Corn Rootworm Adults Are Emerging

June 23, 2000
I've received only one report of rootworm adults in corn thus far (Ford County), but we all know it's time for them to start showing up. As we have stated in previous issues of the Bulletin, rootworm development this year is a bit ahead of schedule, so it's not too soon to start watching for both northern and western corn rootworm adults throughout the state.

Adult western corn rootworm female (left) and male (right).

Western corn rootworm males emerge from the soil 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 2). 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.

Adult western corn rootworm female (left) and northern corn rootworm (right).

Adult northern corn rootworms emerge soon after emergence of adult western corn rootworms 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; they become green as they age. The females are slightly larger than the males.

Leaf-feeding injury caused by western corn rootworm adults.

Many cornfields will not be silking or pollinating when corn rootworm adults begin seeking food, so they will have to eat something other than their preferred food (silks and pollen). 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.

The first reason to scout for rootworm adults in corn will be to watch for silk-clipping injury that interferes with pollination. This type of injury is especially critical in seed-production fields. In seed corn, treatment is justified if the silks on 20% 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. 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.

The other reason to scout for corn rootworm adults in July and August is to assess the potential for injury in corn in 2001. Throughout the Midwest, corn planted after corn has the potential for corn rootworm damage every year, so scouting for rootworm adults in cornfields this year that will be planted to corn next year is very important. Also, in an ever-expanding area in Illinois, Indiana, Michigan, and Ohio (and possibly Iowa) where western corn rootworm adults lay eggs in soybeans, scouting for the adults in soybeans this year will give some insight about the potential for rootworm larval damage if corn is planted next year. Specifics about these scouting programs will be provided in near-future issues of the Bulletin. In the meantime, watch for emerging rootworm adults to get an early indication of their densities this year.--Kevin Steffey

Author: Kevin Steffey