TITLE: Selecting for Pesticide Resistance in Trioxys pallidus Principal Investigator:

The Guthion-resistant strain of walnut aphid parasite, Trio x y s p a II i d us. was mass reared in the greenhouse on potted walnut trees and released into five commercial walnut blocks near Colusa, Stockton, Modesto, and Hanford. Parasites successfully established in four of the five sites, surviving on field rates of Guthion and Supracide. The Guthion-resistant strain persisted in the release orchards throughout the growing season. Samples of parasites from the release sites in October indicated the parasites had maintained their Guthion resistance. In addition, Guthion-resistant parasites were collected from walnut trees in nearby non-release sites, which indicates the Guthion-resistant strain has spread from the release sites. Such natural dispersal will make implementation easier. The Guthion-resistant and a wild strain of T. pallidus were tested with field-collected foliage treated with field rates of Supracide, Zolone, Thiodan, and Lorsban using a clip cage bioassay. The results indicate that the laboratory-selected Guthion-resistant strain is cross resistant to all these pesticides. This is very useful information, as it will help in the establishment of the resistant strain in the field; nearly any pesticide applied in walnut IPM will assist in selecting for the Guthion-resistant strain of T. pall idus. It also makes the Guthion-resistant strain more valuable; even if the grower doesn't use Guthion, this parasite survives better than the wild strain on the major pesticides used in walnut IPM, which should reduce the likelihood of secondary outbreaks of walnut aphids after pesticide use. Tests to determine mode of inheritance of the Guthion resistance are now underway. Complete concentration mortality lines of the pure colonies will be obtained at that time. Objectives: The objectives for the funding interval were: 1) Complete selection for Guthion resistance with the Guthion-resistant colony of I.. p a II idu s and develop concentration/mortality lines for Resistant, Base and Yolo colonies to document the resistance levels achieved. 2) Test Lorsban, Supracide, Thiodan, and Zolone to determine whether the Guthion-resistant strain of T. P a Iii du s exhibits cross resistance to these pesticides. 3) Conduct genetic analyses to determine mode of inheritance of the Guthion resistance. 4) Conduct field evaluations of the Guthion-resistant strain in commercial walnut orchards. Evaluate efficacy overwintering ability and dispersal rates. Results: Objectives 2) and 4) were completed. Objectives 1) and 3) are being conducted currently. The results for Objective 2) are presented in the ID:.afl manuscript below: Toxicity of Pesticides Used In Walnuts to a Wild and LaboratorySelected Azlnphosmethyl-Reslstant Strain of Trloxvs pallid us (Hymenoptera: Aphldlldae)