Efficacy of insecticide-fungicide tank mixes for control of soybean aphid (Aphis glycines)

Soybean producers face pest pressures from soybean aphid (Aphis glycines Matsumura) and foliar diseases (e.g. brown spot, frogeye leaf spot, Cercospora leaf blight). Soybean aphid causes yield reduction of 14-50% and is typically managed with foliar insecticides. Foliar disease impact on yield is not well-established, but fungicides are frequently applied. Pest management programs offering a co-application of insecticide and fungicide (a “tank mix”), based on soybean growth stages, are increasingly popular. However, current integrated pest management (IPM) recommendations emphasize the application of pesticides only when pest pressure exceeds a pre-determined economic threshold. It is unclear if growth stage based applications of tank mixes provide greater benefits than IPM recommendations.

To examine the impacts of fungicide-insecticide tank mixes on soybean aphid populations and soybean yield, we conducted field trials in replicated small plots from 2008-2010 at three locations in Iowa with variable aphid pressure. We tested the impact of a fungicide alone (a mix of a strobilurin and a triazole), an insecticide alone (a mix of a neonicotinoid and a pyrethroid), and a fungicide-insecticide tank mix. Pesticides were applied at bloom or beginning pod set (growth stages R1 and R3, respectively) or when warranted based on IPM recommendations (i.e. 250 aphids per plant). Soybean aphid populations were monitored throughout the growing season (June – September). A preliminary cost effectiveness analysis will be shared. Overall, highest yields were observed in the IPM treatment and in treatments that applied an insecticide, alone or with a fungicide, at the R3 growth stage.