Corn Earworm (a.k.a. bollworm) will feed on leaves, flowers and pods, but it is pod feeding that is the real concern. They can be a serious threat to soybeans beginning at R1 – R5, but they usually get started in earnest during full bloom (R2). The treatment threshold is 36 larvae per 100 sweeps. Fall armyworms will also feed on foliage and pods. Each year I walk into a few fields that have been completely defoliated by fall armyworm (pictured right). Most times these infestations start on weedy grasses and then move to soybean. This “grass strain” of fall armyworms will feed mostly on foliage. Treatment for fall armyworm is recommended anytime they cause or are likely to cause greater than 25-30% defoliation. If pod feeding is being observed, use the threshold above for CEW.
My experience has been that infestations of both fall armyworm and corn earworm are most likely to occur in later planted fields beginning in late July or Early August. It is infestations that go undetected that cause serious losses. The sweep net works well in catching both kinds of larvae. You just have to use it. Mid or high rates of synthetic pyrethroids typically provide good control of both pests, but there are other options listed in UT’s insect control recommendations for soybean. One benefit of using these other insecticide is they are less likely to flare infestations of soybean looper. However, products like Belt, Steward and Tracer also cost more.
Identification: Both cornearworm and fall armyworm have four pair of prologs (besides the pair on the last segment). Fall armyworm will usually have a darker head capsule that has a relatively prominent
inverted, light-colored “Y”. Corn earworm larvae will have more obvious hairs on the body, and in soybean, larger larvae typically have a light green color. Another hint – corn earworms will ball up in the bottom of the net more so than fall armyworm (right) which will try to crawl out more quickly. I’ve included some pictures below to help avoid confusion, and there also are a lot of good web resources to help if you are unsure.