- 7 - 



Due to these observations, insecticide tolerance was strongly 

 suspected. Post-bloom Gutbion* treatments had proved effective in the past 

 as GFW injury was rarely noted in the packhouse. In addition, GFW damage 

 had not increased in other major fruit growing regions of the state despite 

 the use of similar spray schedules. Guthion* was apparently still effective 

 in these areas where GFl! injury averaged less than 0.1% in 1^83. 



Tolerance Study 



To investigate the possibility of insecticide resistance, we collected 

 GFVI larvae from the Shelburne population during bloom of 1984, caged them on 

 abandoned apple trees, and subjected them to various rates of Guthion*. 

 Treatments were made at petal fall when commercial insecticides would likely 

 be applied. A Lorsban* (chlorpyrifos) treatment was included for evaluation 

 as an alternate material should Guthion* prove ineffective. Our results are 

 presented in Table 1. 



Table 1. Results of several organophosphate insecticide treatments against 

 GFV/ larvae (tentatively identified as Orthosia hibisci 'Guenee') 



Material and formulation Lbs/100 gal % Mortality 



Guthion SOUP 0.5 30 



Guthion 50WP 1.0 30 



Guthion 50WP 2.0 55 



Lorsban 50WP 1.0 95 



Control 5 



The data clearly indicate that neither the full label rate nor the 

 double rate Guthion* treatments proved effective against these larvae, 

 resulting in only 30% mortality. If relied upon for control, these treat- 

 ments would have allowed the survival of a residual population capable of 

 subs' antial fruit injury. Even the quadruple rate resulted in only 55% kill 

 and we repeatedly observed larvae continuing to feed on fruit and foliage 

 desp te the visible presence of a heavy spray residue. Lorsban* proved a 

 highly effective material that resulted in 95% mortality. 



Summa ry 



Baseri on the work presented here, we believe that insecticide tolerance 

 to Guthicn* (azinphosmethyl ) was involved in the GFW control failures 

 recently experienced by fruit growers in western Massachusetts. We recom- 

 mend careful monitoring of GFW larval populations on an orchard-by-orchard 

 basis and the use of alternate post-bloom insecticides should damage exceed 

 acceptable levels. 



