June 21, 2002

LINCOLN, Neb. — Summer insects pests are showing up across Nebraska, sometimes aided by crops already weakened by drought.

University of Nebraska entomologists recommend that producers scout their fields for insect damage and make careful treatment decisions if necessary.

European corn borer numbers are up in Nebraska this year. Entomologists say that could be due to natural population swings and a slight drop in the number of acres planted to Bt corn. Since 1998, corn borer moth flights have been relatively low.

"We haven't needed to treat much in the past few years, but that doesn't mean we can become complacent," said Keith Jarvi, a pest management specialist at NU's Northeast Research and Extension Center at Norfolk. Producers should carefully scout their non-Bt corn, especially early planted fields. Early plantings attract female corn borers and they'll lay eggs there.

Recently high humidity and high temperatures in eastern Nebraska are ideal for corn borer larvae. When scouting, producers should watch for white egg masses that look like fish scales on the underside of the leaf. Four days to a week later, eggs will appear spotted as developing larvae become visible. Eggs usually hatch within 24 hours after spots appear.

"As the larvae enter the whorl to feed on developing tissue, the feeding scars appear as the leaves emerge from the whorl," Jarvi said. "Larvae will remain in the whorl for 7 to 14 days before boring into the stalk."

Bt corn provides excellent protection against corn borers, Jarvi said, but that doesn't mean farmers shouldn't scout Bt fields.

"You should still be looking at those fields to check for resistance or other unusual problems," the Institute of Agriculture and Natural Resources specialist said. About 2 percent to 4 percent corn borer damage is normal in Bt fields. If there is more damage, producers should check to be sure it's not from another insect. If corn borers are the problem, they should contact the seed company's representative.

Once producers determine infestation levels, they should analyze treatment costs to find out if its economical. An interactive worksheet for making treatment decisions is available from NU Cooperative Extension at Work sheet for First Generation Corn Borer Larvae.

Treatments are only effective if borers are still in the whorl, Jarvi said. Research shows the best control comes from aerial- or ground-applied granular formulations or liquid applications through sprinkler irrigation systems.

Stinkbugs also are threatening corn in southern Nebraska cornfields this year. Though the pest doesn't usually cause damage to crops, they may be more prevalent because of a mild winter, said Bob Wright, entomologist at NU's South Central Research and Extension Center near Clay Center.

Stinkbugs are one-half inch long, shield-shaped and have piercing-sucking mouthparts. The upper side of the body is light or dark brown and the underside is light yellow to green.

"Treatment thresholds for stinkbugs haven't been developed because it's so uncommon for this problem to arise," Wright said. "Once the injury is done, it's really too late to treat. Most injury happens when plants are small and the stinkbugs can pierce the base of the plant and reach the growing point."

Farmers may notice stinkbug damage as rows of yellow-edged holes on the leaves, Wright said. In more severe cases, plant growth may be distorted and leaves may be wrapped together, an effect called buggy whipping. Stalks may be twisted and leaves may not unroll. In the most severe cases, corn plants may develop tillers.

Spider mites arrived early in corn in southwestern Nebraska and the Panhandle.

"Spider mites overwinter in grasses and wheat and their survival was probably enhanced by a mild winter," said Gary Hein, an entomologist at NU's Panhandle Research and Extension Center at Scottsbluff. "Spider mites also reproduce better in the spring in plants that are drought-stressed."

As grasses dry down, spider mites move into nearby crops, mainly corn. Many producers are noticing damage in cornfields adjacent to wheat, Hein said.

"Normally we don't see spider mites until mid- to late July, but this year we've been seeing them since early June," he said.

Spider mite damage usually appears as discoloration on the lower leaves of corn. Yellow speckling will appear on the upper leaf surfaces and the mites themselves can be seen feeding on the lower surfaces. Good treatment thresholds for early infestations have not been established. Treatment is only necessary when infestations become severe and plant survival is threatened, Hein said.

"Treat only when necessary and only the area of the field that is affected," he said. "Mite problems this early can be serious and we need help from natural predators to keep mite populations at bay for the summer. Don't wipe out all of the natural enemies by treating the whole field when it isn't necessary."

Thrips pose a threat to dry beans, alfalfa, sunflowers and soybeans statewide, Hein said. These pests also overwinter in grasses and wheat and move to irrigated crops after dry down. The most common damage is yellow stippling of the leaves and the thrips leave dark fecal deposits on the leaves. In dry beans, more severe infestations may cause leaf cupping. In alfalfa, severely damaged leaves will curl and pucker.

"Dry beans are small at this point and that leaves them susceptible to damage from thrips," Hein said. "We don't know what the extent of the damage will be yet, but thrips tend to do well when rainfall is limited." If the plants are growing rapidly, there is no reason to treat for thrips even if the pests reach threshold numbers, Hein said.

"Treat only if there are 15 or more thrips per plant plus visual symptoms of damage," he said. "It's rare for thrips to infest alfalfa, so we don't have threshold numbers for alfalfa. Treat alfalfa if numerous thrips are present and plants look stressed. If it's close to cutting time, cut the crop and monitor thrips in the regrowth."

For more information on crop pests, visit Insects & Pests or contact your local Cooperative Extension office.

06/21/02-HC

Gary L. Hein - Ph.D.
Entomology
Professor
(308) 632-1369

Keith J. Jarvi
Entomology
Integrated Pest Management Specialist
(402) 370-4016

Robert J. Wright - Ph.D.
Entomology
Professor
(402) 472-2128

Heather Corley
IANR News and Publishing
(402) 472-3030

Department: Entomology