In last week's newsletter, we reported finding low densities of soybean aphids during late May in Story County. Since then, aphids also have been found in northeastern Iowa, near Decorah, by Brian Lang, extension field crops specialist. It is too early to tell if these early reports will develop into populations that will increase to the economic threshold (80 percent of plants infested and the average number per plant at 250 and rising).
Soybean aphids are small, pear shaped, and range in color from green to yellow. (Marlin E. Rice)
If you are beginning to scout for soybean aphids, be aware of other small, soft-bodied insects that may resemble the soybean aphid. Recently, while visiting a soybean field at the Neely-Kinyon Research and Demonstration Farm in Adair County, one such aphid mimic was found. At first glance, these leaves appeared to have 15 aphids per leaflet (see photo on the following page), and almost every plant within this field was infested. However, these insects were missing a few details from this aphid "wanna-be," i.e., cornicles, or "tailpipes," from the rear. The waxy, oval-shaped body and the very tight attachment to the leaflet suggested that it was another plant sucker: in this case, an immature whitefly. Various whitefly species are known to feed on soybean but never have they been known to cause economic damage in Iowa. Whiteflies in Iowa are most commonly seen on velvetleaf or buttonweed, and this field is likely harboring these whiteflies due to its current weedy state. As the season wears on, these insects will likely switch to a more-preferred host plant.
Whitefly crawlers on the underside of soybean leaflets. (Marlin E. Rice)
Whitefly crawlers may resemble soybean aphids, but they lack visible antennae, long legs, cornicles (tailpipes), and are powdery white in color. (Marlin E. Rice)
Other insects may resemble aphids, such as thrips (see photo). Like the whitefly, these are rarely a significant pest of soybean but overlap at the same time in the field with the soybean aphid. Refer to the pictures for distinctive features of a soybean aphid--cornicles (tailpipes), pear-shaped body, long legs and antennae, and green to yellow coloration. Make certain that your pest identification is correct before making anyinsecticide application.
Soybean thrips have a dark head, three dark bands on the body, and are cigar shaped. (Marlin E. Rice)
This article originally appeared on pages 165-166 of the IC-496(15) -- June 12, 2006 issue.