Comparison of Three Egg Mass Survey Methods in Relation to Gypsy Moth (Lepidoptera: Lymantriidae) Defoliation in Michigan

Abstract

We evaluated 3 egg mass survey methods used to predict gypsy moth, Lymantria dispar (L.), defoliation in 32 oak stands in northwestern lower Michigan. Fixed-radius plots, timed walks, and a recently developed method, the 100-tree plot, were used to assess egg mass density in each stand. Defoliation in each stand was estimated visually the following summer, and associations among defoliation levels and the 3 survey methods were evaluated. Larval cadavers were collected in each stand to determine if the gypsy moth pathogens Entomophaga maimaiga and nuclear polyhedrosis virus (NPV) were present. Egg mass counts from the fixed-radius plot and 100-tree plot methods were significantly associated with subsequent defoliation, but timed walk counts were not. We used common egg mass density thresholds and results of the 3 survey methods to determine if the stands would require treatment in a hypothetical suppression program. Treatment decisions based on results of the fixed-radius plots and timed walks were similar. The 100-tree plot method, however, yielded fewer erroneous decisions to treat stands when suppression was not needed. All stands that sustained >30% defoliation had at least 84% new egg masses and a new egg mass density of at least 6,563 egg masses per hectare as determined by fixed-radius plots. Surveys conducted using fixed-radius plots and 100-tree plots took 2–3 times longer than surveys using timed walks. Infection of cadavers by NPV was higher in stands with high egg mass density and defoliation, whereas infection by E. maimaiga was more prevalent in stands with low egg mass density and defoliation.