Effectiveness of Dimilin™ Against Apple 

 Blotch Leafminers 



Ronald J. Prokopy, Susan L. Butkewich, and Margaret Christie 



Department of Entomology, University of Massachusetts 



In a previous issue of Fruit Notes [52(1):12-15] we 

 reported on one year of preliminary trials using Dimil- 

 in™ against apple blotch leafminers (ABLM) in com- 

 mercial apple orchards. Dimilin is an insect growth 

 regulator which inhibits chitin synthesis, particularly 

 in the egg stage of ABLM. It has a long residual activity 

 and is reported to be relatively non-toxic to beneficial 

 predators, reducing the risk of mite buildup associated 

 with use of synthetic pyrethroids, Vydate™, or Lan- 

 nate™ against leafminers. Here, we report on 3 years 

 of trials (1987, 1988, and 1989) of Dimilin against 

 ABLM in several commercial apple orchard blocks. 



In our trials, Dimilin 25 WP was applied by grow- 

 ers to orchard blocks 2 to 3 acres in size. On opposite 

 ends of each block, approximately 20 trees (10 at each 

 end) were left unsprayed to serve as checks. The only 

 other insecticides used in any of the test blocks were 

 Imidan™ or Guthion™, to which ABLM are resistant. 

 Against first-generation ABLM, Dimilin application 

 occured at tight cluster to early pink, timed to coincide 

 with the onset of ABLM oviposition. Against second- 

 generation ABLM, Dimilin application occured in 

 summer Gate June or early July), timed to coincide 

 with the peak of second-generation adult emergence 

 from pupae. In all cases, only a single pre-bloom appli- 



cation (16 oz/acre in 1987, 8 oz/acre in 1988 and 1989) 

 or a single summer application (same rates as for pre- 

 bloom) was made. 



The results (Table 1) indicate that in 1987, trees 

 treated with Dimilin at 16 oz/acre before bloom aver- 

 aged only one-fourth as many first-generation mines as 

 untreated trees. In 1988, trees treated pre-bloom with 

 Dimilin at 8 oz/acre averaged only about one-sixth as 

 many first-generation mines as untreated trees. In 

 1989, however, trees treated prebloom with Dimilin at 

 8 oz/acre averaged nearly one-half as many first-gen- 

 eration mines as untreated trees. The very frequent 

 and heavy rains from tight cluster through pink in 1989 

 may have reduced the residual activity of Dimilin, so 

 that the pesticide was considerably less effective than 

 in 1987 or 1988. Where Dimilin was applied in summer 

 against second-generation eggs, only one-fourth (1987) 

 to one-sixth (1988) as many mines were present on 

 Dimilin-treated trees as on untreated trees. 



In conclusion, our findings suggest that a single 

 well-timed application of Dimilin, either pre-bloom or 

 in summer, can be very effective against ABLM, pro- 

 vided rainfall is not excessive following application. 

 Possibly, split applications (4 oz/acre twice or 2 oz/acre 

 three times) either pre-bloom or in summer would 



14 



Fruit Notes, Spring, 1990 



