Survival of cabbage looper, Trichoplusia ni (Lepidoptera: Noctuidae), through winter cleanups of commercial vegetable greenhouses: Implications for insecticide resistance management

Abstract

Cabbage looper, Trichoplusia ni (Hübner) (Lepidoptera: Noctuidae), is a highly migratory and destructive pest of various crops in North America. Its infestations in vegetable greenhouses in western Canada are generally attributed to annual immigrations from southern overwintering areas, but whether it survives overwinter in western Canada is uncertain. Bioinsecticides based on Bacillus thuringiensis ( Bt) toxins are commonly used in southwestern British Columbia where some T. ni populations have developed strong resistance to Bt-insecticides. In this study, overwintering success of T. ni populations was monitored outside and inside three commercial greenhouses in British Columbia. In both years, T. ni pupae placed outside greenhouses did not survive although pupae placed inside the greenhouses survived the unheated cleanup period and exposure to organophosphate insecticides commonly used in the cleanup procedure in the greenhouses. During the cleanup period, no moths were caught inside the unheated greenhouses, but once the heating was restored for the new growing season, adults were captured in pheromone traps. No developmental stages of T. ni were detected on nursery seedlings and visual monitoring of transplants at the beginning of the season found no signs of previous feeding damage. This strongly suggests that T. ni were not introduced with the seedlings. Our findings are important for understanding the potential risks of inadequate winter cleanup and the potential carryover of resistance to Bt-insecticides in populations from one season to the next that could complicate pest management.