Issue No. 13, Article 6/June 17, 2005
Caterpillars in Soybeans
A couple of recent reports reminded me that while our surveillance for soybean aphids and twospotted spider mites is ongoing, a few other creatures that enjoy soybeans also may make their presence known. Both Mike Roegge, Extension unit leader in Quincy, and Kevin Black, with Growmark, reported the appearance of thistle caterpillars (the larval stage of the painted lady butterfly) in a few soybean fields in Illinois. These larvae are striking in appearance and are about 1-1/4 to 1-1/2 inches long when fully grown. Their bodies are brown to black with a yellow stripe along each side of the body. They have a thornlike appearance from the spiny hairs that cover the body.
Thistle caterpillar (photo courtesy of Marlin Rice, Iowa State University).
When the thistle caterpillar finishes feeding and completes development, it will pupate, then ultimately emerge as the painted lady butterfly. Though the butterfly is predominantly brown, the wings of the painted lady are red and orange with black and white spots. This strikingly pretty butterfly usually is observed easily in the countryside.
Painted lady butterfly (photo courtesy of www.readingmuseum.org.uk).
The painted lady does not overwinter in the Midwest; rather, it migrates into our area each spring from tropical or subtropical locations. Females lay eggs singly on hosts, and the eggs hatch in about 7 days. The larvae (thistle caterpillars) feed for 2 to 6 weeks on more than 100 species of plants, including Canada thistle, soybean, sunflower, garden vegetables, and ornamentals. When large numbers occur, they can become economic pests of soybeans. They feed on the leaves, defoliating plants and webbing the leaves together. The published threshold is three or more larvae per plant for soybean stages V3 and V4. The threshold should be higher for more mature plants, or you can rely on percentage defoliation (25% to 40% defoliation, depending on the stage of soybean development) as a treatment guide.
Eric Adee, superintendent of the Northwestern Illinois Agricultural Research and Demonstration Center near Monmouth, sent me some information and photos of yellowstriped armyworms feeding on soybeans in his area. A mature yellowstriped armyworm larva is 1 to 1-2/5 inches long and glossy black to gray, with dark triangular spots along the midline of the back. A yellow stripe is evident along each side of the triangular spots, and there is a black spot above the hind legs.
Dorsal view of a yellowstriped armyworm larva, Spodoptera ornithogalli (photo courtesy of John Capinera, University of Florida).
Like the fall armyworm, the yellowstriped armyworm is a tropical and subtropical species that migrates northward as weather conditions permit. When they arrive, the females deposit eggs in irregular masses on plant foliage. The larvae feed on the leaves of numerous hosts, including corn and soybeans.
Low to moderate infestations of both thistle caterpillars and yellowstriped armyworms are no cause for concern. As you know, soybeans can tolerate significant defoliation before yield is compromised. The rule of thumb for most soybean defoliators is that control is not warranted until defoliation exceeds 30%. Of course, such thresholds can and should be adjusted if soybean plants are suffering from other stresses (e.g., lack of moisture, infestations of other insects). So, if you encounter either of these occasional pests in soybeans, use your best judgment.--Kevin Steffey