During the weekend of June 29-30, Joe Spencer, with the Illinois Natural History Survey, reported finding four western corn rootworm adults in a small plot of corn near Thomasboro (Champaign County). He also observed significant rootworm larval damage to the roots in some plots close to campus. John Obermeyer, entomologist at Purdue University, found a western corn rootworm male on June 28 in Tippecanoe County, Indiana. He, too, observed plenty of third-instar rootworms feeding on roots in plots. Although John found no pupae during his digging, he indicated that feeding was almost over with.|
These observations of the first western corn rootworm adults of the season are a little later than they were last year, but it was only a matter of time. After the cooler temperatures during May and early June slowed larval development, the recent hot temperatures sped up larval development, and it's likely that adult corn rootworms will begin emerging rapidly now. Japanese beetles have captured most of our attention thus far, but adult corn rootworms should be monitored carefully during the next couple of weeks.
When cornfields are not silking or pollinating when corn rootworm adults begin seeking food, the beetles will feed on corn leaves. 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. (As indicated in a previous article, Japanese beetles cause the same type of injury.) Northern corn rootworm adults also will feed on the blossoms and pollen of many species of weeds.
Adult western corn rootworm and leaf-feeding injury.
When cornfields begin to pollinate, adult corn rootworms will congregate on silks. As they feed on and clip silks, they might interfere 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.
Silk clipping injury caused by corn rootworm adults.
Most people are familiar with corn rootworm adults, but it's always a good idea to review their emergence patterns, their morphological features, and some of their behaviors. 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 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.
Adult female (left) and male (right) western corn rootworms.
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.
Watch for adult corn rootworms and Japanese beetles ganging up on corn silks when they emerge. People in areas where both pests occur may have their hands full for a while.--Kevin Steffey and Mike Gray