No. 15 Article 5/July 2, 2004

Diligent Scouting for Soybean Aphids Keeps Us Apprised of Developments Throughout the Midwest

As one consequence of last year's outbreak of soybean aphids, many pairs of eyes have been searching for any sign of this invasive pest in soybean fields throughout the Midwest. Fortunately, thus far, although soybean aphids have been "discovered" here and there, densities have been very low in all of the typically affected states. Unfortunately, also as a consequence of last year's outbreak, some people already have applied insecticides to control very low densities of soybean aphids in Illinois. We cannot overemphasize the need to remain patient as soybean aphid populations develop--or do not. We understand that many people believe they waited too late in 2003 to treat for soybean aphids; however, overcompensating for mistakes made last year is not a suitable insect management strategy. Applying insecticides too early will kill predators (e.g., multicolored Asian lady beetle) and parasitoids, and soybean aphid populations can resurge in the absence of natural enemies, possibly necessitating a second insecticide application. Also, multiple applications of insecticides will place a great deal of selection pressure on the soybean aphid population, increasing the potential for the development of resistance to insecticides.

By this time last year, densities of soybean aphids in northern Illinois were increasing rapidly; by early to mid-July, their numbers approached or exceeded economic levels. One recent report from northern Illinois deserves attention. Craig Kilby, a technical service agronomist with Golden Harvest, found high numbers of soybean aphids on individual plants in a field west of Wilmington on the border of Grundy and Will counties on June 28. Most of the aphids were on the stems and petioles rather than on the leaves. Craig reported that many of the plants harbored more than 100 aphids of various sizes.

Given the incredible reproductive capacity of soybean aphids, the numbers of aphids per infested plant could double by week's end, and more plants within the field will become infested. If this type of isolated infestation is occurring elsewhere, economic infestations might develop. As is always our advice and request, remain vigilant, and please let us know what you find.


Soybean aphids on a soybean plant from a field near Wilmington, Illinois, June 28, 2004. (Photo provided by Craig Kilby, Golden Harvest.)

Counter to Craig Kilby's report, Dr. David Onstad, with the Department of Natural Resources and Environmental Sciences at the University of Illinois, has found very few soybean aphids thus far. David and his crew sampled 650 plants in 13 fields in Champaign, Kendall, Tazewell, and Woodford counties, and they had found only one soybean aphid as of June 24. The stages of soybean growth in Champaign, Tazewell, and Woodford counties ranged from V5 to V6, and in Kendall County from V1 to V3. I should point out that Craig Kilby found no other fields in which densities of soybean aphids were high, including fields he sampled in Tazewell County.

The "working" economic threshold (developed at the University of Minnesota) agreed on by entomologists throughout the Midwest is 250 or more aphids per plant through the R4 stage of soybean growth. This threshold incorporates a 7-day lead time during which insecticide application can be scheduled. At the end of the 7-day lead time, soybean aphid densities might reach 1,000 per plant, the density of aphids still believed to cause economic injury equal to the cost of control. Benefits of applying an insecticide to control soybean aphids when soybeans are in the R5 or R6 stages of growth still are being debated. From information we learned in 2003, the yield benefit from controlling aphids during the R1 and R2 stages of soybean growth averaged 10 bushels per acre. The yield benefit from controlling aphids during the R3 and R4 stages of growth averaged 5 bushels per acre.

Insecticides suggested for control of soybean aphids in Illinois are presented in Table 2. Please abide by all label directions and precautions. Once more, don't let emotion control your decision making. Determine the density of soybean aphids within a field by scouting thoroughly, and if the density is below the threshold, keep scouting to determine whether the numbers are increasing or decreasing. Only increasing numbers are threatening.--Kevin Steffey

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