GEOSTATISTICAL ANALYSIS OF GYPSY-MOTH (LEPIDOPTERA, LYMANTRIIDAE) EGG MASS POPULATIONS

Among-sample spatial variation in gypsy moth egg mass population density was quantified from four databases: the Melrose Highlands data (104 plots [0.0405 ha] sampled throughout coastal New England over a 20-yr period in the early 1900s; maximum point separation of almost-equal-to 250 km), Massachusetts state survey data (150 plots of 20 banded trees located throughout Massachusetts and sampled from 1985 to 1987; maximum point separation almost-equal-to 300 km), Fox Chapel Survey Data (517 plots [0.0101 ha] sampled throughout Fox Chapel Borough, Pennsylvania from 1988 to 1990; maximum point separation almost-equal-to 8 km) and Cape Cod within-stand data (groups of 169 plots [0.008 ha] located in a 25-m grid; maximum point separation almost-equal-to 1 km). Sample semivariograms were calculated that quantified spatial dependency in density at a variety of spatial scales. Both the Melrose and Massachusetts data showed evidence of spatial contagion in density of distances ranging from 20 to 100 km. The range and magnitude of this spatial dependence varied considerably from year to year. The extent of small-scale (< 200 m) spatial contagion of egg mass densities in the Cape Cod data was also quite variable. Some of the sites in some years showed evidence of spatial dependence at various distances, whereas data from other years and other sites showed no spatial contagion. In contrast, semivariograms from the Fox Chapel plots were quite similar: in each of the 3 yr the maximum distance of spatial dependence ("range") was almost-equal-to 1 km. In summary, we quantified spatial dependency in egg mass densities at scales ranging from 25 m to 100 km. There was little evidence of spatial dependency at greater distances. The ordinary kriging procedure can use these semivariograms to generate maps of interpolated estimates of egg mass densities. These maps may be valuable in area-wide gypsy moth management programs. Specific recommendations were developed for the spacing of spatially stratified egg mass samples in area-wide management systems.