What did we learn about soybean aphids in 2008?

By David Nicolai, University of Minnesota Extension

Bruce Potter, integrated pest management specialist of southwest Minnesota at Lamberton, reviews the 2008 soybean aphid problem by illustrating several of the core principles of IPM regarding insect control with insecticides which he lists below:

1. Economic thresholds (250 per soybean plant on 80 percent of the plants in the field) based on economic injury levels minimize economic crop loss and prevent unnecessary insecticide applications. The soybean aphid economic thresholds developed by land grant institutions have a lot of biological data behind them.
2. Insecticide applications (foliar, soil and seed applied) reduce natural controls by predators and parasites.
3. Once an insecticide application is made, fields are more susceptible to reinvasion by the pest since biological controls are removed, marketing notwithstanding.
4. Poorly timed or unnecessary insecticide applications can cause economic loss through yield loss or the need for additional applications.

When will it be over?
Winged soybean aphids are still present but aphid flights are down. Fields treated after Aug. 1 or a bit before, should be relatively safe for the remainder of the season according to Bruce Potter. This is better phrased as fields with very low populations at this time should be safe for the remaining month of the 2008 soybean growing season.

There is no magic calendar date when soybeans are out of aphid danger. The most likely end to the problem will be an aphid population crash from disease or when they begin to move to buckthorn. This is when the sexual stage for the soybean aphid occurs. While aphids have been observed on buckthorn in August, typically this does not occur until September with males moving to buckthorn after females.

Insecticide pre-harvest intervals will become important as the season progresses. Most soybeans are in the early R5 (beginning seed) stage and still susceptible to aphid injury. While data is limited, it is probable that soybeans in late R5 and beyond stages can tolerate higher aphid populations without yield loss. On the other hand, while it does not often happen, very large aphid populations can cause yield losses to R6 soybeans into September.

Disease: Fungal diseases can cause rapid aphid population crashes if epidemics occur. This is what stopped the southwest Minnesota outbreak on Aug. 23, 2003. Most species of entomopathogenic (insect disease) fungi require cool temperatures. As you might expect, they also require wet conditions.

It would appear that high aphid populations are required for widespread epidemics. The heavy dews, fog and cool nights may help this form of biological control. Diseased aphids can carry the pathogen to new locations. These entomopathogenic fungi may be most effective while aphids are on buckthorn.

(David Nicolai is a crops educator with University of Minnesota Extension in Hutchinson.)