Mode of inheritance of azinphosmethyl resistance in a laboratory - selected strain of Trioxys pallidus

Homogeneity in azinphosmethyl resistance was assessed in males of a laboratory-selected (Select-H) and susceptible (Yolo) colony of Trioxys pallidus (Hymenoptera: Aphidiidae) using a time response assay. No evidence of heterogeneity within the two colonies was found. Reciprocal crosses between the Yolo and Select-19 colonies resulted in F1 females which exhibited a semidominant response to azinphosmethyl with a dominance value (D) of 0.32, as well as no evidence of maternal effects or sex linkage. Responses of F2 progeny to azinphosmethyl suggest that more than one gene is involved because no inflection was observed in the time response lines of F2 males. Introduction Despite the implementation of integrated pest management programs, the walnut aphid Chromaphis juglandicola (Kaltenbach) remains a persistent pest of Persian walnut trees (Juglans regia L.) throughout California. Prior to the release of the parasitic wasp Trioxys pal/idus, an aphidiid parasitoid of C. juglandicola, springtime damage by the walnut aphid was capable of reducing the developing crop by 50% (Michelbacher & Ortega, 1958). In 1959 a French biotype of T. pallidus was imported and released into the walnut growing regions of California (Schlinger et al., 1960). The French biotype reduced aphid populations in the coastal growing regions; however, it failed to reduce aphid populations in the more extensive walnut growing regions of the Central Valley (van den Bosch et al., 1962). In 1969, a more heat tolerant Iranian biotype was released which rapidly established and dispersed throughout the Central Valley (van den Bosch et al., 1970, 1979). Though the establishment of T. pallidus was successful in reducing aphid populations, aphid outbreaks are not uncommon today. The control of other walnut pests by azinphosmethyl can reduce T. pallidus numbers, resulting in secondary outbreaks of C. juglandicola. T. pallidus was selected in the lab for resistance to azinphosmethyl as part of a genetic improvement project (Hay & Cave, submitted). This parasite is a haplo-diploid endoparasite with a generation time of about two weeks at 25-270C. The azinphosmethyl-resistant strain has been released into commercial walnut orchards where establishment, season-long