Japanese beetles like grassy areas and they also like to feed on outlying areas. Michigan chestnut growers are being advised by Michigan State University Extensions to be on the lookout as it can damage trees – especially young ones.

The adult beetles are considered a generalist pest and affect many crops, particularly irrigated turfgrass. The grubs feed on grass roots in early spring and again in fall, and can cause significant damage. Larvae prefer moist soil conditions and do not survive prolonged periods of drought.

Adult Japanese beetle. Photo: David Cappaert, MSU, Bugwood.org

Japanese beetles typically emerge in early July and feed on hundreds of different plant species, skeletonizing the tissue. If populations are high, they can remove all of the green leaf material from the plants.

They are a substantial beetle and measure 0.375-0.5 inch long. The thorax is green and wing covers are copper. There are five tufts of white hairs on both sides of the abdomen and a pair of tufts on the end of the abdomen that can help distinguish Japanese beetles from other look-alike species. The legs and head are black.

Because of their aggregating behavior, Japanese beetles tend to be found in larger groups and are typically relatively easy to spot. Pheromone and floral-baited traps are available and potentially can help indicate the arrival of adults and help estimate the potential pest pressure, but are not considered effective for trapping out Japanese beetles. Growers considering the traps should remember that they may attract beetle adults from the surrounding area and contribute to damage.

Japanese beetle feeding damage to chestnut. Photo: Erin Lizotte, MSU Extension

There are no established treatment thresholds or data on how much Japanese beetle damage a healthy chestnut tree can sustain, but growers should consider that well-established and vigorous orchards will likely not require protection. Younger orchards with limited leaf area will need to be managed more aggressively.

Managing Japanese beetles can be a frustrating endeavor, as they often re-infest from surrounding areas, especially during peak adult emergence in July. This re-infestation is often misinterpreted as an insecticide failure, but efficacy trials have shown a number of insecticides remain effective treatment options. For more information on insecticides available for treating Japanese beetles, refer to the “Michigan Chestnut Management Guide” by Michigan State University Extension.

Source: MSU Extension