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Issue No. 20, Article 2/August 8, 2008

Outbreaks of Soybean Aphids Underway in Parts of the Midwest

Numbers of soybean aphids infesting soybean fields in Iowa, Minnesota, North Dakota, South Dakota, and Wisconsin have increased dramatically over the past couple of weeks. Insecticide applications for control of soybean aphids are widespread in these states. We have received limited reports of large numbers of soybean aphids in some northern Illinois counties, but densities in our 26 survey fields thus far have not been near the 250-aphids-per-plant threshold. The numbers of soybean aphids found in almost all 26 fields increased from the week of July 21 to the week of July 28, but average densities were still rather low. At the time this article was written, data from the week of August 4 had either not been entered or not been collected, so updated numbers will be telling.

Thus far, soybean aphid numbers have been relatively low in the states to our eastIndiana, Michigan, Ohio. However, we have learned recently that numbers have begun to increase in Michigan and northern Indiana.

We strongly encourage soybean growers and their advisers in the northern third of Illinois to start scouting diligently for soybean aphids. During outbreaks, alates (winged aphids) move from field to field, from region to region, and from state to state if the weather patterns are favorable for their dispersal. Consequently, outbreaks elsewhere in the Midwest are alerts for potential outbreaks in Illinois. Winged soybean aphids (albeit in single-digit numbers) have been captured in suction traps near Freeport (Stephenson County), Metamora (Woodford County), and Monmouth (Warren County), indicating that movement among soybean fields has begun (www.ncipmc.org/traps). If environmental conditions remain favorable, colonization and population increase will occur quickly.

Despite this warning, however, it is equally important not to apply an insecticide if the numbers of soybean aphids do not warrant it. I direct you to an excellent article about economic thresholds and economic injury levels for soybean aphids by Jon Tollefson, Matt O'Neal, and Marlin Rice for the August 4 issue of Integrated Crop Management News. The authors clearly articulate that "The consequences of preventative insecticide applications can be severe." We have received a handful of reports that soybean fields sprayed previously with insecticides (some necessarily for control of Japanese beetles, others treated preventively) have rapidly increasing numbers of soybean aphids, either as a result of resurgence within the same field or immigration of soybean aphids after the residual effectiveness of the chosen product had deteriorated. Declining prices for soybeans will not support the costs of repeated insecticide applications, and the ecological consequences of multiple insecticide applications could be dire.

Some of you may be wondering about the every-other-year aphid outbreak syndrome that has prevailed to some extent in many areas of the Midwest. Since the discovery of the soybean aphid in North America in 2000, widespread outbreaks have occurred primarily in odd-numbered years (e.g., 2003, 2007). However, we have stated repeatedly that it was merely a matter of time before this pattern changed. The pattern has been maintained primarily by the ebb and flow of soybean aphid populations and populations of predators, primarily the multicolored Asian lady beetle. During years when soybean aphid outbreaks have occurred, numbers of multicolored Asian lady beetles have increased dramatically in response to the increased availability of prey (soybean aphids). Consequently, numbers of soybean aphids typically have been considerably lower during the year after an outbreak year.

This alternating cycle, although not necessarily distinct in all Midwestern states, may well have been broken by relatively widespread insecticide applications in both corn and soybeans where predators such as multicolored Asian lady beetles reside. Given the nature of population increase of soybean aphids (doubling time of 3 to 4 days in ideal conditions), the absence of sufficient densities of natural enemies will facilitate rapid increase in soybean aphid populations. David Ragsdale, entomologist at the University of Minnesota, has stated that the every-other-year cycle "no longer holds water."--Kevin Steffey

Kevin Steffey

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