Pioneer Agronomist Offers Advice On Evaluating The Impact Of Japanese Beetles


Japanese beetles are serious and well-established pests of corn and soybeans east of the Mississippi River, with an emerging presence in most other corn and soybean growing states. They are relatively easy to identify by their metallic green heads, bronze wing covers and large spikes. By releasing a strong aggregation pheromone that induces group formation, Japanese beetles are often found feeding and mating in clusters.

Adult beetles are highly mobile and move frequently in the summer. Migrating adults could reinfest a field, even after an initial population is knocked down. Scouting for Japanese beetles in corn should begin in July, while soybean scouting should be done in August. If found, it’s important that growers carefully assess if they meet the damage threshold where an insecticide treatment makes economic sense.

“The biggest detriment to corn is Japanese beetles feeding on silks before pollination is complete,” said Jaime Farmer, Pioneer Field Agronomist. “When determining whether you’ve met the economic threshold for insecticide treatment, look for silks being clipped within a half inch of the ear, three or more beetles per plant and less than 50% pollination.”

For soybeans, use percent defoliation and the amount of pod fill remaining to help decide on the economics of insecticide treatment. For Japanese beetle management, it’s important to note that no transgenic or native gene resistance is currently available for either corn or soybeans. Due to the aggregation pheromone released, trapping is not recommended as it may attract more beetles.