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What's the Scoop on Soybean Aphids?

August 16, 2002
It's no secret to anyone that populations of soybean aphids have failed to build to very large numbers this year throughout most of the Midwest. Even our neighbor states to the north--Michigan, Minnesota, and Wisconsin--have had few threatening populations of soybean aphids. As you will recall, populations of soybean aphids in those three states were quite large in 2001. Interestingly, Marlin Rice, extension entomologist at Iowa State University, recently reported that it was not uncommon to find thousands of aphids per plant in small areas of fields in central Iowa. These populations are more the exception than the rule for most of the rest of the Midwest. Although no one has a definitive answer for the lower numbers of soybean aphids this year, a lot of finger pointing at the hot, dry weather is occurring. Indeed, weather probably has played a major role in suppressing populations of this invasive species in 2002. However, only after considerable discussion among the experts will we learn why soybean aphids were so "quiet" this year.

Despite the relatively low numbers of aphids in most of areas of Illinois, we have news to report. Marion Shier, Extension Educator in Crop Systems in Livingston County, sent us some photographs of soybean aphids distributed along the stems and on the leaves of soybean plants in fields in his area. Based on these photographs, the threshold of 25 or more aphids per leaflet was exceeded, at least in the field in which the photographs were taken. However, you'll note that many cast skins are evident, and it's not clear what percentage of the aphids are alatoid nymphs that will develop into winged adults and fly away. Additionally, the soybeans probably have matured to a stage that will not encourage much more growth of soybean aphid populations.

Soybean aphids on soybean stems in a field in Livingston County. (Photo courtesy of Marion Shier.)

Soybean aphids on soybean stems and leaves in a field in Livingston County. (Photo courtesy of Marion Shier.)

The soybean aphid research team, headed by David Onstad, Department of Natural Resources and Environmental Sciences, and David Voegtlin, Illinois Natural History Survey, continues to sample intensively for soybean aphids in some northern counties. Ron Estes, Research Specialist in Agriculture working for Drs. Onstad and Voegtlin, has led a crew of summer interns who have taken repeated measures of soybean aphid populations within townships of some counties. In each field they sample 25 plants in each of two transects (30 meters apart) from the edge of the field to 50 meters into the field. One plant is sampled every 2 meters.

The populations of aphids they found in Kendall Township in Kendall County (Table 3) show that numbers of soybean aphids have increased from 1.7- to 51-fold from July 23 to August 12. On the per-plant basis, the numbers are not very large. However, because of the increases in densities of soybean aphids in all 14 fields, it's important to keep watching soybean fields for at least the next couple of weeks. Although predators (e.g., multicolored Asian lady beetles) and development of alatoid nymphs may prevent populations from reaching threatening levels, we shouldn't turn our backs on soybean aphids yet.

On Friday, August 9, John Shaw, Research Scientist at the Illinois Natural History Survey, established an insecticide efficacy trial for soybean aphids in a field in Stephenson County. Although the average number of aphids per leaflet (approximately 10 aphids per leaflet) on August 9 was below threshold, we deemed the population density large enough to make it worth the time and energy invested in the trial. Overall, John found an average of more than 200 aphids per plant throughout the plot area. He applied 16 different insecticide treatments and included two untreated check plots within each of four replications.

On Sunday, August 11, John observed that the population density in the untreated check plots had increased to an average of about 14 aphids per leaflet; numbers of aphids in the insecticide-treated plots had decreased dramatically. John will sample aphid populations in the trial area again at 7 and 14 days after treatment. When the results become available, we will share them with you.

It's unlikely that soybean aphid populations will build to economic levels in most fields in most areas of Illinois this year. However, continue to monitor soybean fields for aphids throughout August, especially later-planted fields. A sudden surge in aphid numbers could change our outlook.--Kevin Steffey

Author: Kevin Steffey

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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