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Corn Rootworm, Grape Colaspis, and Japanese Beetle Adults Are Emerging

June 22, 2001
The first report of western corn rootworm adult emergence has been reported by Randy Wright, graduate research assistant, in a University of Illinois research plot in Champaign County, so it's time 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. 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. For more information on monitoring soybean fields for western corn rootworm, refer to the article in last week's Bulletin.

Don Rhodes and Todd Burrus of Burrus Seed have reported finding grape colaspis adults in seed production plots, so anticipate adult feeding on corn leaves in the same fields in which the larvae injured the roots. At some point within the next few weeks, the adults will move into other crop fields, including alfalfa and soybeans, where they will continue to feed and lay eggs.

Pinned grape colaspis adults.

The tan grape colaspis adult is oval and about 1/6 inch long, with rows of tiny punctures on its wing covers, making them appear ridged. They may resemble newly emerged northern corn rootworm adults; however, northern corn rootworm adults have smooth wing covers that eventually turn green. Feeding by grape colaspis adults on corn leaves and silks and on leaves of alfalfa, clover, grape, soybeans, and strawberries causes no economic damage. They also feed on bull nettle and smartweed, and possibly other species of weeds.

We would like to keep track of when grape colaspis adults move into soybeans. Many of you will be scouting soybean fields for western corn rootworm adults, so consider watching for grape colaspis adults, too. If you find grape colaspis adults on yellow sticky traps, in sweep nets, or simply by observation, please let us know. Recording their numbers over time would be especially helpful.

Many of us will remember the very large numbers of Japanese beetles that occurred in corn and soybeans last year, especially in eastern and central Illinois. Well, we need to be ready for them again because early indications suggest they may be numerous in 2001. Omar Koester, crop systems unit assistant at the Monroe/Randolph Extension unit, reported the first Japanese beetles of the season in Monroe County on June 20.

We used to think of Japanese beetles as a problem experienced only by folks in east-central Illinois. However, that has changed in recent years. Our understanding is that Japanese beetles have been reported around homes and gardens in most of the major cities in Illinois. Therefore, we should anticipate that they will show up in nearby crop fields and possibly spread from there. Let us know if and when you find these handsome, albeit irritating, insects this summer. We'll want to keep track of their distribution within the state.

We first reported Japanese beetles found in Madison County last year on June 22, so their emergence right now is occurring at virtually the same time. A sudden and significant attack of these insects in cornfields and soybean fields always seems to catch a lot of people by surprise. And homeowners will also enter the fray when they begin finding Japanese beetles in their flower and vegetable gardens and fruit trees.

Japanese beetle on corn silks.

The adult Japanese beetle is shiny metallic green, with hard, bronze-colored wing covers. Along each side of the abdomen, just below the wing covers, are six tufts of white hair. The adult is about 1/2 inch long. The beetles may feed on leaves if silks are not available; injured leaves appear skeletonized or lacy, similar to injury caused by corn rootworm beetles. However, leaf feeding is seldom economically important. An informational video on Japanese beetles is available at videos/japanese_beetle/japanese_beetle.html. If you have a copy of our IPM On-Line Companion CD, remember to place it in your CD-ROM drive before viewing for a higher-quality video.

More importantly, Japanese beetle adults may clip a sufficient number of silks to prevent proper pollination. Control with an insecticide may be warranted if silks are clipped to less than 1/2 inch, less than 50% of the plants have been pollinated, and beetles are feeding. A rule-of-thumb economic threshold is three or more beetles per ear. Insecticides suggested for control of Japanese beetles in corn are *Capture 2EC at 2.1 to 6.4 oz per acre, *Penncap-M at 2 to 4 pt per acre, Sevin XLR Plus at 1 to 2 qt per acre, and *Warrior T at 2.56 to 3.84 oz per acre. (The use of products preceded by an asterisk is restricted to certified applicators.)

As most of you know, the greatest concern regarding Japanese beetles is their feeding, sometimes in very large numbers, on soybean foliage. Although no one has reported finding Japanese beetles in soybean fields yet, you should be aware of the threshold, just in case the beetles show up in soybeans soon. Treatment may be warranted in soybeans during vegetative growth if defoliation reaches or exceeds 40%. The threshold decreases to 15 to 20% defoliation during flowering, pod set, and pod fill. Insecticides suggested for control of Japanese beetles in soybeans are *Ambush at 6.4 to 12.8 oz per acre, *Asana XL at 5.8 to 9.6 oz per acre, *Penncap-M at 3 to 4 pt per acre, *Pounce 3.2EC at 2 to 4 oz per acre, Sevin XLR Plus at 1/2 to 1 qt per acre, and *Warrior T at 3.2 to 3.84 oz per acre. (The use of products preceded by an asterisk is restricted to certified applicators.)--Susan Ratcliffe and Kevin Steffey

Author: Kevin Steffey Susan Ratcliffe

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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