reduce potential adverse effects of rainfall on the resid- 

 ual activity of Dimilin. Pre-bloom application has the 

 added advantage of being effective against a range of 

 first-generation leafroller larvae which are active at 

 that time, but which are in the pupal stage (unaffected 

 by Dimilin) in late June and early July. 



For the past 3 years, Massachusetts has received 

 an Experimental Use Permit (EUP) for several 

 hundred acres worth of Dimilin use each year on 



apples. We have been much more fortunate than other 

 states in this regard. Still, we are hopeful for full 

 registration of this effective, relatively environmen- 

 tally safe compound in the near future. 



Acknowledgements 



We are grateful to Paul Bohne and Uniroyal 

 Chemical Co. for supporting this work. We also thank 

 all cooperating growers. 



*I* *$* *I* *f* *I* 

 «j» #j» •{• »j» •{• 



Evaluation of Dimilin™ Against Pear Psylla 



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



Department of Entomology, University of Massachusetts 



In the preceding article, we reported on trials of 

 Dimilin™ for control of apple blotch leaf mi ne r . In this 

 article, we report on tests of Dimilin used with oil 

 against pear psylla. This insect is more difficult to con- 

 trol than any other pear pest. Nationwide, it has 

 developed at least some resistance to nearly every 

 pesticide to which it had been exposed during regular 

 orchard treatments. Because Dimilin disrupts insect 

 growth rather than being acutely toxic, it represents a 

 new form of pesticide not previously encountered by 

 pear psylla. 



All of our trials were conducted over 3 years (1987- 

 89) in commercial orchards. Growers applied the 

 Dimilin and/or oil with their own sprayers. Test blocks 

 averaged 1 to 2 acres in size, with Dimilin and oil 

 applied to all trees except 1 to 12 trees left as untreated 

 checks at each end of the block. On each sampling date, 

 10 watersprouts on each of 20 treated and 20 untreated 

 trees were examined for presence or absence of live 

 psylla nymphs or adults. 



In 1987, oil was applied pre-bloom to all test blocks. 

 Dimilin at 16 oz per acre was applied only post-bloom, 

 twice per block in May or June. In 2 orchards where 

 psylla populations were high or moderate (Orchards A 

 and B), Dimilin had little effect (Table 1). In Orchard 

 C, where psylla was relatively low, Dimilin maintained 

 the population at a low level through mid- July. 



In 1988, oil was applied once and Dimilin twice at 

 8 oz per acre pre-bloom in 2 orchards. Where there was 

 a moderate population of psylla (Orchard A), control 

 through mid-June was fair at best (Table 1). Where 



psylla was rather low (Orchard B), control through 

 mid- June was good. In Orchard C, oil was applied once 

 and Dimilin once at 12 oz per acre pre-bloom. Then 

 Dimilin was applied twice postbloom at 12 oz per acre. 

 The psylla population remained low on treated trees 

 through mid-July. Orchard D had a very high psylla 

 population. Two pre-bloom treatments of oil plus 

 Dimilin at 12 oz per acre together with a post-bloom 

 treatment of each in late May failed to show any 

 reduction in psylla when sampled in mid- June. 



In 1989, oil together with Dimilin at 12 oz per acre 

 was applied twice pre-bloom in 2 orchards. In Orchard 

 A, where Dimilin alone at 12 oz per acre was also 

 ap plied post-bloom in late May, treated trees were free 

 of psylla in mid- June. In mid- July, trees had relatively 

 low numbers of psylla (in contrast to rather high 

 numbers on the checks). In Orchard B, where the 

 psylla population was high, treated trees had rather 

 low psylla populations in mid- June but fairly high ones 

 by mid-July. 



Together, these results suggest that Dimilin may 

 indeed be an effective pesticide against pear psylla, but 

 only if a sufficient number of pre-bloom applications 

 are made. Treating with Dimilin strictly post-bloom 

 seems to have little positive effect. The best results 

 (1989 in Orchard A) were obtained where Dimilin was 

 applied with oil twice pre-bloom (mid and late April) 

 and again shortly after petal fall. However, even this 

 usage pattern had no effect where treatment was not 

 begun by mid-April (Orchard D in 1988). One caution- 

 ary note in the interpretation of our results is the lack 



Fruit Notes, Spring, 1990 



15 



