Modeling crop response to multiple sources of injury

Soybean production in the Midwestern United States is at risk of herbivory from two distinct feeding guilds, phloem feeding and defoliation. The economic injury level for soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae) was published in 2007 establishing a framework for soybean aphid management, and economic thresholds exist for many defoliators, like bean leaf beetle, Ceratoma trifurcata (Förster) (Coleoptera: Chrysomelidae). Currently no comprehensive thresholds exist. This forces pest managers to rely on simple thresholds when making recommendations where pest complexes are found. To what extent two feeding guilds interact to impact yield is not clear. In an attempt to shed some light on how injuries interact to cause yield loss we have designed an experiment using modeling techniques found in pharmaceutical and toxicological interaction research (linear isoboles and isobolograms). In this experiment both defoliation and aphid injury are applied using a five by five factorial design. We will model plant injury response to point defoliation event at six levels (0%, 20%, 40%, 60%, 80%, and 100% defoliation), and exposure to soybean aphid (measured in Cumulative Aphid Days, CAD) so that treatments experience one of six levels (0 CAD, 20,000 CAD, 40,000 CAD, 60,000 CAD, 80,000 CAD, and 120,000 CAD). Both forms of injury are applied during the early reproductive stages (R1-R2). We will discuss how this research improves our understanding of how stresses interact to cause yield loss in soybean production systems while taking the first crucial steps towards the development of comprehensive pest management thresholds.