Extension Communications
Extension 4-H Youth Building
Ames, Iowa 50011-3630
(515) 294-9915
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Extension Communications |
9/2/04
Contacts:
Laura Jesse, Entomology, (515) 294-1101, lrahnsen@iastate.edu
Jean McGuire, Extension Communication Services, (515) 294-7033, jmcguire@iastate.edu
Garden Column for the week of Sept. 10, 2004
Corn Rootworm Beetles Invade Iowa Gardens
By Laura Jesse
Entomologist
Iowa State University Extension
Each fall some unlucky gardeners get inundated with hungry refugees from the corn. As the corn plants dry down, insects tend to wander out of the fields looking for a fresh bite to eat. Corn rootworm beetles seem to be a common hungry wanderer in gardens this fall. There are two species of corn rootworm beetles that can cause problems for gardeners, the northern corn rootworm beetle and the western corn rootworm beetle. The northern corn rootworm beetles are small, uniformly pale green and the western corn rootworm beetles are yellowish with black stripes along their back.
If you farm, you probably already know that northern and western corn rootworms are serious pests of corn. The adult female beetle lays her eggs in the soil of the corn field. The eggs hatch the following spring and the rootworm larvae feed on the corn roots. The larvae damage the corn by removing roots that are needed for water and nutrient uptake, and for stabilizing the plant. Corn plants that are heavily damaged by corn rootworms, are prone to lodging. Lodging is when a corn plant falls over, often during heavy winds. The plant usually survives, but it is difficult to harvest the corn with a combine when many of the plants are bent over.
In mid-summer the rootworm larvae transforms into the adult beetles that are approximately one-quarter inch long and slender. The beetles remain in the corn fields and lay eggs in the soil until mid to late August. Since the female lays her eggs in the soil of the corn field, rootworms are a particular problem in continuous corn. However, it has been recently observed that some of the corn rootworm eggs will delay hatching for a year, foiling the effectiveness of the corn-soybean rotation in controlling this pest. Insects always seem to get the upper hand somehow. With roughly 12 million acres of corn in Iowa, there is plenty of food for the corn rootworm beetle larvae, and therefore many adult beetles to invade gardens.
Corn rootworm beetle adults prefer to feed on corn pollen and silks, but when the corn begins to dry down they wander out of the corn fields in search of food. Often they wander right into gardens where they find a lovely array of succulent flowers on which to feast. They may become numerous on a wide variety of plants but seem most attracted to light colored flowers such as dahlias, roses and others. Even small numbers of beetles can ruin flowers by feeding on the moist buds or petals. Rootworm beetles also are common in melon and pumpkin patches where they feed on the fruit.
There are no completely effective control methods for rootworm beetles on flowers in late summer. Insecticides sprays are of limited effectiveness and repeated applications are required for even modest impact. Unfortunately, insecticides applied to blooms may kill bees, butterflies and other beneficial, pollinating insects. Screening highly-prized blossoms with cheese-cloth, nylon netting or other light fabric is one way to avoid attack but this may not always be practical. Ultimately the number of beetles dwindles by late summer and they disappear completely with frost. Learning to tolerate a little damage is probably the most practical response.
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ml: isugarden
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Editors: Two color photos, suitable for publication, are available at right. Click on each thumbnail photo to go to the fullsized photo. The top picture's fullsize photo is 132K and the bottom picture's fullsize photo is 152K. Caption: Northern Corn Rootworm Beetle |
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| Caption: Western Corn Rootworm Beetle |  |