What Iowa Farmers Need to Know as Japanese Beetle Activity Rises

Agronomists and entomologists across the state have confirmed Japanese beetles are present and active in Iowa crop fields. First confirmed in the state in 1994, the beetle has now been identified in most Iowa counties.

Many of the top corn and soybean growing states have a history with this beetle as well. In general, the beetles prefer feeding on soybeans over corn.

Where Are the Japanese Beetles?

Iowa State Extension Entomologist Erin Hodgson said Japanese beetles need about 1,030 growing degree days to complete development. The beetles will continue emergence until around 2,150 degree days. The latest maps from the Iowa Environmental Mesonet indicate most of Iowa is within that growing-degree-day window.

Logan Dinkla works for Syngenta as an agronomist covering western Iowa. He said “there have been a few reports of Japanese beetles in pockets” of soybean fields within his territory.

Hodgson confirmed folks in her lab found adult Japanese Beetles in corn toward the end of June.

Soybean fields with sandy soils seem to be at higher risk of infestation by Japanese beetle, notes Hodgson.

Japanese beetles prefer to lay eggs where grass is present since larvae primarily feed on grass roots. The edge of fields in particular can experience severe defoliation or silk-clipping.

What Do Japanese Beetles Do to Crops?

In soybeans, Japanese beetles cause defoliation. Skeletonized soybean leaves are characteristic of this beetle species. This means the adult beetles feed on the leaf tissue, but leave all the veins intact.

“Aggregations of Japanese beetle adults on soybean plants can cause heavy defoliation in a particular area, but beetles are usually only feeding in the upper canopy and on a few trifoliates,” Hodgson said.

What Should Farmers Do?

Iowa State University experts say, “It is unlikely that Japanese beetle adults will cause economic injury to corn or soybean in Iowa. However, sometimes silk clipping or defoliation could warrant treatment.”

Soybeans

Guidelines from the Iowa State Integrated Crop Management team say the treatment threshold for Japanese beetle in soybean is 30% defoliation before bloom and 20% defoliation after bloom.

The Crop Progress report published June 30 said 22% of Iowa’s soybean crop has reached the bloom growth stage, so far.

Corn

In corn, a foliar insecticide is warranted if these three conditions are met:

• Three or more beetles present per ear
• Silks have been clipped to less than a half inch
• Pollination is less than 50% complete

USDA said just 3% of the Iowa corn crop has reached the silk growth stage, so far this season.

Crop Protection

There are a number of insecticides labeled for Japanese beetle grubs and adults. “Our best recommendation is going to be an application of Endigo ZCX insecticide,” Dinkla said. “With the combination of thiamethoxam and lamda-cyhalothrin we see good control of Japanese Beetles along with other soybean pests.”

In a recent article, Hodgson pointed out, “Because adults are highly mobile, remember to continue scouting until pollination is complete. Adults can continue to reinfest fields even after an insecticide application.”