Viruses to control winter moth, Operophtera brumata (Lepidoptera: Geometridae)

An abandoned apple orchard in Victoria, British Columbia, was used to test winter moth nuclear polyhedrosis virus (NPV) in 1979. Concentrations of 10", 107 and 10· polyhedral inclusion bodies (PIB)/ml were applied at the rate of 1 l/tree using a backpack mistblower, soon after the larvae hatched when buds were pre-pink and 8 days later when the buds were full pink. Each treatment was replicated on 6 trees; 6 trees were untreated checks. Best results were with 10" PIB/ml on pre-pink buds which caused 46% population reduction, a statistically significant saving of foliage and high levels of larval infection with both NPV and cytoplasmic polyhedrosis virus (CPV). Both viruses were found in larvae on the check trees and this was attributed to spray drift. The source of the CPV was investiga ted and found to be a contaminant of the NPV suspension in which the ratio of NPV: CPV PIB was 161 : 1. Despite the low level of CPV applied, up to 65% of the larvae were infected. In 1980, a survey to determine levels of infection in winter moth larvae showed no viruses in 5 untreated sites and only 1% NPV and 5% CPV in the treated orchard. The virus treatment did not initiate a continuing epizootic and the effective concentration of lOS PIB/ml was too costly to produce as a biocontrol agent having an impact only in the season of application