The Japanese beetle is becoming an increasingly prevalent pest in the north-central region of the United States and can occasionally be an economic problem in soybean or corn fields, says Kelley J. Tilmon, South Dakota State University Extension soybean entomologist. For clarification purposes, Tilmon wants to ensure that readers do not confuse the Japanese beetle with the Asian ladybeetle, which is often called Japanese beetle by mistake.
"Asian ladybeetles are familiar to many as the yellow or orange ladybeetles that come into houses in the fall and are beneficial predators of crop pests," she says.
Japanese beetles are large – up to a half inch long – and metallic green and copper colored. Adults feed on the leaves and flowers of more than 300 plant species. They are an introduced pest first found in the United States in 1916 in New Jersey.
"Only in recent years have they become common in the Midwest," Tilmon says.
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The South Dakota Department of Agriculture monitors for this pest with traps, and it has been detected in several South Dakota counties particularly in the southeastern part of the state.
Life cycle of the Japanese beetle
Japanese beetle immatures are soil-dwelling white grubs that feed on roots and organic material and are often pests of turfgrass. The adults typically feed between the veins of leaves causing a characteristic lacy or "skeletonized" damage. They feed on a wide range of plants including various ornamentals, fruits and vegetables.
Though they are more common in horticultural settings, they will also feed in field crops, including corn and soybeans. In soybean fields they cause defoliation of leaves, which reduces photosynthesis, and in corn they feed on silks, reducing kernel set.
Japanese beetles in South Dakota fields
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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