Effect of forest edges on population density of whitefly and parasitoids in soybean fields

A considerable proportion of the Brazilian Cerrado has been converted for agricultural uses such as pasture and multiple species of annual crops.  The predominant crops include soybeans, corn and sorghum under no till soil management during the rainy season and tomatoes under center pivot during the dry season. Little is known regarding how intensive cropping, and the accompanying decline in native vegetation, affects pests and their natural enemies. We evaluated if fragments of natural vegetation and landscape characteristics affect population density of whitefly and beneficial insects in soybean fields. From September 2013 to August 2014 we sampled 20 fields every 15 days. Fields sampled were grown with soybean in the rainy season and tomatoes or corn under irrigation during the dry season. The whitefly population dynamics and nymph parasitism increased with soybean development. The population peak was in February, when soybean harvesting starts and whitefly moves from harvested field to younger soybean. Whitefly nymph parasitism by Encarsia sp. and Eretmocerus sp. also increased with soybean development. Average parasitism across 20 fields was relatively low, ranging from 0.7% (V3) to 19.8% (R6). Whitefly nymph density was higher at field edges, adjacent to natural vegetation, but no edge effect was observed for parasitoid density or percentage of parasitism. Counter to expectations, our first year data suggests that natural vegetation may be a source of pest insects rather than natural enemies moving into these soybean fields.