April 23, 2019
Japanese beetles (Popillia japonica Newman) are a concern every year in West/Central Illinois and other areas. The insect can be a pest in both corn and soybean. The adult beetles can interfere with corn embryo fertilization when they feed heavily on silks during pollination. In soybean, their feeding on flowers can lead to loss of potential pods and extensive leaf feeding can interfere with photosynthesis and plant energy production.
Life Cycle and Identification
The metallic green- and bronze-colored beetle (Figure 1) starts life as a white grub. When spring soil temperatures reach 50 degrees, the larval grubs move up to the soil surface to feed on developing plant roots. Near the soil surface they will feed, pupate, then emerge as adults. Adults start emerging around late May with peak emergence occurring about four to five weeks later.
Figure 1: Adult Japanese Beetle
Corn Scouting and Thresholds
Adult beetles feed on leaves, silks, tassels and pollen. Beetles may leave corn leaves skeletonized or “lacey,” but leaf feeding rarely has an economic impact on yield. Real damage comes from the interruption of pollination and partially pollinated ears from silk clipping (Figure 2). To check for embryo fertilization, carefully remove husks and shake the ear; unfertilized embryos still have silks attached (Figure 3).
Silk clipping after pollination does not affect yield potential or pollination. However, there is some consideration that beetle feeding may leave an “infection point” for ear molds. When scouting for Japanese beetle, a representative portion of an entire field should be scouted as populations of the Japanese beetle are usually higher on the field edges. An insecticide treatment should be considered in corn when the following conditions exist:
- There are three or more Japanese beetles per ear
- Silks have been clipped to less than an inch
- Pollination is less than 50% complete
- Japanese beetles are present
Soybean scouting and thresholds:
- 30% defoliation occurs prior to bloom
- 20% defoliation occurs after bloom
- Japanese beetles are still active
For additional information, please see Channel® Agronomy ADVICE, Control of Japanese Beetle in Corn and Soybean .