Last week we reported  that small populations of soybean aphids were being found in Minnesota and Wisconsin. In Iowa, after our students and summer helpers checked thousands of soybean plants last week, the first soybean aphids (and it was just three) were found near Ames in Story County. Brian Lang, extension field crops specialist, also reports finding small populations of aphids in northeastern Iowa near Decorah. No soybean aphids were found in research plots in Floyd and Lucas counties. So the aphid population in Iowa this spring appears to be progressing "normally" and as expected so far--populations are very small and widely scattered. However, the same cannot be said for soybean fields to the east of us.
On June 8, Tracey Baute, field crop entomologist with Ontario Ministry of Agriculture, Food and Rural Affairs, reported very large populations of soybean aphids in southwestern Ontario near the town of Harrow. For geographical reference, Harrow, Ontario, is almost directly east of Ames, Iowa, and at approximately the same latitude. What Tracey found was quite stunning. She notes that on V2-stage soybeans there was an average of 111 aphids per plant (20 plants assessed) with 17 of those 20 having aphids. The highest aphid count on a single plant was 316! She also scouted a nearby grower's field in the V1-V2 stage and aphids were on every plant with more than 50 aphids per plant.
So what does this mean for Iowa soybean growers? It does confirm that, although rare, early-season populations of soybean aphids can be quite large. A situation somewhat similar to this year's Ontario problem was noted three years ago in a June 14, 2004, ICM article where Doug Tinnes, NK Seeds sales agronomist, scouted a field in southeastern Iowa and noted that in a 140-acre field, the plants consistently had 100 aphids per plant, and one large area of the field had 200-250 aphids per plant.
We typically don't start scouting for soybean aphids until late June or early July and that seems like a realistic approach at this time based upon small aphid populations across Iowa. We'll keep you informed if we hear otherwise.
Marlin E. Rice is a professor of entomology with extension and research responsibilities in field and forage crops. Matt O'Neal is an assistant professor of entomology with research and extension responsibilities in field crops.
This article originally appeared on page 175 of the IC-498(14) -- June 11, 2007 issue.