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Issue No. 11, Article 3/June 9, 2006

Soybean Aphids Already Found in Soybean Fields in Northern States

During the week of May 29, entomologists at the University of Minnesota found soybean aphids beginning to colonize V1-stage soybeans in several locations in the state--southwest, south central, southeast, and northwest. Soybean aphids were found in a field near Ames, Iowa, on May 31, with as many as 40 aphids on one plant (although this density was not common). Kelley Tilmon, entomologist at South Dakota State University, reported the first finding of soybean aphids on V1-stage soybeans in a field in eastern South Dakota on June 6. All of the reports indicate that these are the earliest observations of soybean aphids colonizing soybean fields in each state.

So the big question is what these early observations of soybean aphids on small soybean plants mean for the probability for outbreaks in 2006. Well, to be honest, no one has a very good answer. Entomologists who have observed early colonization of soybean aphids have indicated that sometimes the early populations build to economic levels and sometimes they don't. The truth of the matter is that weather and natural control agents (e.g., insidious flower bugs [Orius insidiosus], lady beetles) usually play major roles in regulating soybean aphid populations. High temperatures and lots of predators will suppress soybean aphids. On the other hand, cooler temperatures and low levels of predation establish the potential for populations to explode. Only time and frequent observations will reveal what is in store.

Adult insidious flower bug on a corn leaf.

Given these early sightings in Iowa, Minnesota, and South Dakota, it's probably not too soon to examine soybean fields for soybean aphids in Illinois, at least in northern counties where buckthorn (the aphids' overwintering host) is prevalent. We will begin weekly surveillance of about 25 soybean fields in Woodford and Stephenson counties (and in between) this week, and we'll keep you apprised of our findings. However, we invite reports, as usual, from anyone who happens to encounter soybean aphids in June.

The earliest date that soybean aphids have been found in soybean fields in Illinois was May 27 in 2005 in Ford County (east-central Illinois), seemingly an unlikely location for a first sighting of that pest. So with no reports of soybean aphids yet in Illinois, we won't set a new record in 2006. But vigilance should be the order of the day.

What should be done if soybean aphids are found on small soybeans early in the growing season? Except for keeping an eye on the trajectory of the population (are numbers increasing, stable, or decreasing?), there is no reason to take action with insecticides. The population of soybean aphids in that field in Ford County last year never developed into a threatening population in 2005. In fact, repeated samples revealed no population growth whatsoever. Entomologists throughout the north-central states strongly advise against applying insecticides to control soybean aphids in June. Insecticides applied too early may kill important early-season predators (e.g., insidious flower bugs), allowing colonizing soybean aphid populations to increase in the absence of predators later in the summer. So hold back on taking action against soybean aphids until you are certain that densities will exceed the economic threshold of 250 aphids per plant, but make certain you are aware of developments with soybean aphid populations in your area and other parts of Illinois and the Midwest.--Kevin Steffey

Kevin Steffey

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