Biological Efficacy of Gypchek Against a Low-Density Leading-Edge Gypsy Moth Population

The USDA’s Slow-the-Spread (STS) program seeks to retard the continued spread of the gypsy moth using ecologically desirable treatments such as Gypchek. At “trace” population levels, evaluation of treatment success by defoliation reduction, egg mass reduction, burlap counts, or larval collection is not feasible. We adapted the “bugs-in-bags” technique to evaluate an operational application of Gypchek against trace populations of gypsy moths in Wisconsin, an STS area. Late firstor early second-instar gypsy moth larvae were placed, 1 per bag or 10 per bag, in sleeve cages placed over treated foliage one hour posttreatment. Mortality observed for larvae placed 10 per bag was equivalent to that recorded for larvae placed 1 per bag, and both should approximate the mortality occurring to the larvae scattered in nature. A single application of Gypchek applied in 9.5 liters of Carrier 038 at 10 polyhedral inclusion bodies per hectacre was found to induce a higher rate of infection in blocks treated in the early morning than in blocks treated later in the morning, correlating significantly with a lowering of relative humidity and an increase in temperature and wind speed. Recorded levels of efficacy (24–67%) did not meet quarantine objectives; however, Gypchek, which kills only the gypsy moth, remains a product of choice by many land managers for use in certain environmentally sensitive areas. These results provide such land managers with a realistic assessment of the level of efficacy that can be expected from this formulation of Gypchek used at the currently recommended dose. North. J. Appl. For. 21(3):144–149.