Role of neonicotinyl insecticides in Washington apple integrated pest management. Part II. Nontarget effects on...

The effect of neonicotinyl insecticides on integrated mite control in Washington apple was examined from 0 In a series of 20 field trials (54 treatments) designed primarily to look at efficacy against the codling moth, Cydia pomonella, nearly half of the treatments using four or more applications of acetamiprid had peak mite densities exceeding the economic threshold of 5 mites per leaf. Overall, acetamiprid treatments had 4.6-fold higher mite densities than the standard organophosphate insecticide treatment. Of the treatments with high mite populations, Panonychus ulmi, the European red mite, and Tetranychus urticae, the twospotted spider mite, were the dominant species in roughly equal numbers of cases. Only 11.1% of the thiacloprid treatments exceeded 5 mites per leaf; these experimental treatments included eight applications, whereas the current label restricts the number of applications at the rate for C. pomonella to two applications. One out of six clothianidin treatments caused a significantly higher mite density than the standard treatment; however, this material appeared to suppress predatory mites. Neonicotinyl insecticides did not eliminate predatory mites, but they inhibited their ability to respond normally to increasing prey populations. In field trials designed specifically to examine mite population densities where neonicotinyl insecticides were used, significantly higher levels of tetranychid mites occurred in one or more acetamiprid treatments (one, two or four applications) in five out of six trials. In the sixth trial (in a commercial orchard), only two acetamiprid applications were made, and mite populations were low in all treatments. While elevated mite densities were more likely to occur with four applications, in one case it occurred following a single application. The predominant tetranychid mite species (either P. ulmi or T. urticae) varied from trial to trial; however, there was no apparent bias regarding stimulation of the two species. Horticultural mineral oil was used with acetamiprid in some trials in an attempt to mitigate mite outbreaks. However, the addition of oil did not counteract the tendency of acetamiprid to increase tetranychid mite populations, and in one trial, had a negative effect on predatory mite densities. Seasonal tetranychid mite density was positively related to the total grams AI (or number of applications) of acetamiprid, thus reducing the number of applications per season should lower the probability of mite outbreaks.