Estimating the Density of Larval Gypsy Moth, Lymantria dispar (Lepidoptera: Lymantriidae), Using Frass Drop and Frass Production Measurements: Sources of Variation and Sample Size

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Abstract
At three sites on Cape Cod, Mass., successive gypsy moth, Lymantria dispar (L.), larval densities were estimated using simultaneous measurements of the number of frass pellets produced per larva (frass yield) and the number of frass pellets falling in the forest per unit area (frass drop). Estimated larval densities declined through the period of larval development at all sites. Frass yield was positively correlated with ambient temperature. Frass drop was positively correlated with the basal area of host trees immediately around the frass trap. At a low-density population, rate of frass drop exhibited a diel periodicity similar to that of larval feeding. Mean-variance relationships of frass drop measurements and density estimates indicated that frass pellets and larvae were spatially aggregated within stands. The optimal ratio of drop to yield samples and the number of replicates of each sample necessary to obtain a given level of precision decreased with increasing density.