Impact of fungicides on soybean aphids (Aphis glycines) and soybean yield in Iowa

In recent years, growing concern about invasive pests such as soybean aphid and soybean rust has led to dramatic increases in pesticide use on soybean. Many agribusinesses are now offering growers a pest management program emphasizing a calendar-based, co-application of a fungicide-insecticide tank mix. However, it is unclear if these co-application methods exhibit improvement upon current recommendations, which apply pesticides as needed.

Entomopathogenic fungi such as Pandora neoaphidis and Conidiobolus thromboides can cause epizootics of soybean aphid in temperate regions and are a potential source of biological control. The impacts of fungicides and fungicide-insecticide tank mixes on these entompathogenic fungal species are not well known and could inhibit or prevent the occurrence of population suppressing epizootics of soybean aphid.

In 2009, we tested the impact of fungicides (strobilurin, triazole, and a combination of the two) applied alone and in combination with insecticides. Pesticides were applied either at bloom (R1) or beginning pod set (R3), or when prescribed based on current recommendations (i.e. at 250 aphids per plant and/or threshold level of foliar disease). Treatments were applied in replicated, small plots (six 12.2 m long rows with 38.1 cm spacing) in which we monitored soybean aphid populations throughout the growing season (June 15 – September 4) and yield. This experiment was conducted at three locations in Iowa where we anticipated variable severity of both foliar disease and aphid pressure. Results will be discussed and compared to preliminary studies conducted in 2008.