No. 16 Article 3/July 8, 2005

Keep Scouting for Soybean Aphids

As we have reported in the past couple of issues of the Bulletin, the high temperatures seemed to have slowed development of soybean aphids in Illinois. For a couple of weeks, densities in fields we have surveyed stalled at fairly low numbers. However, the recent return to more pleasant temperatures could kick-start more rapid development of soybean aphids, so make certain you revisit fields in which you have found aphids already this season.

Reports from other states, with noted exceptions, also reveal that soybean aphids are not quite as threatening as they were in 2003 at this same time of year. However, densities in some Michigan soybean fields have exceeded the threshold of 250 aphids per plant (with 80% or more of the plants infested), and insecticide applications are under way. Fields here and there in Iowa and Ohio also are supporting relatively large numbers of soybean aphids.

Our University of Illinois surveyors inspected one of our research trials in northwestern Illinois (Whiteside County) on July 5 and determined that about 50% of the plants were infested. They found 712 soybean aphids on one plant, and three other plants had more than 100 aphids each. From these plants, the infestations can spread and soybean aphid densities can increase relatively quickly if temperatures are optimum for development of the aphids (75-85°F).

Now is as good a time as any to reiterate the broadly accepted threshold of 250 or more soybean aphids per plant. This threshold is conservative on purpose, with a built-in lead time of 7 days during which an infested field should be treated. So there is no need to lower the threshold to accommodate the time required to line up an applicator. Also, the threshold is based on field density of soybean aphids, not on large numbers of aphids on a few selected plants. As already indicated, these heavily infested plants will serve as a source for population buildup throughout the field, but economic damage in a field of soybeans does not occur until the infestation is more established throughout the field.

Insecticides suggested for control of soybean aphids in Illinois are indicated in Table 1. If we learn anything about insecticide efficacy in the "heat of the battle" (if the battle commences), we will share the experiences in future articles of the Bulletin.--Kevin Steffey

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