean ( c o r r e c t e d ) m o r t a l i t y i n r a p i d b i o a s s a y s was r e g r e s s e d on t h e two measures o f d i c o f o l s e l e c t i o n p r e s s u r e (weighted and non-weighted v a l u e s ) (41) . Results and Discussion. Most T. urticae p o p u l a t i o n s sampled were r e l a t i v e l y s u s c e p t i b l e t o d i c o f o l ( F i g u r e 1)/ t h e a v e r a g e o f t h e mean m o r t a l i t y v a l u e s o f a l l 15 p o p u l a t i o n s was 90%. Four p o p u l a t i o n s had no d e t e c t a b l e r e s i s t a n c e (100% m o r t a l i t y ) , and 12 o f t h e 15 p o p u l a t i o n s had >80% m o r t a l i t y i n b i o a s s a y s . Linear r e g r e s s i o n o f m o r t a l i t y on t h e number o f d i c o f o l t r e a t m e n t s (nonweighted) was done f o r b o t h t h e complete d a t a s e t (15 o r c h a r d s ) , and a f t e r r e m o v a l o f two o u t l i e r p o i n t s ( o u t l i e r s n o t e d as open c i r c l e s on F i g u r e 1) f r o m t h e complete d a t a s e t . A n a l y s e s w i t h o r w i t h o u t t h e o u t l i e r s y i e l d e d s l o p e s o f t h e r e g r e s s i o n l i n e t h a t were not s t a t i s t i c a l l y d i f f e r e n t than zero. Use o f w e i g h t e d v a l u e s o f d i c o f o l s e l e c t i o n pressure a l s o produced a slope f o r the r e g r e s s i o n l i n e equal to zero. T h r e e i m p o r t a n t f i n d i n g s a r e i l l u s t r a t e d by t h e r e g r e s s i o n a n a l y s i s o f d i c o f o l s e l e c t i o n o f T. urticae populations. First, f a c t o r s other than d i c o f o l s e l e c t i o n pressure o v e r r i d e the determination of population s u s c e p t i b i l i t y to d i c o f o l . Populations t h a t had r e c e i v e d 3, 4, o r 5 d i c o f o l t r e a t m e n t s d u r i n g t h e s t u d y p e r i o d were j u s t as s u s c e p t i b l e as some p o p u l a t i o n s t h a t had r e c e i v e d o n l y one t r e a t m e n t . C l e a r l y , f a c t o r s mediating population s u s c e p t i b i l i t y a r e s t r o n g l y c o u n t e r i n g r e s i s t a n c e development. These f a c t o r s a r e v e r y l i k e l y t o be i m m i g r a t i o n o f s u s c e p t i b l e m i t e s and/or r e d u c e d f i t n e s s o f r e s i s t a n t i n d i v i d u a l s . Secondly, p o p u l a t i o n s o f T. urticae were r e l a t i v e l y s u s c e p t i b l e t o d i c o f o l ; most p o p u l a t i o n s had l e s s t h a n 10% r e s i s t a n t s p i d e r m i t e s . Field t r i a l s e v a l u a t i n g t h e p e r f o r m a n c e o f d i c o f o l i n New York a p p l e o r c h a r d s p o s s e s s i n g d i f f e r e n t f r e q u e n c i e s o f r e s i s t a n t m i t e s have shown t h a t d i c o f o l p r o v i d e s v e r y a c c e p t a b l e c o n t r o l on p o p u l a t i o n s p o s s e s s i n g up t o 20% r e s i s t a n t i n d i v i d u a l s (W. H. R e i s s i g , C o r n e l l Univ., unpublished d a t a ) . The t h i r d i m p o r t a n t p o i n t i l l u s t r a t e d i n F i g u r e 1 i s t h a t two p o p u l a t i o n s t h a t our r e c o r d s i n d i c a t e d had e x p e r i e n c e d l i t t l e o r no d i c o f o l s e l e c t i o n had u n e x p e c t e d l y h i g h f r e q u e n c i e s o f d i c o f o l resistance. These d a t a p o i n t s a r e from o r c h a r d s t h a t had z e r o o r one d i c o f o l t r e a t m e n t s i n c e 1982, y e t t h e y show g r e a t e r t h a n 20% d i c o f o l - r e s i s t a n t i n d i v i d u a l s ( JD Q. CD O

(/> O

o o T3 2 C

o r o

4

6

8 10 12 14 16

Generations in cage

Generations in cage

Q. O

T-

2

4

6

8

10

12

Generations in cage F i g u r e 3. P r o p o r t i o n o f d i c o f o l - s u s c e p t i b l e T. urticae i n caged p o p u l a t i o n s shown i n r e l a t i o n t o t h e number o f g e n e r a t i o n s p o p u l a t i o n s were i n cages. Grand mean m o r t a l i t y and 95% c o n f i d e n c e l i m i t s a r e g i v e n f o r groups o f f o u r c a g e s . The s o l i d l i n e denotes t h e f r e q u e n c y o f SS p l u s RS genotypes p r e d i c t e d from t h e model and s i s t h e c o e f f i c i e n t o f s e l e c t i o n a g a i n s t t h e r e s i s t a n t (RR) genotype. S o l i d c i r c l e s depict data f r o m c o n t r o l groups f o r which 100% RR m i t e s were i n t r o d u c e d i n t o cages. F i g u r e s a-d show r e s u l t s o b t a i n e d from cages s t a r t e d w i t h s p i d e r m i t e s from t h e Res and Sus c o l o n i e s . F i g u r e e shows r e s u l t s o b t a i n e d from cages s t a r t e d w i t h s p i d e r m i t e s from t h e n e a r - i s o g e n i c l i n e , IsoR, and t h e Sus c o l o n y .



5. DENNEHY ETAL.

89 Instability ofSpider Mite Resistance to Acaricides

c o l o n i e s , i t i s h i g h l y probably t h a t the d i f f e r e n c e s observed are r e l a t e d d i r e c t l y t o , o r c l o s e l y l i n k e d t o t h e gene c o n f e r r i n g dicofol resistance. I f r e l a t i v e f i t n e s s d i f f e r e n c e s of the magnitude o b s e r v e d i n t h e cage s t u d i e s o c c u r i n t h e f i e l d , t h e n t h i s phenomenon i s a p o w e r f u l f o r c e d r i v i n g i n s t a b i l i t y o f d i c o f o l r e s i s tance. F o r example, i f t h e f r e q u e n c y o f t h e RR genotype i s 0.30, t h e n , assuming a s e l e c t i o n c o e f f i c i e n t o f 0.30, t h i s f r e q u e n c y would d e c l i n e t o 0.22 i n two g e n e r a t i o n s . There a r e , however, u n r e s o l v e d q u e s t i o n s , d i s c u s s e d below, t h a t need t o be a d d r e s s e d b e f o r e we can be c o n f i d e n t t h a t f i t n e s s d i f f e r e n c e s we have measured a r e m a n i f e s t e d comparably under f i e l d c o n d i t i o n s . C o n c u r r e n t w i t h t h e cage s t u d i e s , l i f e t a b l e s were c o n s t r u c t e d f o r RR and SS female T. urticae by o b s e r v i n g i n d i v i d u a l s p i d e r m i t e s r e a r e d on h e a l t h y , undamaged bean s e e d l i n g s . In t h i s c a s e t h e r e l a t i v e f i t n e s s o f t h e RR genotype was e x p r e s s e d as t h e e x p e c t e d p r o g e n y f r o m t h e RR f e m a l e s d i v i d e d by t h e e x p e c t e d p r o g e n y f r o m t h e SS f e m a l e s . The p o o l e d r e s u l t s from t h r e e l i f e t a b l e s p r o d u c e d a s e l e c t i o n c o e f f i c i e n t , s, o f 0.12. In t h e s e s t u d i e s , d e v e l o p m e n t a l t i m e and egg p r o d u c t i o n were not s i g n i f i c a n t l y d i f f e r e n t i n t h e two strains. The cause o f t h e o b s e r v e d v a l u e o f s was increased m o r t a i l i t y o f t h e r e s i s t a n t s t r a i n ( u n p u b l i s h e d r e s u l t s f r o m our laboratory). I n t r i n s i c growth r a t e s f o r r e s i s t a n t s p i d e r m i t e s were always l e s s t h a n t h o s e f o r t h e s u s c e p t i b l e s p i d e r m i t e s (SS g e n o t y p e ) ; however, t h e average d i f f e r e n c e was s m a l l (0.275 v e r s u s 0.282). We must c o n c l u d e t h a t t h e r e l a t i v e l y l a r g e f i t n e s s d i f f e r e n c e s o b s e r v e d i n t h e cage s t u d i e s a r e d e n s i t y dependent. It i s g e n e r a l l y a c c e p t e d t h a t f i t n e s s i s a f u n c t i o n o f d e n s i t y , and i t i s c e r t a i n l y p o s s i b l e t h a t d i f f e r e n c e s i n such t h i n g s as v i a b i l i t y , f e c u n d i t y , and m a t i n g s u c c e s s v a r y as a f u n c t i o n o f d e n s i t y i n d i f f e r e n t ways f o r RR and SS a n i m a l s . D e n s i t i e s i n t h e cage s t u d i e s were r e l a t i v e l y h i g h . T h e r e f o r e , t h e dynamics o f l o s s o f r e s i s t a n c e o b s e r v e d i n our cage s t u d i e s may r e f l e c t an upper l i m i t f o r what might be e x p e c t e d , under f i e l d c o n d i t i o n s , from t h e p r o c e s s o f d i f f e r e n t i a l f i t n e s s o f d i c o f o l - r e s i s t a n t and s u s c e p t i b l e T. urticae.

Conclusions From t h e s t a n d p o i n t o f managing r e s i s t a n c e t o d i c o f o l i n s p i d e r m i t e p e s t s o f New York a p p l e o r c h a r d s , our f i n d i n g s f o r T. urticae are positive. Most p o p u l a t i o n s had fewer t h a n 10% r e s i s t a n t m i t e s , and our l a b o r a t o r y d a t a i n d i c a t e d t h a t t h e r e s i s t a n t g e n o t y p e s o f T. urticae a r e s u b s t a n t i a l l y l e s s f i t t h a n s u s c e p t i b l e ones. Field s t u d i e s of the c r i t i c a l frequency f o r d i c o f o l r e s i s t a n c e ( i . e . , the f r e q u e n c y a t which r e s i s t a n c e r e s u l t s i n a p p r e c i a b l e r e d u c t i o n s i n c o n t r o l ) i n d i c a t e t h a t f r e q u e n c i e s o f r e s i s t a n c e must be >20% to impair performance of d i c o f o l treatments. Therefore, d i c o f o l t r e a t m e n t s would be e x p e c t e d t o p r o v i d e a c c e p t a b l e c o n t r o l o f 9-12 o f t h e 15 T. urticae p o p u l a t i o n s we sampled. However, P. ulmi i s t h e dominant s p i d e r m i t e p e s t o f a p p l e i n w e s t e r n New York and, as we have d e m o n s t r a t e d , t h e s p e c i e s most l i k e l y t o e x h i b i t r e s i s t a n c e to d i c o f o l . In New York a p p l e o r c h a r d s , i t a p p e a r s u n l i k e l y t h a t s u s c e p t i b i l i t y t o d i c o f o l can be s u s t a i n e d i n P. ulmi p o p u l a t i o n s which a r e t r e a t e d w i t h d i c o f o l d u r i n g s u c c e s s i v e y e a r s . Mean m o r t a l i t y i n t h e 9 o r c h a r d s t h a t had r e c e i v e d o n l y 1 d i c o f o l t r e a t m e n t d u r i n g 1982-85 p e r i o d was 0.86 (SEM 0.30). G i v e n a c r i t i c a l f r e q u e n c y o f 20%, p o p u l a t i o n s showing >80% mean m o r t a l i t y i n r a p i d b i o a s s a y s would be a d e q u a t e l y c o n t r o l l e d by d i c o f o l . However, t h e mean m o r t a l i t y f o r p o p u l a t i o n s t h a t r e c e i v e d 2




90

t r e a t m e n t s (6 l o c a t i o n s ) o r 3 t r e a t m e n t s (7 l o c a t i o n s ) o f d i c o f o l was 0 . 6 3 (SEM 0.05) and 0 . 5 1 (SEM 0 . 0 8 ) , r e s p e c t i v e l y . I t i s very u n l i k e l y t h a t d i c o f o l would a d e q u a t e l y c o n t r o l t h e s e p o p u l a t i o n s . In New York a p p l e o r c h a r d s , i t appears a d v i s a b l e t o d e v e l o p a c a r i c i d e r o t a t i o n s t h a t u t i l i z e d i c o f o l no more f r e q u e n t l y t h a n once e v e r y 3 - 5 y e a r s t o c o n t r o l P. u l m i . F i e l d s t u d i e s a r e ongoing to evaluate t h i s preliminary conclusion. Our f i n d i n g s r e g a r d i n g t h e dynamics o f r e s i s t a n c e t o d i c o f o l i n P. u l m i and T. urticae i l l u s t r a t e the c a s e - s p e c i f i c nature of the s t a b i l i t y of resistances. Even i n t h e same c r o p p i n g system, r e s i s t a n c e dynamics were v e r y d i f f e r e n t f o r two s p e c i e s , T. urticae and P. ulmi. T h i s f i n d i n g s h o u l d s e r v e as w a r n i n g a g a i n s t g e n e r a l i z i n g about r e s i s t a n c e s t a b i l i t y a c r o s s d i f f e r e n t s p e c i e s and c r o p p i n g systems. Though i t appears t h a t r o t a t i o n s o f d i c o f o l i n New York a p p l e s h o u l d i n v o l v e use o f d i c o f o l no more o f t e n t h a n once e v e r y t h r e e y e a r s , major c r o p p i n g systems e x i s t t h r o u g h o u t t h e w o r l d where d i c o f o l has been u s e d on much more i n t e n s i v e b a s e s w i t h o u t e x p e r i e n c i n g t h e r e s i s t a n c e problems documented h e r e i n f o r P. u l m i . Such examples have been a l r e a d y c i t e d from Japanese c i t r u s and C a l i f o r n i a cotton. In each i n s t a n c e t h e same p r i n c i p l e s a p p l y f o r e x p l o i t i n g i n s t a b i l i t y o f s p i d e r m i t e r e s i s t a n c e , however, t h e m i n i m a l t r e a t m e n t i n t e r v a l f o r which d i c o f o l t r e a t m e n t s must be spaced i n order t o a v o i d e s c a l a t i n g r e s i s t a n c e frequencies i s different. Acknowledgment We thank J . Minns, L. Fergusson-Kolmes, K. Wentworth and L. C l a r k for assistance with these studies. F u n d i n g was p r o v i d e d , i n p a r t , by t h e Rohm and Haas Company.

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Characterization and Exploitation of Instability of Spider Mite

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