Selecting for Pesticide Resistance in Trioxys pallidus

During the past year, we reared and released approximately 40,000 parasites of the Guthion-resistant strain of walnut aphid parasite, Trioxys pallidus, into a walnut orchard near Red Bluff, CA. Parasites were recovered three times during the season following release and results of bioassays indicate the resistant strain has established and persisted through its first field season in this orchard. The overwintering success and persistence of the Guthion-resistant strain was evaluated in the Hanford and Gridley sites where releases had been made during 1988 and 1989. The results of the laboratory bioassays support the conclusion that the Guthion-resistant strain has overwintered and persisted at the Hanford release site (Bairstow orchard) through three seasons (1988-1991) and the Gridley orchard through two seasons (1989-91) since its release. Rates of diapause induction and termination in the Guthion-resistant strain of T. pallidus were compared to that of the base and a recently-collected field strain. The resistant strain has retained its ability to enter and terminate diapause. Objectives: The objectives for the 1991-92 funding interval were: 1) Maintain the Guthion-resistant strain of Trioxys pallidus. 2) Release resistant T. pallidus in selected walnut orchards. 3) Provide starter cultures of the resistant colony to commercial producers and training on how to rear the colony. In addition, the following objectives were completed: 4) Monitor walnut orchards where the Guthion-resistant strain was released in 1988 and 1989 to determine if the resistant parasites continue to overwinter and persist. 5) Complete studies of diapause induction and termination of the Guthion-resistant strain. .