No. 20 Article 3/August 5, 2005

Soybean Aphids in Illinois: An Update

At this time, it seems that a soybean aphid outbreak akin to the outbreak that occurred in 2003 is unlikely. By this same time in 2003, the densities of soybean aphids in central Illinois soybean fields were increasing regularly and rapidly. And although some fields in central Illinois will have economic levels of soybean aphids this summer, a widespread problem is improbable.

The message that "each field should be scouted for soybean aphids" rings very true in 2005. It is not common for densities of any corn or soybean pest species in the Midwest to be uniform among fieldsexcept during an outbreak. So one field of soybeans may be heavily infested with soybean aphids this summer, but the adjacent field may have very few aphids. Dave Feltes, Extension IPM educator in the Quad Cities, sampled a few soybean fields in Carroll, JoDaviess, Stephenson, and Whiteside counties on August 1, and he found densities of soybean aphids ranging from well below threshold to near threshold to well above threshold. It seems apparent that widespread application of insecticides for control of soybean aphids in Illinois will not be necessary in 2005.

We shifted the geography of our weekly survey for soybean aphids from northern to central counties during the week of July 25. On July 28, our surveyor sampled 10 fields of soybeans for soybean aphids in DeWitt, Logan, McLean, Piatt, and Sangamon counties. The data gathered from this survey are presented in Table 1. None of the fields sampled had densities of soybean aphids that exceeded the economic threshold of 250 aphids per plant, with at least 80% of the plants infested. All three fields in McLean County had individual plants on which the numbers of aphids were in triple figures, but the average field densities were well below the economic threshold. The numbers of aphids found in DeWitt, Logan, Piatt, and Sangamon counties were quite low.

It's too early to stop scouting for soybean aphids yet, or for other insect or mite pests for that matter. Some projected cooler temperatures could accelerate soybean aphid development, so soybean fields in central Illinois are not out of the woods yet. And once again, as a reminder, please make decisions about whether to control soybean aphids on a field-by-field assessment. As another reminder, please note the preharvest interval for any insecticide selected for control of aphids (or other insect or mite pests in soybeans). A table of preharvest intervals was published in the Bulletin article "All Soybean Producers in Northern and Central Illinois Should Be Alert for Soybean Aphids" (issue no. 19, July 29, 2005). It's getting a little late in the season in some areas of Illinois for application of products with 45-day preharvest intervals.--Kevin Steffey

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