Though still a minor field crop pest, Japanese beetle outbreaks are becoming more common in Illinois and Iowa soybeans and corn. So far, in South Dakota, most reported problems with Japanese beetles have been in gardens near urban centers, but as it becomes more common in South Dakota, producers should also be on the lookout for this insect in crops.

Japanese beetles have one generation per year and overwinter as grubs in the soil. Adults emerge from the soil in late May or early June and can be found through early September. Feeding damage is most noticeable in July and August.

"Japanese beetle feeding damage in soybean may be confused with bean leaf beetle feeding because both make holes in the leaves," Tilmon says.

The difference she says is that bean leaf beetle feeding produces more smooth-edged "shot-holes" in the leaves, whereas Japanese beetles create a lacy patchwork of holes between the veins.

"Also, unlike bean leaf beetles, Japanese beetles are not shy or skittish and are usually found easily at the scene of their crimes. Damage often appears first at field edges," she says.

Soybeans can bear a fair amount of defoliation before yield is lost, so modest numbers of Japanese beetles and other defoliators can be tolerated.

Tilmon says to consider management when total defoliation from all leaf-feeding pests reaches 40% in pre-bloom, 20% during bloom and pod-fill and 35% from pod-fill to harvest. Consider the whole plant when making this decision, not just upper leaves. If beetles are aggregated in border rows, consider an edge treatment first.

A number of pesticides are labeled for Japanese beetle control in soybean. See the SDSU Extension 2013 South Dakota Soybean Crop Protection Guide for examples.


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