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Corn+Soybean Digest

Baffling Beetles

A variant of the western corn rootworm is spreading farther and faster than expected. And entomologists are scrambling to learn more about its behavior.

Since the early 1990s, rootworm beetles in east-central Illinois and adjacent northwestern Indiana have been moving out of cornfields into nearby soybean fields to lay their eggs. They aren't supposed to do that.

As a consequence, rootworm larvae can attack first-year corn planted the next season in those fields. Affected farmers have been forced to shell out thousands of dollars for rootworm soil insecticides they hadn't previously needed.

The rootworm behavior has astounded entomologists. And, unfortunately, the problem has now spread into western Ohio, southern Michigan, farther north and west in Illinois and into northeastern Iowa.

Joseph Spencer and colleagues at the Illinois Natural History Survey and University of Illinois have been studying the problem, striving to understand the factors and conditions that cause egg-laying female beetles to move out of corn. Their work is funded by the Illinois Council on Food and Agricultural Research and the Illinois Soybean Program Operating Board.

"We've never found any evidence that the western beetles are attracted to soybeans per se," Spencer reports. "Rather, given the way crop rotation detrimentally affects western rootworm populations, western individuals that leave corn to lay eggs are better able to survive.

"Soybeans happen to occupy most of the cropland that is not growing corn. However, we find western beetles in other crops, and they lay their eggs in those crops. In 1999 we saw the most western beetles in soybeans, but also noted significant numbers in corn and alfalfa plots. This suggests that western adult beetles are not moving into soybeans for egg-laying as much as they are moving out of corn and into a number of other crops to deposit their eggs."

The Illinois researchers have found consistently high numbers of beetles at least 400' into soybean fields and in soybean fields 1/2 mile or more from the nearest cornfield.

"At one time we believed that the western beetles moved between corn and soybean fields to lay eggs only after corn silks, tassels and foliage were either no longer available or attractive as food," says Spencer. "However, more recently we have observed that beetles frequently move between corn and soybeans and feed on soybean foliage in areas where they are laying eggs."

Spencer says lab studies have shown that these beetles don't remain vigorous. In fact, if they feed exclusively on soybean plants, they'll die. He suspects that they move back into cornfields so they can eat corn plant parts and restore the vigor they lost eating soybeans.

"Laboratory results demonstrate that feeding on corn plants after eating soybeans makes western beetles better able to withstand physiological stress. Soybean-eating females die much sooner than corn-fed females under the same conditions."

Examination of stomach contents of 6,600 adult females suggests that beetles tend to eat from corn plants shortly before moving into soybean fields for egg laying. After depositing eggs, they eat from the less nutritious soybean plants.

Even though the rootworm beetles feed on soybean plant parts, Spencer and colleagues have found no evidence that such feeding has a significant effect on soybean yields.

The Illinois scientists have noted that soybean varieties resistant to feeding by Mexican bean beetles also are significantly less preferred by western corn rootworm adults. ?

Monitor For Beetles Entomologists urge farmers to monitor their soybean fields for rootworm beetles. Here are University of Illinois guidelines for 2000:

Use Pherocon AM sticky traps. They're available from Gemplers at 800-382-8473 and Great Lakes IPM at 800-235-0285.

Start using the traps the last week of July and continue through the third week of August. If you're in an area where these new beetles have already been found, place 12 traps evenly throughout the interior of each soybean field, regardless of field size. Place them just above the canopy on metal fence posts. Each week, count all western corn rootworm beetles on each trap in each field. Replace the traps weekly.

Corn and soybean growers in areas where the problem has not yet been confirmed should place four sticky traps throughout each soybean field. If any beetles are detected, go to the 12-trap procedure.

For each field scouted (12 traps), divide the total number of beetles collected by 12, then by the total days to get the average number of beetles per trap per day. If the average is two or more, there could be economic root damage to corn plants in that field the next year. A soil insecticide should be considered.

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