Insect Distribution and Insecticide-induced Mortality in Corn and Potatoes as Affected by Plot Size and Location

This study was designed to determine if results of insecticide tests were affected by size and location of field plots used. Population density of insects in plots treated with carbaryl varied with plot size. Feeding damage by the potato leafhopper, Empoasca fabae (Harris), to potato foliage dropped rapidly as the treated plot size increased from 4 to 99 m2 and then continued to decline gradually to 1,000-m2 plots. Thus, a plot of ca. 100 m2 may be the minimum size to accurately determine insecticidal effect on the potato leafhopper. Corn earworm, Heliothis zea (Boddie), egg numbers changed the most sharply as plot size increased from 3 to 31 m2. Larval numbers dropped most sharply when plot size was increased from 3 to 14 m2. A minimum plot size of ca. 30 m2 appears to be necessary for corn earworm infesting sweet corn for tests done under similar conditions. Taylor’s index of aggregation and Lloyd’s mean crowding index and patchiness index were calculated for potato leafhopper injury in potatoes, caterpillar feeding damage in sweet corn ears, and numbers in sweet corn plots of Orius insidiosus (Say), Coleomegilla maculata (De Geer), corn earworm eggs and larvae, and European cornborer larvae, Ostrinia nubilalis (Hubner), in treated and untreated areas of plots.