Japanese Beetles Considerations for Corn and Soybeans

The high number of Japanese Beetles emerging might result in some corn and soybean fields, or parts of some corn and soybean fields needing to be treated with an insecticide. This is not a message to get out and spray your fields with an insecticide to control Japanese beetles, if numbers in your fields are low enough there is no need to. The take home message is keep an eye on fields and know what the thresholds are to justify a treatment. Margins are too tight to be spending money on treatments that are not necessary.

The recent Wisconsin Department of Agriculture Pest Management Bulletin survey staff indicated that moderate counts of the beetles were found in the edges of a few corn fields this past week, and some beetles were found at about 35% of sites scouted.

For corn, the primary concern is to protect the silks from clipping since heavy beetle feeding on corn silks can impair pollination. Treatment may be justified for fields with three or more beetles per ear and silks that have been clipped to ½ inch when pollination is occurring (less than 50% complete).

Japanese beetles aggregate on plants in the edge rows, emphasizing the importance of obtaining a representative sample from several areas throughout the field before making control decisions. Border row spot treatments may be sufficient if the beetles and damage are confined to the field edges. Beetles must be on the outside of the ear to be killed by contact insecticides.

Soybeans across the southern half of the state are showing 2-18% of plants with light to moderate leaf injury by a combination of Japanese beetles, bean leaf beetles, grasshoppers and various caterpillars.

Leaf injury by these defoliators should not be allowed to exceed 20% (incidence and severity) between the bloom and pod-fill stages and 30% in the pre-bloom soybeans.

The UW-recommended sampling method is to select 10 plants throughout the field, choosing a trifoliate from the upper, middle and lower canopy on each plant, for a 30-leaf sample. Compare the 30 leaflets with an online defoliation estimating guide to determine the average percent defoliation, and if feeding is progressing through the canopy. Scouting several areas in the field interior, in addition to field edges where beetles are most numerous, is required for an accurate assessment. Spot treatment may be considered for severely defoliated border areas. Recall that soybeans can tolerate considerable defoliation without yield loss and defoliation is commonly overestimated.