Variables that impact soybean aphid populations 

Is 2024 going to be another bad year for dealing with soybean aphids? While that may be difficult to predict – time will tell – there are some factors that have been known to increase or decrease aphid production heading into the growing season, including the prior winter, stormy spring weather and planting timing.  

Soybean aphids are native to Asia and were first discovered in North America in Wisconsin in the year 2000. They winter on buckthorn in nearby wooded areas and slowly make their way into soybean fields throughout the summer. The aphids use their needle-like mouths to extract the sap out of the soybean plants. In large numbers, they can greatly reduce soybean quality and yield.   

While there’s no real way of determining if the soybean aphids will be a significant problem again this year, Dr. Robert Koch, University of Minnesota Extension Specialist in the Department of Entomology, did share some of the known variables that can determine the size of the potential threat heading into summer. For starters, the mild winter of 2023-24 may have led to a higher survival rate, but Dr. Koch believes a more important factor in aphid survival is the spring conditions and how far along the soybeans are when they start migrating. 

“What matters most is the ability for the aphids to go from the buckthorn, where they hatch and develop wings, to the soybean plant,” he said. “Are the soybeans planted and coming up when they are making that move? Has there been torrential downpours and wind that can increase mortality or have the conditions been super favorable?” 

In the beginning of summer, when the soybean plant is still in its early development, it offers little protection against thunderstorms. However, later in the season when soybean plants are further along in development, they may offer a nice sanctuary for the aphids from the severe weather. 

As the calendar shifts to mid to late June, farmers can begin the process of scouting for soybean aphids checking their earliest planted fields first, especially those next to wooded areas with abundant buckthorn. Finding actual aphids may be a challenge, but the presence of lady beetles and ants are an indicator that the aphids are in the early stages of colonization. At that point, farmers should be scouting for aphids on a weekly basis. When applying insecticides, Koch recommends sticking to the proven economic threshold of 250 aphids per plant. Before reaching that threshold, there’s no noticeable yield loss and it’s not economically feasible to spray. 

“Prior to hitting that economic threshold, we can’t be certain that it will actually get to a damaging level because there are other forces out there working against the aphids, such as disease, lady beetles, parasitic wasps and other predators,” said Koch. “We’re seeing more often that fields are reaching 50 or 100 aphids per plant and then plateauing there as the other factors are keeping them in check, but if you do reach the 250 aphid threshold it seems there’s enough momentum to reach a damaging level,” Koch said.  

Learn more about soybean aphids by visiting the University of Minnesota Extension website. 

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