STIMULATION OF PEAR RUST MITE POPULATIONS DUE TO APPLICATION OF SPINETORAM IN SOUTHERN OREGON AND NORTHERN CALIFORNIA

Abstract

Spinetoram is currently being used in pears for control of Lepidoptera and other pests. Over the last ten years, results of repeated trials in southern Oregon and northern California examining the effect of spinetoram use in pears have shown elevations in the population level of pear rust mite, Epitrimerus pyri. In both regions, the increase in pear rust mite populations due to the application of spinetoram has been statistically significant in multiple years. In 2013, an evaluation in northern California resulted in both significantly higher levels of pear rust mite throughout the sampling period and significantly higher levels of russet on the fruit at harvest. In the spinetoram treatment, 8.4% of the fruit sampled had greater than ¼ of the fruit surface with russet as compared to 0.2% of the fruit in the untreated check. Reduced levels of some natural enemies have been observed in southern Oregon trials, specifically parasitic hymenoptera, Encarsia sp., and the common earwig, Forficula auricularia. Whether the increase in pear rust mite resulting from spinetoram applications in pear is due to a reduction in natural enemies or to hormoligosis remains undetermined.