University of Illinois

No. 16/July 11, 1997

Time to Begin Watching for Corn Rootworm Adults

In last week's issue of the Bulletin (no. 15, July 3, a, b), Mike Gray brought you up-to-date on the progress of corn rootworm larvae feeding below ground. Reports about rootworm larval damage to corn roots have begun to trickle in. We always anticipate some reports about rootworm damage in fields of corn planted after corn if an insecticide was not applied at planting, or if the insecticide failed to control the larvae adequately. However, in recent years, concern about rootworm larval injury to corn planted after soybeans has increased dramatically in east-central Illinois, where western corn rootworm females are laying eggs in soybeans (refer to related article). Consequently, people in the affected area usually are on alert to determine if the problem has reoccurred or spread. Howard Brown with Pioneer Hi-Bred International already has observed numerous corn rootworm larvae in part of a field in Champaign County that had not been treated with a soil insecticide. He observed as many as seven larvae per plant. A grower in Iroquois County discovered some lodged corn in spots in one of his fields and determined that rootworm larvae had severely pruned the root systems. So it's time to make an extra effort to determine whether corn rootworms have injured roots in cornfields.

It's been stated more than once, but egg hatch of corn rootworms this year was delayed. Although we generally would have found corn rootworm adults in cornfields by now, their appearance is delayed, just as it was last year. However, during the next couple of weeks, both western and northern corn rootworm adults should begin emerging in fields throughout the state, so we should be vigilant. When emergence of large numbers of rootworm adults coincides with silking and pollination, the threat of the beetles' clipping silks becomes a reality.

Western corn rootworm (left), northern corn rootworm (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. Western corn rootworm beetles are about 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, nearly the entire front half 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.

Figure 1. Western (top), northern (bottom left), and southern (bottom right) corn rootworm adults (illustrations from Seed Corn Pest Management Manual for the Midwest, Purdue University).

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

You also may encounter 12-spotted cucumber beetles (also called southern corn rootworms, although technically only the larvae are called southern corn rootworms). These insects do not overwinter in Illinois. They arrive from southern states early enough in the growing season that they frequently are found alongside the western and northern species. The adult southern corn rootworm is slightly larger (3/8 inch long) than the western and northern corn rootworms, is yellow-green, and has 11 conspicuous black spots on the wing covers (Figure 1). They rarely cause much silk-clipping injury.

If cornfields are not silking or pollinating when corn rootworm beetles begin seeking food, 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 looks fairly ugly. The northern species also will feed on the blossoms and pollen of many species of weeds.

Start making plans now to scout for rootworm beetles. The first objective of scouting for rootworm adults is to determine their potential 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. A second objective of scouting for rootworm beetles is to determine the potential for rootworm problems next year, either in corn after corn or corn after soybeans. Guidelines for corn after corn have been determined and will be offered in a future issue of this Bulletin. However, research on scouting soybeans to determine whether or not rootworm larvae will be a problem in next year's corn is still under way (refer to related article).We'll give you as much information as we can as our summer research projects progress.

A third objective of scouting for rootworm beetles is to determine whether you should prevent them from laying eggs this year to prevent rootworm larval injury next year. This practice has specific guidelines for corn after corn (to be printed in a future issue of this Bulletin), but not for corn after soybeans. Again, we'll offer any suggestions we can ascertain during this year's research activities, which now include a large-scale areawide study in which the primary objective is to prevent corn rootworm adults from laying eggs. Keep watching for new information.
Kevin Steffey, Extension Entomology, (217)333-6652