Here's one for the record books: cotton aphids (also known as melon aphids) in soybean fields in Illinois. I first became aware of this occurrence when I read a recent issue of the University of Wisconsin's Wisconsin Crop Manager. John Wedberg, Extension entomologist at the University of Wisconsin (and former Extension entomologist at the University of Illinois a looooong time ago; sorry, John), had first observed aphids in soybeans in the southern tier of counties in Wisconsin in late July. Several plants, in a plot in which John and Craig Grau (plant pathologist) are investigating insect/disease/herbicide interactions, were "plastered" with aphids on leaves, stems, and petioles. The diagnostician at the University of Wisconsin has identified the aphids as cotton aphids, or melon aphids, Aphis gossypii. The folks at Wisconsin have sent samples of the aphids to David Voetglin, an aphid specialist in the Center for Biodiversity at the Illinois Natural History Survey, for his taxonomic opinion. We will either verify, or correct, the species in a future issue of the Bulletin, after David has had an opportunity to see the aphids.
Since reading John's article in Wisconsin's newsletter and receiving some information from him via e-mail, we also have received reports of these aphids in fields of soybeans in some counties in northern Illinois, including Kane and Grundy counties. Some FS field men have reported to Ria Barrido with Growmark, Inc., in Bloomington that they can find aphids in soybean fields in the northern tier of counties in Illinois.
What do cotton/melon aphids look like? Cotton aphids are very small (about 2 millimeters long) and may be winged or wingless. The aphid usually is pale to dark green but may be yellow to dark brown. Its cornicles ("tailpipes" on the rear of the abdomen) are black.
If cotton/melon aphids occur in soybeans, what are the noticeable signs of their presence? According to John Wedberg, the first thing you notice in infested fields, in addition to the "plastered" plants, is the sticky honeydew, a common aphid secretion, on your pants when you exit the field. Leaves on injured plants are cupped or crinkled; they turn yellow if the infestation is severe. John also reported that in their plots sprayed with Warrior T, the healthy plants are taller and the canopy is closing. Injured plants are shorter and less full.
Do cotton/melon aphids cause economic injury? Your guess may be as good as ours. Obviously there is no threshold for this insect in soybeans. John Wedberg has suggested spraying a few fields when the fields had signs of severe stress. However, he indicated that most infested fields look fine. We usually expect natural enemies such as lady beetles to keep aphids in check, but John reports that natural enemies have not done their job in Wisconsin; the same probably is true in northern Illinois.
Are certain types of soybean fields more likely to be infested with cotton/melon aphids? John Wedberg indicated that infestations of these aphids are heaviest in late-planted and double-cropped soybeans; however, earlier-planted soybeans also are infested in some counties. If you want to look for aphids in soybeans, focus on later-planted fields first.
Are any insecticides labeled for control of aphids in soybeans? The insecticides registered for use on soybeans that also have "aphids" listed on their labels for use in cotton or melons include *Asana XL, dimethoate, Lorsban 4E, *Penncap-M, *Pounce 32.EC, and *Warrior T. (*Use restricted to certified applicators.) However, we can find no information about control of cotton/melon aphids in soybeans. So, you're on your own if you believe an insecticide application is warranted. Check appropriate labels for footnotes regarding control of aphids in cotton or melons (Does the label indicate suppression?), and don't exceed the highest labeled rate of the insecticide for use in soybeans. Follow all label directions and precautions.
We'll provide updates as appropriate. Let us know if you are finding aphids in soybeans and the conditions of fields in which you find them.--Kevin Steffey and Mike Gray