RANDOM AMPLIFIED POLYMORPHIC DNA MARKERS TO MONITOR LABORATORY-SELECTED, PESTICIDE-RESISTANT TRIOXYS-PALLIDUS (HYMENOPTERA, APHIDIIDAE) AFTER RELEASE INTO 3 CALIFORNIA WALNUT ORCHARDS

A nearest-neighbor discriminant analysis based on the frequencies of o ccurrence of six random amplified polymorphic DNA markers was used to identify individuals from an insecticide-resistant laboratory biotype (SEL) of the walnut aphid parasite Trioxys pallidus after release into three California walnut orchards. Releases of SEL individuals were ma de in a Hanford, CA, orchard in 1988 and in a Gridley, CA, orchard in 1989. Nearest-neighbor analysis of postrelease collections from both t he Hanford and Gridley orchards showed a continuing influence of the S EL biotype up to 3 yr after the release. SEL individuals also were rel eased in a Red Bluff, CA, walnut orchard in 1991. At this orchard, SEL males were recovered from the release area throughout the season of r elease, but a June 1992 sample indicated that no SEL individuals remai ned in the orchard. Comparison of observed data at Hanford and Gridley to simulated hybrid data provided evidence that there was interbreedi ng between the SEL and the native field biotypes. Bioassays indicated that the level of azinphosmethyl resistance in these two orchards was intermediate between the level in the orchards before release and the level in the pure SEL biotype. Therefore, the release of pesticide-res istant T. pallidus in two California walnut orchards significantly inc reased the level of resistance, but did not completely displace the na tive populations, despite multiple treatments of postrelease populatio ns with azinphosmethyl.