Evaluation of the Timed-Walk Method of Estimating Gypsy Moth (Lepidoptera: Lymantriidae) Egg Mass Densities

Abstract

Timed walks are widely used by pest managers for estimating gypsy moth, Lymantria dispar (L.), egg mass densities. According to this method, an individual(s) walks through a woodlot for a fixed length of time (typically 5 min) and counts all egg masses observed. These counts are then used in one of several previously developed linear regression equations to estimate egg masses per unit area. In this study, we evaluated the statistical properties of these estimates. At several 25-ha woodlots, we estimated true egg mass densities by counting egg masses in multiple fixed-radius plots. Two observers then made four 5-min walks through each woodlot. These counts were more highly correlated with estimated density when expressed as numbers per walk than as number per distance walked. Analysis of variance indicated that timed counts varied significantly among observers. For individual observers, counts of egg masses per walk were used to develop linear regressions for predicting egg mass densities. These regressions were characterized to determine the confidence intervals associated with density estimates obtained from a given number of samples. Results indicate that even if separate regression equations were developed for each observer, the timed-walk method provides imprecise estimates. Lack of precision and observer effects indicate that fixed-time and fixed-distance methods of estimating gypsy moth egg mass densities should be discontinued. Analyses indicated that even one O.OI-ha fixed-radius plot per woodlot will usually provide a more precise estimate of density than four or more 5-min walks.