Selection for Pesticide Resistance in Trioxys Pallidus

During the past year, we collected walnut aphid parasitoids, !. pallidus, from 8 walnut orchards. Parasites from 2 sites were maintained as individual colonies, those from'the six other sites were pooled into one colony. These colonies and one started in 1985 from three sites were screened with Guthion. We found substantial variability in responses to Guthion by these colonies, which suggests that natural selection is operating in walnut orchards. Our laboratory selections have yielded increases in tolerance to Guthion in the colony that has been selected 11 times. Selection of two other colonies has occurred two and four times, and a small response has occurred in both. The Valley-Christensen colony has had its LC50 increased ca. 1.7-fold over that of the base colony from which it was derived. The survival rates of the newly-collected Aggregate and Los Banos colonies are nearly as high as that of the selected Valley-Christen colony, so we have special hopes for the selection responses of these two colonies. We plan to evaluate whether the increases in tolerance to Guthion that are obtained will be useful in the field. To do so, we plan to spray potted walnut trees and test different aged residues for their toxicity to the selected and susceptible strains. Because T. pallidus has such a high reproductive rate, with each female able to produce an average of 300 progeny, we believe that if a selected colony of T. pallidus were able to reenter walnut orchards several weeks sooner than is the case currently, then a genuine improvement in walnut aphid control would occur.