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Issue No. 23, Article 2/October 6, 2006

Soybean Aphids Showing Up in Large Numbers on Buckthorn, Increasing the Potential for Problems in 2007

The days are becoming shorter, the temperatures are cooling, and winged soybean aphid females (gynoparae) are dispersing from soybean fields to buckthorn. After finding buckthorn, females feed and begin producing nymphs that mature into adults (oviparae) that mate with males, which also have dispersed from soybean fields. After mating, females deposit eggs on buckthorn, and the eggs overwinter. The most common overwintering host in Illinois is the common buckthorn (Rhamnus cathartica L.), a woody perennial with European origins considered invasive.

During the week of September 25, Dr. Dave Voegtlin, research entomologist with the Illinois Natural History Survey, reported that captures of winged soybean aphids in suction traps had increased noticeably in many areas of the Midwest. To view the numbers of aphids captured in a network of suction traps in 10 states, visit the Regional Soybean Aphid Suction Trap Network on the North Central IPM Center Web site at www.ncpmc.org/traps. Dave also observed soybean aphids flying around the University of Illinois campus as well as buckthorn leaves "covered" with soybean aphids. He says he has not seen an infestation like this on buckthorn since the fall of 2002. Dr. Bob O'Neil, Purdue University, and Dr. Ron Hammond, Ohio State University, also have reported finding large numbers of soybean aphids on buckthorn in Indiana and Ohio, respectively.

On September 28, 2006, Mike Gray observed very large numbers of soybean aphid females and nymphs on buckthorn growing on the southern edge of the University of Illinois campus. Although some predators, primarily multicolored Asian lady beetles, Harmonia axyridis (Pallas), were present on the buckthorn, their numbers were not nearly as high as the numbers of soybean aphids. Densities of the multicolored Asian lady beetle have been noticeably low this year in most areas of Illinois. However, it is possible that large numbers of soybean aphids on buckthorn could attract large numbers of natural enemies (e.g., multicolored Asian lady beetle, flower flies), which ultimately could have an impact on overwintering populations of soybean aphids.

Large numbers of winged (alate) soybean aphids on buckthorn leaves, Champaign County, Sep 28, 2006 (University of Illinois).

Winged (alate) and wingless (apterous) soybean aphids on buckthorn leaves, Champaign County, Sep 28, 2006 (University of Illinois).

Multicolored Asian lady beetle (right) and flower fly adult (right) on buckthorn, Champaign County, Sep 28, 2006 (University of Illinois).

A lot can happen between now and the 2007 growing season, but the numbers of soybean aphids being observed on buckthorn right now deserve our attention. If natural enemies do not exert much impact on soybean aphid populations this fall, numbers of overwintering soybean aphid eggs could be quite large. The potential for a rapid increase in numbers of soybean aphids next spring is feasible, given the right environmental conditions. And if temperatures next summer are not very hot (more than 90°F), an outbreak is possible.

We'll keep you posted about the progress of the fall migration of soybean aphids from soybean to buckthorn in Illinois and in other states.--Mike Gray and Kevin Steffey

Kevin Steffey
Mike Gray

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