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Issue No. 21, Article 3/August 15, 2008

A Full-Blown Soybean Aphid Outbreak in the Northwestern Midwest

As I reported in last week's issue of the Bulletin (no. 20, August 8, 2008), soybean aphid populations have reached economically damaging levels over a large area of the northwestern Midwest--North Dakota, South Dakota, northeastern Nebraska, Minnesota, northern Iowa, and western Wisconsin. In some very large areas, 90% to 100% of the fields have been sprayed. In Nebraska, this year's outbreak of soybean aphids is the worst they have experienced, and the range of this pest has expanded farther west and south in that state this year. During a weekly teleconference on August 11, some entomologists indicated that insecticides have been applied to soybean fields at least twice already this year. Delays of 10 days to 2 weeks between notification and insecticide application have occurred.

We are keeping our eyes on soybean aphids in Wisconsin and northern Illinois right now because there is evidence that the numbers continue to increase. Eileen Cullen, extension entomologist at the University of Wisconsin, reported that numbers in southern Wisconsin have been increasing during the past two weeks, and we can say the same for Illinois. University of Illinois Extension educators who are monitoring sentinel soybean plots for evidence of Asian soybean rust also are counting soybean aphids on 20 plants at each visit. The numbers of aphids, which have been very low throughout the season, are starting to increase in several northern Illinois sentinel plots. Dave Feltes, University of Illinois Extension IPM educator in Morrison, on August 12 found a marked increase in soybean aphids in a field of R3-stage soybeans in northern Stephenson County. The average density of soybean aphids in the field was less than threshold, but scattered numbers of aphids exceeded the economic threshold on scattered plants. Many of the aphids were winged, and Dave found very few predators in the field, a finding that is in line with what extension entomologists in the north-central states are saying.

We also have noted a continued increase in numbers of soybean aphids from our weekly samples in 26 commercial soybean fields from Woodford County to Stephenson County. You can view the data from these fields on our Bulletin web site. The average densities of soybean aphids in the 10 fields in Stephenson County were between 2.45 and 59.45 aphids per plant on August 5, still well below the economic threshold of 250 aphids per plant. However, the average densities of soybean aphids in most of those fields had increased from the numbers recorded on July 29. The average densities of soybean aphids in Bureau, Lee, Marshall, Ogle, Putnam, Woodford, and Whiteside counties are considerably smaller, but numbers increased in almost all fields from July 31 to August 6-7. Because recent temperatures (and other weather conditions, for that matter) have been ideal for development of soybean aphids, the numbers of aphids likely will continue to increase, particularly in the relative absence of natural enemies.

It is important to note that Eileen Cullen reported that she has received some inquiries about the "white morph" soybean aphids, which, as the name indicates, are much lighter in color than the typical Mountain Dew-colored aphids. The white morphs are also noticeably smaller. They begin to appear as a consequence of some type of stress (e.g., high temperatures or other unfavorable environmental conditions, poor nutritional quality of soybeans), and they do not develop as well as "normal" soybean aphids, with as much as 70% less production of nymphs. However, a revised economic threshold for white morphs has not been developed, so use your best judgment when assessing the potential need for an insecticide application. If the population is composed primarily of white morphs, a higher economic threshold likely is justified.

It's also important to note the presence or absence of insect predators and the occurrence of alatoid nymphs, which have broader "shoulders" than non-alatoid nymphs, indicating that they will molt into winged adults. If a large percentage of a population is composed of alatoid nymphs, it is entirely possible that winged aphids will leave the field shortly, so an insecticide application may be unnecessary. Otherwise, it's business as usual--scout diligently and repeatedly, and hold off with an insecticide application until the average density reaches or exceeds 250 aphids per plant. If an insecticide is justified, several products provide very good control of soybean aphids (Table 2, page 13, of the 2008 Illinois Agricultural Pest Management Handbook). Please follow all label directions and precautions. And please note that spraying blossoming soybeans can be extremely hazardous to bees. Before applying sprays, coordinate with local beekeepers; their names and colony locations may be obtained from your local University of Illinois Extension office.--Kevin Steffey

Author:
Kevin Steffey

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