- 8 



a mist blower (1x or 6x) accumulate on the ground cover beneath sprayed 

 trees. That is precisely the place where A^ fa 1 lac i s begins to move up into 

 the trees. It seems almost certain, therefore, that the initial high toxi- 

 city of SP ' s to A^ fal lac i s in the ground cover and the tree foliage poses 

 an insurmountable barrier to effective predator suppression of spider mites. 



Besides the devastating effects of SP ' s on mite predators, SP's may 

 promote apple pest buildup yet in other ways. For example, Frank Hall in 

 Ohio (pers. commun.) has found that SP's stimulate spider mite dispersal, 

 leading to colonization of trees that were previously free of mites. Also, 

 data from Weires in New York (pers. commun.) implicate use of SP's in the 

 buildup of woolly apple aphids, rosy apple aphids, and mealybugs. In addi- 

 tion, several researchers believe (though there is no proof) that SP's 

 biochemically alter the physiology of the apple tree in a way that pranotes 

 greater and more rapid spider mite reproduction. We must also recognize 

 that the more often spider mite populations rise above damaging levels, the 

 more miticide is necessary to control them, and hence the more rapidly mite 

 resistance to miticides will develop. The end result is an upward spiral of 

 the cost of mite control over the long term. 



Finally, recent data from Ontario (McKay, pers. commun.) confirm that 

 leafminer adults develop at least partial resistance to SP's after only 1 SP 

 application per year for 5 consecutive years. This very rapid rate of 

 resistance development may be due in large part to pre-adapta t ion on the 

 part of leafminers to detoxify SP's. It turns out that SP's are detoxi'^ied 

 primarily in the same way as DDT and its relatives. Extensive exposure of 

 leafminers and other pests to ODT-like materials in apple orchards in the 

 1950's and 1960's may very well have given such pests a big head-start in 

 ability to rapidly detoxify SP's. The more frequent the application of SP's 

 in any one year, and the more consecutive years they are used, the greater 

 the likelihood of rapid resistance development, as in Ontario. 



Cone 1 us ion . We have tried to present the evidence in favor of, as well 

 as against, the use of SP's for pre-bloom control of plant bugs and leaf- 

 miners. We hope each grower will weigh the evidence and make a decision 

 that best fits his overall operation. In our opinion, use of SP's pre-bloom 

 against plant bugs and leafminers can save money in the short run, but is 

 very likely to be more costly (in terms of greater need for miticides and 

 aphicides) in the long run. Under no cirumstances do we believe it advi- 

 sable to use SP's after bloom. Such use would only promote more rapid 

 development of leaf miner resistance and cause additional harm to mite and 

 aphid predators. 



Reference 



1. Bostonian, N.J. and A. Belanger. 1985. The toxicity of three 

 pyrethroids to Amblysei us fa 1 lac i s . Agricul. Ecosyst. Environ. 

 T4:2'43-250. 



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