Influence of Irrigation and Maturity Group on the Seasonal Abundance of Soybean Arthropods

The effects of irrigation and soybean maturity group (Group V ‘Forrest’ variety vs Group VII ‘Braxton’ variety) on the seasonal abundance of arthropod pest and beneficial population densities were examined in large-plot field tests at the Attapulgus Research Center in Attapulgus, GA, in 1987 through 1990. In general, soybean plant growth was more abundant, yields were higher, and canopy closure indices were lower in irrigated than in non-irrigated plots for both varieties. Irrigation and the resultant increased soybean vegetation supported a greater overall seasonal abundance of all five of the insect pests monitored: stink bugs, primarily Nezara viridula (L.), velvetbean caterpillars, Anticarsia gemmatalis Hübner, soybean loopers, Pseudoplusia includens (Walker), threecornered alfalfa hoppers, Spississtilus festinus (Say), and green cloverworms, Plathypena scabra (F.). No overall varietal effects were noted for N. viridula; however, each year, population densities were higher in the early-maturing ‘Forrest’ up to julian day 250 then densities were higher in the later-maturing ‘Braxton’ for the remainder of the grown season. Mean estimates of the seasonal abundance of A. gemmatalis and P. scabra populations were similar between the two varieties; however, P. includens and S. festinus were more abundant on Forrest than on Braxton. Total arthropod predators, including Nabis spp., Geocoris spp., and spiders, also were more abundant on irrigated soybeans, while overall varietal means were similar. Yearly analyses revealed significant irrigation and variety effects for most of the species sampled.