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Issue No. 7, Article 2/May 11, 2007

Soybean Aphid Spring Densities Not as Great as Anticipated: Cold Snap in Early April Likely Responsible

Last fall (issue 23, October 6, 2006) we reported on the impressive densities of soybean aphids that were present in several areas of Illinois. The potential was set for economic infestations in 2007, particularly in northern Illinois. Last week (May 2), David Voegtlin, an entomologist in the Center for Ecological Entomology with the Illinois Natural History Survey, and Bob O'Neil, a professor in the Department of Entomology at Purdue University, surveyed for aphids on buckthorn in northern Illinois (Joliet, Aurora, and Quad Cities).

They found lower numbers of winged aphids than expected based on the large quantity of overwintering eggs they had previously discovered in these areas. Recall that these entomologists had found record levels of eggs on buckthorn in the Quad Cities area. Although they found aphids at all sampling sites this spring, every buckthorn plant was not infested. David and Bob believe that the cold weather in early April may have directly killed aphids that hatched from eggs in late March or that the buckthorn foliage was killed by the hard freeze, leaving the aphids without any nutritional source. Even though overall aphid densities are lower than expected, aphid colonies were abundant, particularly in the Quad Cities area. David and Bob also reported that some winged aphids were present. This was most evident with the larger colonies.

Because of the delays in field work this season, soybean planting and emergence are still most likely several weeks away. This may reduce survivorship of soybean aphids this spring as well. Time will tell. Both entomologists agree that soybean aphid densities are still likely to be high in some areas of the state this yearbut they may have been much higher had we not experienced those frigid temperatures in early April.--Mike Gray

Mike Gray

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