by Dr. Ian MacRae, U of MN Extension Entomologist
There has been increasing pressure to apply insecticide and tank mixed pesticides at lower thresholds based on claims of increased yield benefits. While increased commodity prices can stimulate the desire to decrease risk tolerance and increase the use of pesticides, this is not always a paying proposition.
The current treatment treatment threshold for soybean aphids (250/plant when most plants have aphids) has been adopted and is recommended by the Extension services of all of the soybean producing states in the North Central region. It is the result of many site-years of data collected from field plots mirroring commercial; production. It incorporates data from a variety of geographic locations, climate conditions, commodity prices and management costs. Most importantly, at the treatment threshold, NO economic injury is yet occurring!
Insurance or just-in-case treatments tend to be applied prior to the optimum treatment timing. Generally they're a tank-mixing of pesticides applied to save application costs, but often carry unforeseen results.
Treating early can often result in the field needing treatment again later in the season. If soybean aphid populations are heavy enough to require treating (they are at or above threshold) then treatment is obviously required. However, treating low populations of soybean aphids in an attempt to avoid later high populations can backfire badly. Treating early can wipe out natural enemies, it does not protect plant material developing after the application has been made and the insecticides used rarely have the residual to prevent aphids from establishing much beyond 10-14 days.
Predation is a significant cause of mortality in aphid colonies. Larvae of lady beetles, hover flies and lacewings all feed extensively on aphids and can often keep smaller populations in check, especially colonizing aphids (Figure 1). Predation and parasitism are extremely important in preventing immediate colonization of fields by soybean aphids.
Over the years, it has been noted in the North Central region that successful overwintering is not necessary for soybean aphids to become a problem. These insects readily disperse, aided by the wind, and high populations in one area can often move and establish in another. The changing aphid populations of July and August are very familiar to growers in the region. In the absence of any factor slowing colonization, soybean aphids will become established and populations will grow quickly.
In addition, data from several states (including trials on this station) have shown that low, sustained aphid populations DO NOT cause significant yield loss. Soybean aphid populations have also been seen to collapse under the right environmental conditions, so low populations of aphids do not always become large populations...
In light of these considerations, one must question just what economic return do the various aphid management programs offer. In an effort to answer this, we designed an experiment to isolate and evaluate different management programs.
View the treatments and outcomes here: