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Corn+Soybean Digest

Bean Leaf Beetle Blitz

Chewing pest hit record levels in 2000 Bean leaf beetles had a banner year in 2000. A rather sissy winter that didn't kill many insects was coupled with super-early soybean planting last spring. An early generation of beetles feasted on emerging soybean plants; a later generation mutilated pods in August and September.

Jim Ganschow got hit with the spring siege.

"We had ideal early soil conditions and began planting soybeans on Apr. 5," says Ganschow, of Walnut, IL. "As the beans emerged, the bean leaf beetles moved in. They were cutting off the cotyledons and killing the plants."

About 40% of the plants were showing severe damage. A local certified crop advisor recommended spraying.

"We applied 4 oz/acre of Pounce," says Ganschow. "By the next day the damage had stopped and within three days the beans had really taken off. If we hadn't sprayed and stopped the damage, I'm sure we would have had to replant."

Some other growers who planted ultra-early had similar experiences. "Those early planted fields drew bean leaf beetles like magnets," points out independent crop consultant Dave Mowers, Toulon, IL. "We saw several cases of economic damage last spring, although in most years that's rare."

But later-season feeding on the pods has become quite common and can cause serious damage, say entomologists.

Larry Herrman, Pioneer agronomist in north-central Illinois, reports that bean leaf beetles are now the No. 1 above-ground insect problem he sees in soybeans. "They were especially bad last year," says Herrman. "We were still getting heavy pod-feeding damage in mid-September."

Pod-feeding damage was seen over much of the Midwest last year. Iowa State University entomologist Marlin Rice reports being in a severely damaged field in central Iowa in early September. He says it wasn't uncommon for 60-70% of the pods on the upper half of a plant to be heavily scarred or flat with no developing seeds.

The beetles also can spread a soybean disease: bean pod mottle virus, Rice notes. Diseased plants have crinkled or mottled leaves and may have mottled seeds. Yield losses vary but in some cases have exceeded 50%.

The first step in combating bean leaf beetles is to scout fields throughout the season, emphasizes Pioneer's Herrman. Regular scouting can help determine if treatment is needed.

The bugs look much like black-spotted yellow, brown or brick-red lady beetles, says Rice. But bean leaf beetles each have a prominent black triangle at the front of the wing covers that points backward. Most have a series of rectangular spots on the wing covers.

Check with your state extension service for treatment thresholds and the latest information on insecticides.

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