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Issue No. 12, Article 4/June 12, 2009

Soybean Aphids Found in Michigan Soybean Fields

On June 2, Chris DiFonzo, extension entomologist at Michigan State University, found soybean aphids in an early-planted soybean research plot located near her campus. She estimated that approximately 5% of the plants were infested with aphids. Chris observed mature nonwinged mothers and nymphs, and she estimated that the flight into the field probably occurred 7 to 10 days earlier. Winged aphids also were present on plants, suggesting that a more recent flight had taken place.

At present it remains a bit uncertain how significant soybean aphid infestations might become across Illinois and the north-central region of the United States. Since the pest's discovery in North America in 2000, soybean aphid outbreaks have been most intense in odd-numbered years. For instance, in 2003, producers in Illinois encountered widespread economic infestations of this insect pest. More localized outbreaks took place in 2005, and in 2007 the northern third of the state had significant management challenges with soybean aphids.

This seemingly predictable every-other-year cycle ended in 2008 for Illinois and many other north-central states. Why? Some entomologists have speculated that low populations of natural enemies in soybean fields may have triggered increased densities of soybean aphids during 2008. Just how important are these populations? A paper by some Canadian scientists helps shed some light on this question. Experiments conducted in their laboratory (Department of Environmental Biology, University of Guelph, Guelph, Ontario) revealed the significance of two important predators: the sevenspotted lady beetle and the multicolored Asian lady beetle.

Based on their investigations, the researchers calculated a "theoretical maximum daily predation rate" of adult soybean aphids by sevenspotted lady beetles to be 204, 277, and 166 aphids consumed by third instars, adult females, and adult males, respectively. Among multicolored Asian lady beetles, the third instars, adult females, and adult males consumed (daily predation rates) 244, 156, and 73 adult soybean aphids, respectively.

The findings attest to the importance of both of these predators in suppressing densities of soybean aphids. For additional details about this research, please consult the authors' scientific paper, published by the Entomological Society of America: Y. Xue, C.A. Bahlai, A. Frewin, M.K. Sears, A.W. Schaafsma, and R.H. Hallett. 2009. Predation of Coccinella septempunctata and Harmonia axyridis (Coleoptera: Coccinellidae) on Aphis glycines (Homoptera: Aphididae). Environmental Entomology 38(3): 708-714.

Conserving populations of beneficial arthropods (insects and spiders) should be a cornerstone in the sound management of many insect pests of field crops. Eliminating prophylactic insecticide treatments within crops as well as enhancing (reducing or eliminating insecticides and short mowing practices) populations of natural enemies in noncrop areas would go a long way in conserving beneficial insect populations. In early June, it was easy using a sweep net to find many beneficial insects inhabiting grassy areas surrounding corn and soybean fields in central Illinois. These noncrop areas serve as reservoirs for many species of predators and parasitoids. As field crops mature during the growing season, many species of natural enemies will begin to move into these fields in search of prey. Let's hope the natural enemy populations help keep soybean aphid densities in check this growing season.

Sweepnet contents from roadside ditch in Champaign, County on June 5, 2009 (note the abundance of natural enemies including the green lacewing and the twelvespotted lady beetle).

--Mike Gray

Mike Gray

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