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Caterpillars in Corn Whorls

June 21, 2002
During June, many people focus on looking for whorl-feeding injury while scouting cornfields for first-generation European corn borer (refer to preceding article). As you scout, keep in mind that several species of caterpillars can be found feeding in corn whorls. First-generation southwestern corn borers require attention in southern Illinois, and first-generation European corn borers can be found throughout the state. Other caterpillars--armyworm, corn earworm, fall armyworm, stalk borer, yellowstriped armyworm--also feed in corn whorls, so you should be able to distinguish among these species.

For the most part, European and southwestern corn borer larvae cause similar injury to corn whorls. The injury becomes evident as leaves unroll from the whorls--pinhead-sized or small, circular holes to "windows" on leaf surfaces to large, elongated holes. Fine, sawdustlike frass (caterpillar excrement) usually can be found in the whorls. The leaf injury caused by armyworm, corn earworm, fall armyworm, stalk borer, and yellow-striped armyworm larvae is more ragged than injury caused by corn borers, and the frass is messier.

Although the injury to the corn may be a clue to the culprit causing the injury, the best way to diagnose the problem accurately is to identify the insect. Following are descriptions of the insects in question. The best photos, as usual, come from Marlin Rice, extension entomologist at Iowa State University. His article "Insect Injury to Mid-Season Corn" in the June 28, 1999, issue of Integrated Crop Management has excellent photos of the different types of injury and some of the species of caterpillars. You can find it on the Web at http://www.ipm.iastate.edu/ipm/icm/ 1999/6-28-1999/midscorninj.html. If you want to try out a dichotomous key to caterpillars on the Web, check out a "Simple pictorial key for identifying some common late-instar caterpillars found on corn" at http://www.ent.iastate.edu/ pest/cornborer/key/. This key, prepared by George Godfrey, an entomologist formerly with the Illinois Natural History Survey, is a great way to learn to identify caterpillars, with step-by-step comparisons that usually lead to a species. The key also has very good illustrations and photographs (Marlin Rice's).

Armyworm. A full-grown larva is about 1 1/2 inches long and green-brown, with varying degrees of black mottling and white flecks. Two orange stripes along each side and two dark stripes on the back are characteristic. The head is yellow-brown, with a brown, netlike pattern of dark lines.

Corn earworm. A full-grown larva is about 1 5/8 inches long, varying from yellow, brown, and red to green, with prominent bands of cream, pink, green, or yellow. The head usually is dark yellow or orange. The cuticle ("skin") is covered with microspines.

European corn borer. Small larvae (first and second instars), the ones usually found in whorls, have dark brown heads and somewhat translucent white bodies. A full-grown larva is 3/4 to 1 inch long, with a medium to dark brown head and a creamy white to gray body. Raised, sometimes slightly darkened, tubercles are evident on the body.

Fall armyworm. A full-grown larva is about 1 1/4 inches long and varies from light tan or green to black. The caterpillar is smooth skinned, with three yellow-white lines along the back and a wider dark stripe on each side of the yellow-white lines. Below the dark stripe on each side is a wide, wavy yellow stripe with red splotches. The head is dark brown with a white inverted Y on the front (although this may be lacking in some specimens). Fall armyworm larvae can be confused with corn earworm and armyworm larvae. However, a corn earworm larva has a yellow-brown head and rough skin, covered with microspines; armyworm larvae do not have the white inverted Y on the head. If you want to compare the heads of armyworms, corn earworms, and fall armyworms, go to the article "Corn Earworms in Whorl-Stage Corn" in the July 12, 1999, issue of Integrated Crop Management at http://www.ipm.iastate.edu/ipm/icm/ 1999/7-12-1999/cewinwhorl.html.

Kevin Black, an entomologist with Growmark, may have found fall armyworm larvae in whorl-stage corn in Macoupin County during the week of June 17. However, the identification has not been verified. Interestingly, no one has reported captures of fall armyworm moths in pheromone traps, including Ron Hines, senior research specialist at the University of Illinois Dixon Springs Agricultural Center. (Check out his weekly moth-capture report at http://www.ipm.uiuc.edu/ publications/hines-report/.) Nevertheless, because corn was planted late in many areas of Illinois this year, it's important to be watchful for fall armyworms. The damage often looks worse than it is, but an insecticide may be justified if 75% of the plants have whorl-feeding damage and larvae are still present.

Southwestern corn borer. A full-grown larva is 1 to 1 1/4 inches long, with a white body and brown to red-brown head (although young larvae have black heads). A pattern of large, raised, black tubercles on each body segment is evident.

Stalk borer. Larvae are 1/12 to 1 3/4 inches long, depending on instar, and purple to black, with five longitudinal white stripes (one on top, two on each side), broken by a purple band encircling the body just behind the head. Older larvae lose the distinct striping, but they usually are not found in corn whorls.

Happy caterpillar hunting. Please don't hesitate to keep us apprised of your findings in your neck of the woods.--Kevin Steffey

Author: Kevin Steffey


The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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