Nebraska

Corn and dry edible bean growers need to scout fields now for western bean cutworms, a University of Nebraska entomologist said.

High numbers of western bean cutworm moths were caught in light traps across central Nebraska starting over the July Fourth weekend, said Bob Wright, entomologist in the university's Institute of Agriculture and Natural Resources. Moth numbers should now be close to peaking, which means farmers and crop consultants should be scouting fields throughout this month.

As moths emerge, they will mate and the females will lay eggs on corn and dry beans, Wright said.

"These eggs are laid in clusters of five to 200 on the top surface of the upper leaves of a corn plants and on the lower leaf surfaces of dry beans," he said.

The white eggs take five to seven days to develop, and change to tan and then purple before they hatch.

"Right after hatching is the best time for control," Wright said.

"That's why it's important to keep checking fields. As soon as eggs start disappearing, you'll find the caterpillars at the base of the leaves and in the ear tips."

If 8 percent of corn plants have eggs or newly hatched caterpillars and 95 percent of the corn had tasseled, an insecticide should be applied while the caterpillars are exposed, he said.

Light trap information for scouting corn fields is available on UNL's entomology website at http://entomology.unl.edu/fldcrops/index.htm. Milk jug type pheromone or scent traps are recommended to monitor potential infestations in dry edible beans, Wright said. If the number of moths accumulated up to the peak of the moth flight totals more than 1,000, the risk of damage is significant.

"If moth flight is moderate, around 700 to 1,000, use the infestation in a nearby cornfield as a decision-making guide" for dry beans, Wright said. If the adjacent corn needs an insecticide treatment for western bean cutworm, the beans also should be treated. Dry beans should be treated 10 to 20 days after peak moth flight.

There is only one generation of western bean cutworms per year.

After the eggs hatch on corn plants, the caterpillars move to the whorl or tassel to feed on the tender yellow leaf tissue or on the tassel itself, he said. Once the tassel emerges, or if it has already emerged when the eggs hatch, the caterpillars will move to the ear tip and feed on the green silks and then enter the ear to feed on the developing grain.

"Once the caterpillars are in the ear, they are hard to control," Wright said. "It's important to scout for the eggs now and not wait until the caterpillars move into the ear. By the time growers see the caterpillars in the ear, it's too late."

Western bean cutworms that hatch on dry bean plants feed on blossoms on young, tender leaf material. The caterpillars will attack bean pods as they develop, eventually feeding on the developing seeds.

For more information about the life cycle, scouting and treatment of western bean cutworm, consult UNL Extension NebGuide G98-1359-A, Western Bean Cutworm in Corn and Dry Beans, available at local extension offices or on the web at http://ianrpubs.unl.edu/insects/g1359.htm.

More information about western bean cutworms also can be found in Crop Watch, extension's crop production newsletter, at http://cropwatch.unl.edu/main.htm on UNL's Entomology website at http://entomology.unl.edu/fldcrops/pestipm.htm.

Date: 7/20/05