Agronomists: Scout Now For Japanese Beetles

A number of trappings and large swarms of Japanese beetles are surfacing in central portions of the Corn Belt, say experts at Pioneer Hi-Bred, a DuPont business. Growers should scout both corn and soybeans now for potential crop infestations.

“The heart of the Corn Belt encounters the pest each year,” says Marlin Rice, Pioneer senior research scientist. “Missouri and Illinois, specifically, see heavy Japanese beetle infestations. Growers need to do a complete evaluation of fields before deciding to treat for Japanese beetles because that may not be necessary.”

Scott Dickey, Pioneer area agronomist in Missouri, says currently populations of the pest vary across Missouri with large numbers in the eastern, central, and southwestern portions of the state.

“Typically, the pest populates east-central and southwestern Missouri, but this year we’ve seen reports of the pest in multiple counties across the state,” he says. “Even driving through portions of southern Missouri, large groups of the pests are visible from the roadside.”

Growers should scout multiple areas in a field to determine infestation levels. Japanese beetles are extremely mobile, and once feeding begins, the pests emit feeding or aggregation pheromones attracting other beetles to the same location.

“Japanese beetles are more prevalent in sandy or lighter soils,” Rice says. “Growers should focus their attention on fields with a history of the pest.”

Japanese beetles defoliate soybean plants and clip corn silks. According to the University of Missouri, growers should look for a lace-like pattern on soybean foliage, starting at the top of the plant moving downward. Adult Japanese beetle feeding can cause severe damage to tassels and corn silks.

“Adults are about ½-inch long, metallic-green beetles with copper-brown wing covers,” Dickey says. “Five white hair tufts projecting under the wing covers on each side and two white hair patches at the tip of the abdomen distinguish Japanese beetles from similar beetles.”

Without careful observation, growers can confuse Japanese beetles with adult green June beetles. Japanese beetles are smaller than June beetles.

“Adult Japanese beetles emerge from the soil from late May to early July and feed for approximately 60 days,” Dickey says.

According to the University of Missouri, silk feeding can disrupt corn pollination and result in substantial yield losses. Foliage feeding is less damaging to soybeans, although newly planted double-crop soybeans may sustain yield loss.

“Applying an insecticide is a difficult decision,” Rice says. “For growers who have seen yield loss in the past, an insecticide might be necessary.”

Rice says soybean plants can withstand a fair amount of damage without affecting yields.

“Plants may take up to 20 percent defoliation,” he says. “Seeing the pest is not justification for an application.”

Rice says control may be necessary when plants reach 30 to 40 percent defoliation prior to bloom, 15 to 20 percent defoliation from bloom to pod fill or 25 percent defoliation from pod fill to maturity. Growers should consider spot-spraying areas of the field with heavy infestation.

The grub stage of this pest feeds on both corn and soybean roots, with most feeding occurring in late June through August. Damage to plant root hairs may result in poor uptake of water and nutrients, or it could reduce stands through plant mortality.

“In soybeans, the usual pattern of Japanese beetles is acute damage to a few plants or in localized areas of the field,” Dickey says. “Since Japanese beetles contribute to the general defoliation level, the economic threshold is similar to many other foliar-feeding insects.”

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