A pest prevalent in eastern Corn Belt states is on the move. The Japanese beetle, a dime-sized insect, has now packed its bags and settled into corn and soybean fields in central Iowa.

Traditionally, the beetle has called Ohio, Indiana and eastern Illinois home. It has, however, colonized in urban areas of Omaha and St. Louis. But it's been building up its numbers in new areas.

Marlin Rice, professor of entomology at Iowa State University, discovered one of those areas this summer. He found fields infested with Japanese beetles seven miles southeast of Ames.

“It was startling to find them in an ag setting this far west,” Rice notes. “Ag infestations had been pretty much along the Mississippi River.”

Their movement may have something to do with the nursery industry, estimates Kevin Steffey, University of Illinois extension entomologist.

Japanese beetles tend to first appear in fields near metropolitan areas and spread from there, he says. The shipping of ornamental plants, fruit trees, roses and turf — the beetle's other hosts — could explain how the insect spreads.

“I think some of these Japanese beetles have spread accidentally this way,” Steffey says. “What seems to happen around metropolitan areas is the beetles move from what would have been their ornamental host into the countryside because they like corn and soybeans as well.”

Rice says he found a tree nursery within a mile of the infected fields near Ames. “It wouldn't be much of a stretch to assume that they came in on some horticultural stock, established themselves and are now spreading,” he says.

While it's a new insect to put on the scouting radar screen, it's more about awareness than action at this point.

The Japanese beetle starts appearing in mid-June and tends to peak just after corn pollination — fortunate considering their preferred meal is corn silks.

Silk clipping from insect feeding must be very severe to warrant treatment, advises Larry Bledsoe, Purdue University extension entomologist.

“You want to keep about a ½" of silk coming out of the corn ear,” Bledsoe says. “If 50% of those ears are being cut back to less than ½", then you've probably reached a point where you should consider treatment.” He cautions farmers to know if the crop has been pollinated. If it has, then treatment isn't warranted. If it hasn't, then it becomes an economic call.

Beetles tend to congregate in a small area of the field, often along the border. That means if there is destruction, it's usually not widespread. Bledsoe adds that often up to 10 male beetles are attracted to an ear where a female is feeding. The ear looks covered in insects, but only the female is doing significant damage.

“It looks like your crop is being destroyed,” he says. “But if you look, you'll see that it's in a small part of your field, and only one of those beetles, the female, is eating. The others are interested in mating with that female. The feeding looks bad, but it's not as bad as it looks.”

Feeding rates of the male beetles typically decline while looking for a receptive female, Bledsoe notes. If this coincides with the short period when fresh corn silks are available in a field, it's unlikely that all of the beetles on an ear are feeding.

In soybeans, the Japanese beetle's appetite is a threat. It likes to feed on the tender leaves of the plant. Treatment is warranted only when the plant reaches critical stages of defoliation.

“If they are defoliating during the vegetative growth stage then soybeans can stand about 30-40% defoliation,” says Steffey. “But if they're defoliating during blooming and pod fill, that threshold drops to about 20-25%.”

The pest is considered a minor one in most areas and since yield loss has been hard to quantify, treatment for Japanese beetle damage isn't a top concern — yet. But that doesn't mean entomologists aren't keeping an eye out.

“It will be one we'll watch in both corn and soybeans,” says Rice.