The first reports of emerging Japanese beetles came just a little over 2 weeks ago. In last week's Bulletin (issue no. 15, July 3, 2003), we reported that western corn rootworm adults also had begun to emerge. Not surprising, the frequency of emergence of both of these pests has increased over the past week, and both of them pose a threat to pollinating corn.
Japanese beetle biology was reviewed in issue no. 14 (June 27, 2003) of the Bulletin, and management of Japanese beetles in pollinating cornfields is discussed in another article in this issue of the Bulletin. So we'll concentrate on corn rootworms in this article.
Corn rootworms begin emerging from the soil in late June and early July in Illinois, males first, followed by females. Female and male western corn rootworm beetles are similar in appearance. Females have a yellow body, with three distinct black stripes on their wing covers, whereas males are slightly smaller, with predominantly black wing covers with a small portion of yellow visible at the tips. Northern corn rootworm beetles begin to emerge after the western corn rootworm beetles. They are cream or tan colored immediately after emerging and turn green as they mature. Northern corn rootworm beetles feed on the blossoms and pollen of many weed species.
When cornfields are not pollinating, Japanese beetles and western corn rootworm adults feed on foliage, stripping leaves of plant tissue. This type of injury does not generally cause economic damage. However, as corn be-gins to pollinate, both insects begin to congregate on silks. As they feed on and clip silks, they may interfere with pollination. This is especially important in seed corn production fields.
During the next couple of weeks, it will be very important to know what is going on in your fields. Just a reminder, a treatment may be needed for Japanese beetles during pollination when there are three or more beetles per ear, silks are being clipped to less than 1/2 inch, and pollination is not complete. Treatment for corn rootworm beetles in field corn may be required if there are five or more beetles per plant, silk clipping is observed, and pollination is not complete. In seed corn, treatment may be warranted 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.
Western and northern corn rootworm adults feeding on silks on an ear of corn.
Japanese beetles feeding on corn silks.
Surprisingly enough, grasshoppers have been fairly abundant in the southern half of the state, despite the wet weather we've experienced the past couple of months. Entomologists at the University of Missouri also have noted significant populations of grasshoppers. We've received a few notes about people having applied insecticides for grasshopper control. Grasshoppers usually thrive in hot, dry weather. Grasshopper nymphs feed in grass waterways, ditches, and other noncrop areas. When these plants are consumed, grasshoppers move into nearby fields of corn and soybeans, potentially causing serious defoliation as they go. Later in the season, they may feed on corn ears and soybean pods. Take some time to keep an eye on field edges as we progress into the summer months. These creatures can consume large amounts of plant tissue in relatively short periods of time.
Although no one has reported finding fall armyworms yet, the prevalence of late-planted corn in southern counties certainly calls for monitoring for their presence. Fall armyworms do not overwinter in Illinois. Rather, they fly northward from the Gulf Coast states and points even farther south early in the summer. Consequently, they pose more of a threat to late-planted corn than to early-planted corn. Keep your eyes peeled, and let us know if you be-gin to find these hungry critters within the next few weeks.--Kelly Cook and Kevin Steffey