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Impact of Japanese beetles in corn and soybeans

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When Japanese beetles invade fields, the result is unsightly defoliation and clipping that can interfere with pollination and curb yield — especially with infestation during the reproductive stages. However, determining whether to adopt control methods can be a tough call.  

Three-step Action Plan

  1. Correctly identify Japanese beetles.
    Japanese beetles are metallic-green-colored, relatively large (⅜ inch long by ¼ inch wide) herbivorous insects. White tufts of hair on either side of their abdomen serve as their key defining feature.       

  2. Scout fields for signs of damage.
    Japanese beetles are highly mobile and feed in groups predominantly during summer months. Beginning at the top of a plant, feeding occurs between leaf veins, resulting in a skeleton like appearance of damaged tissue. They also often aggregate and feed on corn silks, especially after clipping during pollination. Because populations are generally concentrated, farmers will want to assess several locations throughout the field and estimate and record defoliation on a whole-plant basis — meaning the length of silks remaining on each plant as well as the maturity of the silks. By recording estimates of beetle numbers each week, growers can gauge population fluctuations, helping them make management and treatment decisions.

  3. Determine if insecticide treatments are warranted.
    Soybeans: If the defoliation percentage during vegetative growth stages reaches or exceeds 35%, growers should consider an insecticide application. However, the likely economic threshold drops to only 20% defoliation during bloom and pod-fill stages. Treatment will have no economic benefit once the plant reaches the R7 stage.
    Corn: Japanese beetles create the most injury during clipping silks during pollination, concentrated along the borders. If there are three or more beetles per ear, silks have been clipped to less than ½ inch AND pollination is less than 50% complete, consider a foliar insecticide during tasseling and silking. Keep in mind that migrating adults could re-infest the field after taking care of an initial population.

Japanese beetles are easy to identify but can be hard to get rid of in soybeans and corn. The best method for determining insecticide need is to estimate defoliation, identify the crop’s growth stage and monitor fluctuations in beetle populations. This can help growers make economically sound decisions for their fields.

For more information, contact your local Mycogen Seeds customer agronomist or your local ag retailer.
Impact of Japanese beetles in corn and soybeans

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