Western Corn Rootworms Are Emerging

July 2, 1999

· Male western corn rootworms have been observed in cornfields in Champaign, McLean, and Moultrie counties.

· Northern corn rootworms and female western corn rootworms should be observed soon.

· Watch for silk clipping as corn enters the critical pollination period.

· Wait to scout for western corn rootworms in soybeans.

Almost as if on cue, western corn rootworms began emerging in central Illinois cornfields during the week of June 28. I received three reports from people who had observed at least one adult corn rootworm during scouting forays. John Thieme with Zeneca Corporation and David Fathauer, a farmer in Macon and Moultrie counties, each found one male western corn rootworm on June 28 in cornfields in McLean and Moultrie counties, respectively. John observed the adult corn rootworm in corn planted after soybeans. John Shaw, Senior Research Specialist with the Illinois Natural History Survey, found male western corn rootworms near or in our research trials in Champaign County on June 29. Although some of you might believe that it is awfully early for emergence of corn rootworm adults, it really isn't. Although adult corn rootworms have emerged mostly in July during the past couple of years or so, we have records of emergence of corn rootworm adults as early as the third week in June.

Western corn rootworm female (left) and male (right).

Western corn rootworm males emerge first, followed by western corn rootworm females. After emergence and mating, about 14 days elapse before the females begin laying eggs. Adult western corn rootworms are approximately 1/4 inch long. The background color for both males and females is yellow, but the two sexes differ somewhat in their markings (Figure 1). On males, much of each wing cover is black; only the tips of the wing covers are yellow. Females usually are slightly larger and have three distinct black stripes on the wing covers, one on the outside of each wing cover and one in the middle. Gravid (pregnant) females have distended abdomens.

Northern corn rootworm adult.

Northern corn rootworm adults emerge soon after emergence of western corn rootworm beetles begins. Northern corn rootworms also are about 1/4 inch long, but they have no distinct markings. Newly emerged northern corn rootworms are cream or tan in color, but they become green as they age. The females are slightly larger, and pregnant females have distended abdomens.

Dennis Bowman, Extension Educator/Crop Systems at the Champaign Extension Center, also has seen 12-spotted cucumber beetles (the larvae are called southern corn rootworms) in some cornfields in east-central counties. Although these insects do not overwinter in Illinois, they arrive from the South early enough in the growing season that they frequently are found alongside the western and northern species. The adult is about 1/4 inch long and is yellow-green with a black head and antennae and 12 black spots on the wing covers. They rarely cause much silk-clipping injury.

Injury to corn leaf caused by feeding by western corn rootworm adults.

Many cornfields will not be silking or pollinating when corn rootworm beetles begin seeking food. What will the beetles feed on if neither silks nor pollen are available? Both western and northern corn rootworms will feed on the epidermal layer of corn leaves and partially or totally strip the leaves of green tissue. This type of injury seldom is economically important, but it may look fairly ugly. The northern corn rootworm also will feed on the blossoms and pollen of many species of weeds.

Start making plans now to scout for rootworm beetles. There are three different reasons for scouting for adult corn rootworms throughout most of the state, and a fourth reason in east-central Illinois:

1. Determine the potential for adult corn rootworms to interfere with pollination. In commercial field corn, treatment may be justified if you find five or more beetles per plant, pollination is not complete, and silk clipping is observed. In seed corn, treatment is justified if the silks on 20 percent of the plants have been clipped to a length of 3/4 inch or less, pollination is still taking place, and rootworm beetles are present.

2. Determine the potential for rootworm problems next year if corn is planted after corn. These guidelines have been established for a long time and will be offered in a future issue of the Bulletin.

3. Determine whether you should prevent rootworm adults from laying eggs in corn this year to prevent rootworm larval injury next year. This practice also has specific guidelines for corn after corn, but not for corn planted after soybeans. As we have stated emphatically in the past, we strongly discourage attempts to prevent western corn rootworms from laying eggs in soybeans until more information about the movement and egg-laying behavior of this biotype is known.

4. Determine the potential for rootworm problems next year if corn is planted after soybeans in east-central Illinois. I have been asked if yellow sticky traps should be placed in soybean fields right now. The answer is no; it is too early. Wait for female western corn rootworms to emerge and initiate egg laying before you start monitoring for their presence in soybean fields. In the past we have suggested that trapping for western corn rootworms in soybeans need not begin until the last week of July. However, because emergence started in June, we may suggest trapping a bit earlier this year. Stay tuned.--Kevin Steffey

Author: Kevin Steffey