Issue No. 15, Article 2/July 3, 2008
Corn Earworms Found Feeding in Corn Whorls
A few days after we published "Captures of Corn Earworm Adults Are Worth Noting" in issue 13 (June 20, 2008) of the Bulletin, we received a report of corn earworm larvae feeding in corn whorls. Mike Roegge, University of Illinois Extension educator in Quincy, observed corn earworm larvae in a seed cornfield ("not even close to silking") in Adams County on June 24. The larvae ranged in size from 1/4 to 1 inch long and were feeding on leaves still rolled up in whorls. A couple of days later, Dan Schaefer with Illini FS in Tolono reported corn earworms feeding in "hot spots" in a cornfield (V8 stage) in Champaign County.
Injury to corn leaves caused by early instar corn earworm larvae, Adams County, June 24, 2008 (photo courtesy of Mike Roegge, University of Illinois Extension).
Injury to corn leaves caused by corn earworm larvae, Adams County, June 24, 2008 (photo courtesy of Mike Roegge, University of Illinois Extension).
Injury to corn leaves caused by corn earworm larvae, Champaign County, June 26, 2008 (photo courtesy of Dan Schaeffer, Illini FS, Tolono, Illinois).
The corn earworm (also known as the cotton bollworm, soybean podworm, and tomato fruitworm) is well known as a pest of seed corn and sweet corn. However, as I indicated in the article in issue 13, we largely ignore corn earworms in field corn. But when the adults arrived in Illinois this year, they found corn in vegetative stages of growth. Given the broad range of acceptable hosts for the larvae, vegetative-stage corn was perfectly acceptable as an oviposition site for the females.
At this time of year, signs of insect feeding in corn whorls usually suggest the presence of European corn borers, southwestern corn borers (in southern Illinois), or armyworms. So it's important to link injury to the leaves with the appropriate culprit before making a control decision. Following are descriptions of a few species of caterpillars that could be found in corn whorls at this time of year (from the Handbook of Corn Insects, published by the Entomological Society of America; www.entsoc.org/Pubs/Books/Handbooks/Corn.htm or www.shopapspress.org/haofcoin.html).
Armyworm. "The full grown larva is 1 1/2-1 3/5 in. [3.7-4.1 cm] long and green-grown 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." There are dark bands on the abdominal prolegs.
Armyworm larvae parasitized by tachinid fly; note the tachinid eggs near the head and the dark bands on the abdominal prolegs (University of Illinois).
Corn earworm. "Newly hatched larvae are translucent cream to white with a black head. Larger larvae vary from yellow, brown, and red, to green with prominent bands of pink cream, pink, green, or yellow. The head usually is dark yellow or orange. The cuticle is covered with microspines." Larger, erect hairs are surrounded by dark tubercles ("bumps" on the cuticle).
Corn earworm larva, Adams County, June 24, 2008 (photo courtesy of Mike Roegge, University of Illinois Extension).
European corn borer. "Newly hatched larvae are 1/32-1/16 in. [1-2 mm] long with a dark brown head and a somewhat translucent white body. Mature larvae are 3/4-1 in. [19-25 mm] long with a medium to dark brown head and a creamy white to gray body. Raised, somewhat darkened tubercles are evident on the body."
Early instar European corn borer larvae (University of Illinois).
Southwestern corn borer. "Larvae are white with a pattern of large, raised black tubercles on each body segment and are 1-1 1/4 in. [25-30 mm] long when full grown. The head of first through third instars is black, whereas the head of older larvae is brown."
Southwestern corn borer larva (photo courtesy of Tom Riley, Louisiana State University).
Because we infrequently experience corn earworm injury to corn leaves rolled up in whorls in the Midwest, an economic threshold has not been established. The action threshold for fall armyworms, which often feed on corn leaves rolled up in whorls, is 75% of the plants with whorl injury and worms still present. If control with an insecticide seems justified (and in most instances, it isn't), the insecticides suggested for control of corn earworm larvae are listed in Chapter 1, Table 1 (page 6) of the 2008 Illinois Agricultural Pest Management Handbook (www.ipm.uiuc.edu/pubs/iapmh).--Kevin Steffey