Graduate Thesis Or Dissertation

Monitoring pesticide resistance in Psylla pyricola Foerster from western Oregon pear orchards

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  • A modified slide-dip bioassay was used to survey and establish baseline resistance levels in Psylla pyricola Foerster from western Oregon orchards to four pesticides. Orchards selected in the Hood River, Rogue River and Willamette Valleys spanned a wide range of environmental and management conditions. During the study period of 1982-1983, management conditions present at these sites included intensively sprayed, minimally sprayed (IPM), recently abandoned, and long abandoned orchards. Lethal concentration (LC₅₀) values determined for psylla populations ranged from 0.31-12.64 g AI/1 for azinphosmethyl, 0.14-1.73 g AI/1 for endosulfan, 0.02-0.10 ml AI/1 for fenvalerate, and 0.41-1.52 ml AI/1 for Perthane. Pear psylla from all orchards showed some level of resistance to azinphosmethyl (12- to 41-fold) and endosulfan (2- to 12-fold) when compared to the susceptible OSU Entomology Farm strain. Comparative differences in susceptibility among populations to fenvalerate and Perthane, calculated by comparing each strain to the most suceptible strain, were between 1- and 5-fold. Resistance was not correlated with insecticide usage in individual orchards in the Hood River and Rogue River Valleys, where commercial pear production is intensive. Psylla from unmanaged orchards had similar or slightly higher LC₅₀ values than managed trees for each compound. In contrast, in the Willamette Valley, where pear production is much less intensive and orchards are widely scattered, psylla resistance levels were better correlated with local insecticide usage patterns. Pairwise comparisons of mean LC₅₀ values for a region indicated that psylla resistance levels were significantly different at the regional level for azinphosmethyl and endosulfan. The mean LC₅₀ value for Rogue River Valley populations to azinphosmethyl of 9.00 g AI/1 was significantly higher (p<.05) than Hood River Valley and Willamette Valley psylla, which had mean values of 4.93 and 4.23 g AI/1, respectively. The mean LC₅₀ value of 1.29 g AI/1 for Hood River Valley psylla to endosulfan was significantly higher (p<.05) than mean values determined for Rogue River Valley and Willamette Valley psylla, which were 0.64 and 0.36 g AI/1, respectively. These regional trends in resistance accurately reflect differences in the intensity of use of the two compounds in these regions. Regional differences in mean LC₅₀ values for fenvalerate and Perthane were not statistically significant. A regional hypothesis of resistance development and population movement of pear psylla was proposed to explain the observation that managed and unmanaged populations were equally resistant to azinphosmethyl and endosulfan in regions of intense pear production (i.e. Rogue River and Hood River Valleys). An overwhelming proportion of the psylla in a region develop in well-managed trees in commercial orchards and thus experience selective pressure from insecticides. In unsprayed orchards, which are few in these regions, tree vigor is low and natural enemies are abundant, which limits psylla reproduction and development during the season. During fall migration by overwintering psylla, populations from managed orchards and the limited populations from unmanaged orchards probably mix. The abundance of psylla produced in managed orchards and the psylla dispersal behavior appear to have combined to produce populations possessing regional resistance characteristics in these regions. In areas of less intense pear production (i.e. Willamette Valley), orchards are more scattered and population mixing is probably less extensive. Levels of azinphosmethyl and endosulfan resistance better reflected orchard-specific management in the Willamette Valley.
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