In central and western Illinois, the summer's swarms of Japanese beetles have farmers walking their fields and punching their calculator’s key pads in search of an answer:

To spray or not to spray.

The bugs’ presence is not consistent, but where they are, the damage they’re doing to corn and soy crops threatens to strip away much of an otherwise promising harvest, agriculture professionals say.

“This is the critical time” for farmers to decide whether the damage the beetles cause as they dine on emerging corn silks warrants the cost of insecticide applications, said Stephanie Porter, a Jacksonville-based agronomist with Burrus Seed Co.

Where the pest is present, this season’s infestation is every bit as heavy as in 2010-11, when the half-inch, metallic green bugs struck central Illinois as they migrated west, Porter said.

“Western Illinois seems to be the hardest hit” area of the state, Porter said. This spring, based on her own research and farmer’s reports of beetle larvae in their fields, “I said, ‘Oh, we’re going to be in for it.’” She was right. 

Around Roseville, west of Farmington, “the beetles were the highest in numbers (that) I’ve ever seen in my life,” said Porter, who also specialized in plant diagnostics with the University of Illinois Extension.

"They were attacking soy beans two weeks ago, just waiting, lingering” to strike fields of corn silks, she said.

In Tazewell County’s fields, “There’s some damage, we know,” said County Farm Bureau Manager Doug Godke.

“The soy plants are being hit,” he said. “Some farmers are treating with pesticides” on both soy and corn, “but we’re not too sure yet on the severity of crop damage.
“The question is, how much damage can you take before you treat (the crops) with chemicals,” Godke said.

Because the beetles tend to mass around a field’s fringes, he and Porter said farmers need to inspect through their crops to learn how deep the damage is spreading.

For corn, industry guides say damage calls for insecticides if there are three or more beetles per ear. Silks have been clipped to less than a half inch and pollination is less than 50 percent complete.

The beetles are also devouring ornamental plants and trees in backyards in the Pekin-Peoria area – again, in so-called hot spots where their larvae were heavy and survived last winter’s mild temperatures.

To thwart them, Godke suggested an old-fashioned bug deterrent.

“Dawn dish washing soap mixed with water” provides a safe leaf coating, he said. The insecticide Sevin does the same while killing the bugs, said a spokesman for Kelly Seed and Hardware in Peoria.

The beetles’ feeding season typically stretches from mid-June to August. Until it ends, the fight continues.

Follow Michael Smothers at Twitter.com/msmotherspekin