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Japanese Beetle Identification and Management in Soybeans

Once it’s July, Japanese beetles start laying eggs and will continue to into August. But whether larvae or adults, Japanese beetles are problematic at both stages.

Life Cycle and Pest Identification

Japanese beetles overwinter in the soil, and as soil temperatures warm, larvae move closer to the surface and pupate. Adults begin to emerge in mid-June, females lay eggs in July and August, and as eggs hatch in the soil, larvae feed on roots and decaying plant material. Since larvae are about ½ to 1 inch long and cream-colored with a brown head, they can be confused with other common soil grubs. Adult Japanese beetles are about ½ inch long and are shiny metallic green with metallic bronze wings. Adult Japanese beetles also have 6 white tufts of hair along their abdomen. Japanese beetles complete one life cycle each year. 

Injury to Soybean Fields
Japanese beetles feed on more than 300 plants, including field crops and ornamental plants. As pictured above, Japanese beetles feed on leaf tissue between the veins, resulting in a distinctly “skeletonized” look, with leaf veins remaining and the leaf tissue removed. When Japanese beetles continue feeding on leaves, they remove more leaf tissue and reduce the leaf surface area needed for photosynthesis, potentially leading to reduced grain fill and lower yields.

Scouting Tips
Japanese beetles provide the greatest risk of injury during crop reproductive stages. Scout entire fields for signs of Japanese beetle damage. Japanese beetles tend to gather in sections of fields so if only portions of fields are scouted, this may give the appearance that infestation levels are higher than they actually are. Scouting the entire field will give you a better view of just how much pressure is present. Because of how mobile Japanese beetles are, feeding damage is generally higher along the borders, as they move into the field from other host plants. 

Japanese beetles have several look-alikes, including the false Japanese beetle, or sand chafer, so be careful when scouting. While false Japanese beetle adults are about the same size as Japanese beetles, they don’t have a metallic green head. Another difference is that false Japanese beetles may range from coppery brown to black. And although false Japanese beetles may have some white hairs along the abdomen, they aren’t organized into tufts.

Treatment Thresholds
To prevent yield loss, consider an insecticide treatment if defoliation is 30% or greater during vegetation, and more than 20% during the reproductive stages. If your soybean fields hit these thresholds, apply an insecticide like Endigo® ZC, which combines 3 industry-leading technologies for quick knockdown and extended residual control. Keep in mind that because of their mobility, Japanese beetles may repopulate an area that was recently treated. If that’s the case, retreatment may be necessary to protect leaf tissue during pod fill.

Contact your Golden Harvest Seed Advisor with questions or for additional agronomic insights.

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