ACARICIDE RESISTANCE IN POPULATIONS OF EUROPEAN RED MITE IN NEW YORK APPLE ORCHARDS

Acaricides are commonly applied in commercial apple orchards in New York to manage populations of European red mite, Panonychus ulmi (Koch), an important foliar pest. This thesis project was undertaken to document and characterize resistance of P. ulmi to cyhexatin, an acaricide that has been widely used in New York orchards since 1972 but that was reportedly failing to suppress some field populations of P. ulmi in 1984. The susceptibility of cyhexatin of adult female mites was evaluated with several bioassay techniques. In 1985, susceptibility of two field populations differed by ca. 12-fold after exposure to cyhexatin residues in 72-hour leaf bioassays and by 3-fold after topical treatment in 24-hour slide-dip bioassays. In 1986, approximately the same range of differences in susceptibility to residues detected by the leaf bioassay technique was detected with a 24-hour leafless bioassay technique. Choice of the test concentration that best discriminated between resistant and susceptible populations in leafless bioassays depended on whether moribund individuals were scored as alive or dead when evaluating mortality. A wide range of susceptibility to cyhexatin was detected among populations from 36 orchards surveyed in 1986 with leafless bioassays. Field trials conducted in 18 of these orchards in 1986 and six orchards in 1987 showed that efficacy of cyhexatin was related to bioassay response in most but not all cases, and the relationship between laboratory and field estimates of susceptibility was influenced by population density, and the timing and number of applications. Resistance to cyhexatin appeared to be stable; there was no increase in susceptibility of two field populations after 2 yr without exposure to cyhexatin. Resistance was less intense in larval than in adult populations.;The activity against P. ulmi of two newly-developed acaricides, hexythiazox and clofentezine was also characterized. Bioassay methods were developed and baseline susceptibility data obtained for ovicidal, larvicidal, and ovo-larvicidal effects on both winter and summer generations of P. ulmi. The variability in susceptibility to clofentezine of winter eggs from 30 orchard populations was evaluated. This baseline information provides a foundation for future monitoring for potential resistance to these acaricides.