The effect of cropping systems on the pest complex in Virginia soybeans and calibration of the sweep net and ground cloth sampling methods for use in these cropping systems

Increasing numbers of soybean fields in Virginia are double-cropped, immediately following spring harvest of barley or wheat. Most of these fields are planted in notillage systems. Early planted soybeans are sometimes planted with a grain drill in very narrow rows (ca. 18 cm. apart). The effect that these practices have on arthropod populations is not well understood. Also, sampling methods developed for conventionally tilled, wide row systems may be more or less efficient in the double-cropped and drilled cropping systems.

Ten sample pairs were obtained from each of 150 fields between 1980 and 1982 in conventional, drilled, and double-cropped soybean fields. The first sample of each pair was collected with a 38 cm. diameter sweep net (25 sweeps per sample) and the other sample was collected with a 0.91 meter ground cloth. In addition, after each ground cloth sample was obtained, the foliage and base of the plants in the 0.91 m sample were searched for additional arthropods not counted on the ground cloth. Regression relationships were developed and are presented which relate sweep net counts to ground cloth counts. Also, estimates of the percent of selected species missed with the ground cloth may be used to obtain absolute estimates of arthropod densities.

Throughout the 1981 and 1982 growing seasons weekly sweep net samples were obtained from three fields planted to each of the following cropping systems: full season, conventionally tilled; full season, drilled; no-till, double-cropped after barley; and no-till, double-cropped after wheat. Analyses were perforrried to determine which species were most abundant in which cropping systems. Also, a principal components analysis was performed to determine the factors that caused the observed differences.