Modeling soybean yield response to multiple types of insect injury

The development of comprehensive thresholds encompassing multiple types of insect injury has remained an elusive goal of Integrated Pest Management (IPM). This is an especially important goal with the addition of the soybean aphid, Aphis glycines Matsumura (Aphididae: Hemiptera), which represents a feeding guild (assimilate removal) that was previously of little importance to Midwestern soybean production. In 2008 and 2009 the injury response of soybean to two sources of injury: assimilate removal from soybean aphids, and leaf consumption (simulated by leaf removal) was determined. Treatments were applied in a five-by-five factorial design with experimental units experiencing one of five levels of aphid exposure (measured in cumulative aphid day exposure; 0, 20,000, 40,000, 60,000, and 80,000 CAD), and one of five levels of defoliation (0, 20, 40, 60, and 80 percent). In 2008, yield declined at a rate of 5.2 percent per 10,000 CAD at all levels of defoliation, and in 2009 yield declined at a rate of 3.2 percent per 10,000 CAD at all levels of defoliation. Overall the treatment levels, we did not observe evidence of an interaction between plant exposure to soybean aphids and defoliation on seed yield. This was also the case for densities of aphid (<60,000 CAD) and defoliation (<60%) that are more likely to be experienced by growers in North America. In the more restricted range of our model, yield losses were additive and declined at a rate of 4.5 percent per 10,000 CAD and 2.7 percent per 10 percent defoliation.