Soybean aphid populations have increased dramatically in fields across the state with growing populations noted in central, western, and northwest counties. Populations may be peaking in eastern and northeastern county fields. Numbers now being reported in some fields have exceeded 1,000 to 2,000 aphids per plant. With numbers like this on R2-R4 stage plants, economic losses are likely and treatments are merited in most cases.
Considerations for assessing your fields:
Beans in the full bloom (R2), beginning pod set (R3), and full pod (R4) are particularly vulnerable under certain conditions. Generally increased numbers of aphids at these phenological stages can justify treatments. Recent Wisconsin observations suggest that more than 200, 1,000, 1,500 at these stages may call for treatments. Further indications from their fieldwork now suggest that 250 aphids per plant at these stages under moisture stress conditions may be a logical economic threshold as we enter mid-August conditions.
Only about one-fifth of the state received appreciable moisture on the average during the weekend, and most of the state may not benefit from timely rains for another seven days or so. Under these conditions beans will be moisture-challenged and many aphid populations will go unhindered with continued rapid population expansions.
Be aware that soybean aphid populations may vary greatly across the field with hot pockets scattered throughout the field. At the Iowa State University Extension Education Farm some of the highest populations are near field edges. This means that good scouting throughout a field is a must for an accurate assessment of damage potential.
We may expect a population crash in August. This is not unusual, however the time is quite difficult to predict. Populations may persist longer in later-planted fields, especially if temperatures remain seasonable or cooler than normal. Frequent scouting is the only way to ensure that your field deserves a treatment based upon what's happening today, and not what was there several days ago. Watch for winged adults. These are the ones moving to other plants and indicate spreading or growing populations.
For additional information refer to the ICM newsletter (16), July 14 for recommended treatments. As we approach the end of the growing season, be sure to check insecticide labels for pre-harvest intervals if treatments are made for soybean aphid control.
This article originally appeared on pages 145-146 of the IC-490(20) -- August 18, 2003 issue.