Soybean aphid populations continue to build, although somewhat slowly, across the state. I scouted three blooming soybean fields in Ames on July 8 and 80 to 100 percent of the plants had aphids. Populations averaged 15 aphids per plant with the high population being 125 per plant.
|A population of 125 soybean aphids on the underside of two soybean leaflets.|
Brian Lang, Iowa State University Extension field specialist-crops, notes that in northeastern Iowa near Decorah, aphid numbers are still behind last season's and still below what we anticipate as a threshold (an average of >100 aphids per plant). Populations range from 0 to 470 aphids per plant on July 3, but the average at three locations was 1-17 per plant. Brian commented that they still could be a major problem at treatable levels by the week of July 21.
In southern Iowa, Mark Carlton, Iowa State University Extension field specialist-crops, found a colony of 36 soybean aphids in northwestern Wayne County. This is the southernmost report for Iowa this year. In northwestern Iowa, Hal Tucker (Tucker Consulting, Storm Lake) reported on July 1 that he was finding 300 aphids per plant in a field near Newell in Buena Vista County.
I have not had any reports of fields being treated for this pest, but it is critical that fields with aphids be watched closely during the next week or two to determine whether populations reach treatable levels. Information on economic thresholds and treatment options were presented in previous ICM newsletters.
This article originally appeared on page 143 of the IC-488(17) -- July 15, 2002 issue.