Title: Selecting for Pesticide Resistance in Trioxys pallidus

The Guthion-resistant strain of walnut aphid parasite, Trioxys pallidus, was reared in the greenhouse on potted walnut trees and released into a commercialwalnut block near Gridley during the 1989 growing season where it established, survived Guthion,and spread to an adjacent blocktreated with Lorsban. Aphidand parasite counts suggest the parasites controlled the aphids in both the Guthionand Lorsban-treated blocks. The overwintering success of the Guthion-resistant strain was evaluated in four sites where releases had been made during 1988. The results of the laboratory bioassays suggest the Guthion-resistant strain overwintered at the Hanford release site (Bairstow orchard) and perhaps at a nearby nonrelease site (Barker orchard). Survivalof parasites from the Stockton release orchard (Podesta) was poor, and no evidence was found that the parasites overwinteredat the Colusa Northand Colusa South sites. Aphid-parasite interactions were monitored during the growing season at four orchards (Podesta, Bairstow, Barker and Paramount). Resistant parasites had been released at the Podesta and Bairstowblocks during 1988, but not at the Barker or the Paramount orchards. The Paramount orchard had a portion treated with Guthionso we could evaluate the impact of Guthionon wildparasites; the Barker orchard could have had the Resistant strain immigrate into it from the nearby Bairstow site during 1988. Parasite-aphid densities at the Paramount and Barker orchards were maintained below the 15 aphids/leaflet level and the mummycounts "tracked" the aphid counts. Mummy counts did not "track"aphids at the Podesta or Bairstoworchards, however. Parasites were recovered from the Podesta, Bairstow, and Gridley orchards during the fall 1989 to determine the Guthion resistance levels in the parasite populations at sites where Resistant parasites had been released in 1988 (Podesta and Bairstow) or 1989 (Gridley). The results of plastic'cup bioassays suggest that the Resistant strain is present in these sites. Surprisingly,however, colonies collected from the Paramount and Westside Field Station orchards, where the resistant strain was I never released, had high survivalrates in one bioassay. We therefore do not know whether: 1) the Resistant strain has dispersed to these nonrelease sites; 2) selection in the field with Guthion has resulted in a "wild"strain with Guthion resistance; 3) something is wrong with our bioassay. A mode of inheritance test was conducted in the laboratory using a time response technique; evaluation of the reciprocal F1 and backcross progeny indicate that Guthion resistance in this strain is polygenically inherited. While the resistance is polygunic, the F1 progeny are more like their Resistant parents than their susceptible parents. (Dominance is 0.33; a value of 0 would indicate intermediate inheritance while +1 would indicate a complete dominant and -1 a complete recessive.) This means that the F1 progeny should survive in the field relativelywell. Analysis of esterase banding