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Now Is the Time to Scout High-Risk Fields

03 June 2011

US — Now is the time for grain farmers to scout fields at risk of insect infestations and potential pest problems, according to a Purdue Extension entomologist.

Corn planted into grass and wheat in areas of dense growth poses a high armyworm risk, said Christian Krupke, associate professor of entomology. Corn where weedy growth existed could potentially face cutworm troubles, and soybeans first emerging could face bean leaf beetle pressures.

"Corn that has been no-till planted into an abandoned wheat stand or a grass cover crop should be inspected immediately for armyworm feeding - especially in southern Indiana," Krupke said. "Hatched larvae will move from dying grasses to emerging or emerged corn."

Armyworm feed from the leaf margin toward the midrib and give corn a ragged appearance. In some cases damage may be extensive enough that most of the plant, except the midrib and stalk, is consumed.

"A highly damaged plant may recover if the growing point has not been destroyed," Krupke said.

If growers find that more than 50 percent of the corn plants show armyworm feeding damage and there are numerous live larvae less than 1.25 inches long, Krupke said a control method may be necessary.

"If farmers detect armyworm migrating from border areas or waterways within fields, spot treatments are possible if the problem has been identified early enough," he said.

Farmers with wheat should examine plants in various areas of the field - especially where plant growth is dense. Signs of armyworm include flag leaf feeding, clipped heads and insect droppings on the ground.

They also should shake plants and count the number of armyworm larvae on the ground and under plant debris because armyworm will take shelter under crop residue or soil clods on sunny days, Krupke said.

"If counts average about five or more per linear foot of row, the worms are less than 1.25 inches long and leaf feeding is evident, control is likely justified," he said. "If a significant number of these larvae are present and they are destroying the leaves or the heads, wheat growers should treat immediately."

Larvae longer than 1.25 inches consume larger amounts of leaf tissue and are more difficult to control.

While scouting, corn growers also should be looking for black cutworm, a pest that has arrived in Indiana in record numbers this year.

Moths that arrived in the state were attracted to winter annual weeds for egg laying. Now that those eggs have hatched, caterpillars are in the fields feeding either on dead and dying weeds or starting to move onto emerged corn, Krupke said.

"Insecticides, whether soil- or seed-applied, should not lull producers into a false sense of security, as larger larvae are able to continue their feeding," he said. "Timed scouting and careful assessment of damage can go a long way in preserving a stand of corn."

Finally, soybean growers need to keep a close eye out for bean leaf beetle. The beetles awoke from the winter and have been feeding on forages and other legumes, such as clover, while waiting for soybean to be planted and emerge.

"Amazingly, these insects are able to detect and subsequently find first-emerging soybean, whether near a wood's edge or in the middle of a large field," Krupke said. "Once plants have emerged throughout the field, beetles will typically dissipate to non-economic levels."

Farmers should be scouting their first-planted and first-emerged soybean seedlings for signs of bean leaf beetle feeding on cotyledons and unfoliate leaves. While the initial leaf feeding may look serious, Krupke said, only extensive cotyledon damage is cause for serious concern. "If cotyledons are being destroyed before the unfoliate leaves fully emerge or if the growing point is severely damaged, reduced yields are likely."

He pointed out that once trifoliate leaves have unrolled, soybean can tolerate up to about 40 percent defoliation without yield loss.

"It may look ugly, but they can take a beating," he said.

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