Combined effects of host-plant resistance and foliar insecticides for soybean aphid management

Soybean aphid (Aphis glycines Matsumura) is a major pest of soybean in the Midwest, and insecticides such as pyrethroids and organophosphates are used to suppress soybean aphid outbreaks to prevent yield loss. Another management tactic is host-plant resistance (e.g., Rag1 and Rag2 genes). However, aphid populations on aphid-resistant plants can still build to economically damaging levels, which require treatment with insecticides to protect yields. Other pest-resistant crops are known to increase insecticide susceptibility of insect pests feeding on the plant, but has yet to be demonstrated when using foliar insecticides against soybean aphid.  To determine if there is increased soybean aphid insecticide susceptibility on resistant plants, a factorial design consisting of near isoline susceptible and resistant (e.g., Rag1) isoline plots were planted and were either untreated, or treated with an organophosphate or a pyrethroid in mid-July after aphid establishment. Aphids per plant were recorded daily between June and July. After insecticide application, aphid numbers and population growth rates were lower in resistant plant plots treated with insecticide than plots with untreated resistant plants or susceptible treated plants. In situations where resistant soybeans need to be treated with insecticides, reduced aphid population growth rates on the aphid-resistant soybean might allow for the use of less toxic insecticides that typically would be less effective against soybean aphid on susceptible soybeans, and would be more compatible with natural enemies of the soybean aphid. The increased susceptibility may also aid insecticide resistance management as more resistance traits become available for growers.