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Japanese beetles eating leaves
HUNGRY PEST: These Japanese beetles are having a family dinner in a Missouri farm field. However, despite their voracious appetites, they may destroy leaves during summer but have minimal effect on yield in the fall.

Japanese beetles don’t always cause crop yield loss

Hold the insecticide. Determine when beetles are eating before spraying.

Farmers scouting crop fields may see an abundance of Japanese beetles either flying around or feeding on leaves. Depending on the time of year, it may not be enough to sound the alarms or signal a treatment application.

Japanese beetles may not be as damaging as farmers think to corn or soybean crops. It all depends on feeding time, University of Missouri entomologist Kevin Rice said. Understanding when damage occurs and thresholds for treatment is key to staying ahead of this pest in the future.

Corn recon
When it comes to corn, the primary economic damage is at silk, Rice says.

In Missouri, adult beetles emerge in June and feed for 45 days. Feeding on the silk reduces pollination, which results in reduced ear fill and ultimately yield loss, he explains.

When farmers see an average of three or more adults per ear feeding on silk, they should treat with a foliar insecticide. Rice warns that one spray may not keep the beetles at bay for the entire season.

Japanese beetles move from one field to the next. Just because a farmer sprays one field and kills hundreds of them, he says, the beetles may move right back in from a neighboring field.

One more threshold to remember is that if there are other silk feeding insects present, farmers should spray when the silk is clipped to less than a half-inch and pollination is less than 50% complete.

Soybean scouting
It makes farmers uneasy walking through a field of soybeans seeing lacelike skeletons on leaves. Rice says fear not.

When it comes to soybeans, extensive defoliation looks bad, but rarely causes yield damage. “It takes a whole lot of damage before yield loss occurs (in soybeans),” he says.

The greatest effect occurs during flowering and pod fill. Soybean growers should consider insecticide treatments when damage is more than 30% at vegetative and 20% at reproductive stages of the plant.

Help on the way
Initially, the Japanese beetle was introduced in New York in the 1900s and started making its way across the country. It arrived in Missouri about a decade ago.

Rice, who came from Pennsylvania, says damage here is worse. He explains that parasitoids were released along the East Coast and worked to control the beetle. Those parasitoids have not caught up to the beetle’s movement in the Midwest. “We are at a worst-case scenario,” he says. “We hope the parasitoid will follow and knock them down.”

Until the predatory pests arrive, farmers may have one more ally in the fight against the beetle: the weather. “Populations of Japanese beetle are lower in years following a drought,” Rice says.

He suggests scouting both corn and soybeans this year for Japanese beetle, paying special attention to when they are feeding. Save money and only spray when they start to affect yield.

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