Resistance Management Update on Organophosphate Resistance in Codling Moth

In 2003, organophosphate (Guthion) resistance was surveyed in codling moth from populations throughout Washington State. Populations were collected using pheromone traps during first and second flights from apple and pear and sampled for Guthion resistance using the standard adult topical bioassay. Select populations were also sampled for pyrethroid and chloronicotinyl resistance. All orchards sampled during the first flight were Guthion susceptible, despite some extremely high trap catches (both from trap counts provided by referring fieldmen and from bioassay trap yield). Thus, Lorsban and esfenvalerate (Asana) were tested only in orchards with susceptible populations, and data regarding cross-resistance were not found. Assail bioassays were also conducted against Guthion-susceptible populations during the first flight. However, during the second flight, a Guthion-resistant population was identified near Manson, WA. This allowed testing of Assail to determine potential levels of cross-resistance. Fenpropathrin (Danitol) and thiacloprid (Calypso) were not tested against any populations (trap nights were limited in the Guthion-resistant orchard, as the grower treated immediately after samples were collected). Guthion resistance was found to be low to moderate in orchards sampled throughout the state. The lack of significant resistance in many orchards with high trap catch and unacceptably high infestation point toward operational factors as a likely cause. These factors include possibilities such as inadequate pest control measures leading to higher populations, failure to respond to higher populations with increased rates and decreased spray intervals, and low levels of resistance reducing residual activity of Guthion. Nevertheless, it appears that a rapid shift to Guthion resistance is not the cause of increased codling moth damage in many orchards. The adult bioassay for Assail appears to be adequate; however, adjustments to the method should be examined in the future (such as increasing the exposure time, altering the moth holding temperature, etc.). Bioassay results for Assail were quite variable in this study. Unfortunately, the results for the Manson Guthion-resistant population were not precise enough to determine statistical confidence of cross-resistance with Assail. Thus, it cannot be concluded from this study that cross-resistance exists between the compounds. However, with the inference of crossresistance between Guthion and Assail, additional studies should be conducted on field efficacy of Assail in Guthion-resistant orchards.