by Robert Koch (Extension Entomologist)
Japanese beetle, an invasive pest from Asia, is making its presence known. These large beetles with shiny green- and copper-colored bodies can be found feeding on many plants, including soybean, in agricultural areas that are in proximity to the Twin Cities, Rochester, and other urban areas in southeastern Minnesota. This beetle is not yet widely distributed in the state and is not likely to be in fields outside of these areas in southeastern Minnesota.
The identification, biology and management of Japanese beetles are reviewed in “Japanese Beetle in Minnesota Soybean.” Larvae of the Japanese beetle feed on roots of grasses and other plants. The adults emerge from soils in July and August and feed on many species of landscape plants, fruits, and field crops, such as soybean and corn. The feeding injury on leaves results in a characteristic lace-like pattern as the beetles feed on leaf tissues between the leaf veins. In Minnesota soybean, economic infestations of Japanese beetle have not yet been documented; however, the abundance of the beetles and their accumulating feeding injury to leaves could reach economic levels in some fields this year.
Treatment thresholds for soybean are 30% defoliation prior to flowering and 20% defoliation after flowering. To obtain an estimate of the level of defoliation for a field:
- Select at least ten plants (more for larger fields) spread throughout the field.
- From each plant, select a leaf from the top, middle and bottom third of the plant.
- Use the “Visual Guide for Estimation of Soybean Defoliation” to estimate percent defoliation for each leaf. Average the percent defoliation across the three leaves from each plant and then across the multiple plants to obtain the average percent defoliation per field.
- The average percent defoliation per field can be compared to treatment thresholds for the decision about pest control.
If threshold levels of defoliation are reached and Japanese beetles are still present, most labeled insecticides will effectively suppress this pest (follow instructions on the insecticide label). However, continued scouting is required because this mobile pest can recolonize previously treated fields. When multiple defoliating pests occur (e.g., Japanese beetle and green cloverworm), defoliation from both pests should combined and related to the thresholds mentioned above. In corn, Japanese beetle can be a concern when they clip silks.
Additional information about Japanese beetle management in yards and gardens can be found in “Japanese Beetle in Management in Minnesota.”