Infestation Rates and Biological Observations After Harvest of Codling Moth (Lepidoptera: Tortricidae) in Stored In-Shell Walnuts

Abstract

Infestation rates of codling moth, Cydia pomonella (L.), in commercially produced walnuts were determined over a 3-yr period. Infestation rate averaged 0.245 (0.11-0.46) codling moth per kilogram of walnuts. Only the larval stage (fourth [4%] or fifth instars [96%]) was found infesting walnuts at harvest, indicating the potential for diapausing codling moth larvae to occur in stored walnuts. In laboratory tests in which dehydrated walnuts were infested with nondiapausing larvae, < 1% of first, second, or third instars completed development to adult. Significant survival was observed for fourth and fifth instars. Laboratory trials with diapausing larvae in walnuts stored for 30, 60, or 90 d showed that survival was reduced to an average of only 35%. Tests designed to simulate very high levels of infestation of nondiapausing fifth instars inside bags of walnuts showed that 99% of the larvae remained inside the bag and pupated inside or among the walnuts. Most of these larvae survived to adult, but only 18% of the female moths successfully mated. In further laboratory investigations in which we controlled distances of separation between male and female moths in walnut masses, mating was reduced to 9 or 3% when single pairs of moths were separated by only 0.17 or 0.33 m, respectively. No mating occurred when moths were separated by distances of greater than or equal to 0.67 m. We discuss implications of these data in relationship to biological approaches to quarantine treatments for codling moth in in-shell walnuts.