Title: Selec~ing for Pesticide Resistance in Trioxys pallidus

During the past year, we evaluated the fitness and likelihood of establishing the Guthion-resistant strain of walnut aphid parasite, Trioxys pallidus, in California walnut orchards by conducting a combination of laboratory and field experiments. We compared the stability of the azinphosmethyl resistance in the resistant strain and in progeny derived from crossing the resistant and the base colonies in population cages held in the laboratory; the results indicated there is a reduction in Guthion resistance in the outcrossed progeny when they were held for13 generations without selection with Guthion. However, the resistant strain maintained a high level of Guthion resistance over 13 generations without selection with Guthion, which is approximately equivalent to the number of generations that would occur during two field seasons. The diapause incidence under laboratory conditions in the resistant strain, the base colony, and a wild colony collected from the field during 1990 are not qualitatively different. This indicates that the selected strain has the capacity to overwinter in California walnut orchards in diapause, which is confirmed by the apparently successful overwintering of the Resistant strain during 1988-89 and 1989-90 in the Hanford walnut orchard. The overwintering success and persistance of the Guthion-resistant strain was evaluated in the Hanford (Bairstow)and Gridley sites where releases had been made during 1988 and 1989. The results of the laboratory bioassays strongly support the conclusion that the Guthion-resistant strain overwintered at the Hanford release site (Bairstow orchard) during 1988 and 1989 and the Gridley orchard during 1989-90. The population genetics/ecology model, developed jointly with M. Caprio and B. Tabashnick, indicates that establishment of the Guthion-resistant strain of T. pallidus in the San Joaquin Valley may require more than ten years to achieve, assuming no additional resistant parasites are released and that current spray programs are continued. More rapid establishment over a wide area could occur if additional resistant parasites were released and Guthion applicationswere increased. Conversely, if Guthion applications are reduced, establishment of the resistant strain would be delayed, but probably would still occur, because the Guthion-resistant strain is cross resistant to other pesticides used in walnut IPM. Objectives: The objectives for the 1990 funding interval were: 1) Monitor the walnut orchards where the Guthion-resistant strain was released in 1988 and 1989 to determineif the resistantparasites have successfully overwintered. 2) Survey resistance levels of parasites from release and nonrelease sites during the 1990 growing season to determine distribution patterns of Guthion resistance. 3) Complete a model for implementing the Guthion-resistant strain in California walnut orchards. 4) Provide guidelines for implementation. In addition to the above objectives, we also conducted tests in the laboratory to compare fitness of the resistant strain with the Base colony under unsprayed conditions