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Issue No. 17, Article 2/July 20, 2007

Numbers of Soybean Aphids Are Increasing

While soybean aphid activity has slowed down in Michigan (read Chris DiFonzo's article in Michigan State University's Field Crop Advisory Team Alert Newsletter, soybean aphid numbers are slowly increasing in Illinois soybean fields. Although our weekly surveys of 26 fields in north-central and northwestern Illinois were interrupted by significant thunderstorms early in the week, we were able to obtain average densities of soybean aphids in seven fields on July 16. In all seven, the average density of aphids had increased from July 9 to July 16, although the average densities are still well below levels of economic concern:

  • Stephenson County 1--20.15 aphids per plant (3.85 on July 9)
  • Stephenson County 4--18.6 aphids per plant (1.85 on July 9)
  • Stephenson County 5--13.85 aphids per plant (3.35 on July 9)
  • Bureau County--0.05 aphid per plant (0 on July 9)
  • Lee County--14.8 aphids per plant (3.2 on July 9)
  • Ogle County--1.65 aphids per plant (0.2 on July 9)
  • Whiteside County--4.05 aphids per plant (0 on July 9)

Most notable were the relatively larger numbers of aphids on individual plants sampled: e.g., 79 on one plant in Stephenson County 1; 48 in Stephenson County 4; 56 in Stephenson County 5; 42 in Lee County. Japanese beetles also were evident in these fields, although numbers were low.

This slow, steady increase of soybean aphids in soybean fields in northern Illinois bears watching. Only repeated samples will let us determine what, if any, effect the heavy rains had on populations. In the meantime, we need to keep our eyes open for potential rapid increases in soybean aphid densities. As we have indicated before, soybean aphid densities can double within 2 to 4 days, depending on temperatures. If natural enemies are not abundant and temperatures remain relatively cooler, we could still encounter economically threatening levels of soybean aphids in 2007.

You can find a broader overview of the soybean aphid situation at the USDA's Pest Information Platform for Extension and education (PIPE) Web site. The site, which you may have visited already, was developed initially to provide an overview and outlook for Asian soybean rust. The site was expanded in 2006 to include information about soybean aphids, too. When you first visit the site, information about soybean rust is the default setting. In the upper right corner of the page, however, you can select "Soybean Aphid" to obtain up-to-date information about this pest throughout the Midwest, the Northeast, and some Canadian provinces. Repeated sampling for soybean aphids in sentinel plots established for monitoring for the presence of soybean rust provides some of the data for the current soybean aphid situation, but data from other sampling efforts also are provided. For example, data from our weekly surveys of soybean aphids from Woodford County to Stephenson County are represented on the Illinois map. Current commentary about the situation and outlook for soybean aphids and about management guidelines also are included.

It is important to note that the densities of soybean aphids indicated on the maps are field-specific densities and are not necessarily representative of soybean aphid densities within a county or a region. Decisions about managing soybean aphids also must be field-specific, i.e., an economically threatening infestation of soybean aphids may be observed in one field at the same time that very low numbers of soybean aphids are observed in an adjacent field. The soybean aphid data displayed on the PIPE Web site provide an overview of the situation, but decisions to manage soybean aphids must be made for individual fields.

UPDATE FOR JULY 19: Our weekly survey resumed on July 18 after a 1-day interruption because of the thunderstorms. The numbers of soybean aphids counted in some of the seven Stephenson County fields sampled on July 18 were even larger than the numbers counted in three Stephenson County fields July 16:

  • Stephenson 2--5.45 aphids per plant
  • Stephenson 3--41.3 aphids per plant
  • Stephenson 6--48.3 aphids per plant
  • Stephenson 7--5.25 aphids per plant
  • Stephenson 8--8 aphids per plant
  • Stephenson 9--22.4 aphids per plant
  • Stephenson 10--19.15 aphids per plant

The surveyor found as many as 232 aphids on one plant in Stephenson 3 and more than 100 aphids on three of the 20 plants sampled in Stephenson 6. Remember, the economic threshold is 250 aphids per plant (at least 80% of the plants infested) in R1 through R5 soybeans.--Kevin Steffey

Kevin Steffey

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