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Soybean Aphid Observed On S.D. Soybean

Jul 08, 2016

By Adam J. Varenhorst

While scouting for insect pests in soybean we observed several small populations of soybean aphids near Volga, South Dakota. Although these populations were well below the economic threshold of 250 aphids per plant, it was a reminder that soybean aphid scouting should be underway. Since soybean aphids were first discovered in the U.S. in 2000, they have been the most economically damaging insect pest of soybean in the Midwest and Northern Plains. Yield losses begin to occur when soybean aphids exceed the economic injury level of 675 aphids per plant. If soybean aphid populations are not managed, they can reduce soybean yield by as much as 40 to 50%. Typically, soybean aphid populations do not reach these densities until late July and early August.
Soybean Aphid: Identification & General Biology

Soybean aphids are small green to yellow insects that are approximately 1/16 of an inch long or less. Soybean aphids are pear or tear drop shaped, and have distinct black cornicles (tailpipes) present on the end of their abdomen (Figure 1). On soybean, populations are comprised of females that reproduce asexually. This characteristic allows for rapid population growth during ideal weather conditions. It is possible to observe soybean aphid nymphs, and both winged and wingless adults on one plant.

Figure 1. Soybean aphid nymphs and adults. Photo by Adam Varenhorst.
 
Scouting


Early season populations are not likely to be accompanied by honeydew, one of the more common indicators of soybean aphids. However, there is the possibility that ants may be tending to them and natural enemies such as lady beetles may be hunting them, so the presence of these other insects can be helpful in determining if soybean aphids are present in a field.

To begin scouting walk through a field in a “W” or “Z” pattern. Along the path search random plants for the presence of soybean aphids. As mentioned in a previous article there are two methods that can be used to scout for soybean aphids. The traditional method involves scouting 20 to 30 plants and counting the soybean aphids present on each plant. When 250 aphids are counted on at least 80% of the plants the threshold is reached. The alternative is referred to as speed scouting. For this method, you first sample 11 plants. If you observe more than 40 soybean aphids on all 11 plants the threshold has been reached. If only 7 to 10 plants had more than 40 aphids, then you would scout additional plants. This method saves time, and is comparable to the traditional method for making management decisions to prevent yield loss.
 

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