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Entomologists: Scout fields for armyworms
Michigan Correspondent

EAST LANSING, Mich. — Armyworms are on the march in Michigan. Experts are advising farmers to scout fields and consider control methods if necessary.

Michigan State University Field Crops Entomologist Chris DiFonzo said she has received several reports of armyworms in northern Ohio and southern Michigan, some in corn but more in wheat. She said the best way to determine if there is a problem is to regularly scout fields.

“There is no substitute for walking into a field to see what is going on,” DiFonzo said.

Armyworms traditionally will feed on a field at night, which can make early detection difficult. However, younger insects are easier to eradicate, thus making the early detection of their presence so important.

“Larger larvae eat more and are often responsible for the majority of damage,” DiFonzo said. “Catching an infestation early prevents loss from larvae and improves control.”

In wheat, DiFonzo said in many cases caterpillars are still small and feeding on leaf margins, but feeding on the flag leaf is starting.

Treatments are targeted not only to prevent defoliation of the critical flag leaf but also to prevent larvae from clipping heads later in June and then “marching en masse to neighboring fields.”

DiFonzo said the threshold in wheat is two worms per square foot. If a grower sees feeding injury but cannot find larvae easily, DiFonzo said to shake the plant vigorously so that larvae fall to the ground, and look at the base of the plant.

“Armyworms often hide out at the base of plants during the day, moving up on the foliage in the evening to feed,” she said.

DiFonzo said at this late stage of the wheat season the threat of armyworms moving into wheat fields has decreased.

“If the wheat is drying down it probably isn’t as attractive anymore,” she said.

But, she warned growers to keep an eye on their cornfields.

“Look for foliar feeding on corn,” she said. “Check corn with poor weed control and corn that was planted later.”

She said larvae hide in the whorl or the base of corn plants. A sure sign of infestation is large cylindrical frass, or excrement pellets in the whorl.

“Armyworms feed on the leaf margins. In severe cases all leaf material is eaten, leaving only the midrib,” she said.

According to DiFonzo the threshold for a corn infestation is 25 percent of plants with at least two larvae per whorl or 75 percent of plants with a single larva per whorl.

She said directing spray into the whorl should kill the larvae hiding there.

Dan Rossman, MSU Gratiot County Extension director, said he has seen some armyworm destruction this summer in Gratiot County wheat and cornfields.

Two infested cornfields he said were planted where a cover crop had been.

“The armyworms were attracted to that growing grass when they laid their eggs,” Rossman said.

“We have not been above the threshold in the wheat, but we have found a few fields that have armyworms present.”

Rossman said after the insects were found in the corn, the fields were treated.

“One wheat field that I’m aware of was treated. The other wheat fields we’ve been watching,” he said.

Rossman said his next concern is whether the armyworms will move from the wheat fields into neighboring cornfields.

“The key is that normally where we have problems with armyworms is where there was a grass cover crop or there’s grass in the corn,” he said.

“Armyworms can really sneak up on you,” he said. “If you drive by in a pick-up you really don’t see what’s going on.”

This farm news was published in the June 28, 2006 issue of Farm World, serving Indiana, Ohio, Illinois, Kentucky, Michigan and Tennessee.