No. 23 Article 4/October 3, 2008

Soybean Aphids in Review

A great deal of attention was focused on soybean aphids in 2008, representing a major change in the every-other-year outbreak pattern. Since 2001, soybean aphids have reached relatively widespread outbreak levels during the odd-numbered years (2001, 2003, 2005, 2007) and relatively low levels during the even-numbered years (2002, 2004, 2006). There were local exceptions to this pattern, but the pattern was reasonably consistent in Illinois. The pattern was due, in large part, to the ebb and flow of populations of soybean aphids and their principal predator, the multicolored Asian lady beetle, Harmonia axyridis. In the past, when captures of winged soybean aphids in suction traps were high in the fall (mid-September through October), we could be reasonably certain that soybean aphid outbreaks were likely the following year. Numbers of multicolored Asian lady beetles typically have been low during years when numbers of soybean aphids were low, allowing soybean aphid numbers to build up late in the summer and early fall.

Vice versa, when captures of winged soybean aphids in suction traps were low in the fall, we could be reasonably certain that soybean aphid outbreaks were not likely the following year. During outbreak years, numbers of multicolored Asian lady beetles usually increase dramatically toward the end of the growing season, suppressing the numbers of winged aphids that make their way back to their overwintering host, buckthorn. It seems apparent now that numbers of winged soybean aphids captured in suction traps may be useful for predictive purposes only if we have data about the prevalence or relative absence of predators, including the multicolored Asian lady beetle. Captures of soybean aphids in suction traps throughout the Midwest can be viewed at the North Central IPM Center's "Regional Soybean Aphid Suction Trap Network."

The widespread and economically significant outbreak of soybean aphids in 2008 will be discussed in significant detail at educational meetings throughout the Midwest this winter. In the meantime, if you want to look at the population dynamics of soybean aphids in Illinois in 2008, visit our tables of soybean aphid densities in 26 commercial soybean fields. Peak numbers of aphids in Stephenson County typically occurred in mid- to late August, although some populations peaked in early September. The peak densities of soybean aphids in all 10 fields in Stephenson County came close to or exceeded the economic threshold of 250 aphids per plant. In Woodford County, peak populations generally occurred later (several in early September) than peak populations in Stephenson County, and densities were generally lower in Woodford County (economic threshold exceeded in only two fields) than in Stephenson County. The population dynamics in the individual fields in counties comprising the "transect" (Bureau, Lee, Marshall, Ogle, Putnam, and Whiteside) reflected the population dynamics of the 10-field clusters in Stephenson and Woodford counties. Please note that funding for this survey, as well as for several other research projects, was provided by the Illinois Soybean Association. We sincerely appreciate their continuing support of our efforts.

The soybean aphid has earned its place as the key insect pest of soybeans in the Midwest, with lots of money being spent to control it and with costs associated with yield losses. Much has changed with management recommendations in the nine years soybean aphids have been known to occur in North America, and it's likely that new information will continue to flow from the multiple research projects directed toward this insect. We will have much more to divulge about management of soybean aphids after all of our plots are harvested and we have shared information among states.--Kevin Steffey

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