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Issue No. 23, Article 2/October 9, 2009

Soybean Aphids Reach Impressive Levels on Buckthorn in Late September

In late September, reports of "swarms" of aphids became common across much of central and southern Illinois. On our campus, it was common to see students walking to class swatting at clouds of "gnats," as they referred to them. Treatment decisions for producers were made difficult, as many soybean fields were in late reproductive stages of development. As temperatures decline and day length shortens, winged soybean aphid females (gynoparae) abandon maturing soybean fields and fly to buckthorn. Upon their successful search for buckthorn plants, the females feed and begin producing nymphs that develop into oviparae. Late in the growing season, winged soybean aphid males also are produced on soybean plants. The males leave soybean fields and attempt to find buckthorn plants and begin mating with the oviparae. The oviparae lay eggs that overwinter on buckthorn.

In North America there are two overwintering hosts, the common buckthorn (Rhamnus cathartica L.) and the alderleaf buckthorn (Rhamnus alnifolia L'Hér). In contrast to the native alderleaf buckthorn, the common buckthorn is invasive and originated in Europe. This annual fall dispersal of soybean aphids (sexual morphs) to their primary host has been described as a "biological bottleneck" by Ragsdale and others (Annals of the Entomological Society of America 97: 204-208). This so-called bottleneck could be readily observed across many areas of central and southern Illinois in late September. Aphid densities on buckthorn leaves were as high as many observers had ever witnessed.

Soybean aphids on buckthorn leaf, September 18, 2009 (photo courtesy of David Voegtlin).

Soybean aphids on buckthorn leaf, Champaign County, September 24, 2009 (photo courtesy of David Voegtlin).

As we have learned, accurately predicting soybean aphid infestations for the upcoming growing season has proven to be challenging. Certainly the stage has been set for abundant egg-laying on buckthorn plants this fall. Next spring, producers would be well advised to scout their soybean fields for aphids. If overwintering survival is good, natural enemy densities are low, and the growing season is relatively mild, we could see significant management issues develop with this pest in 2010.--Mike Gray

Mike Gray

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