Control of Japanese Beetles in Corn and Soybean

Japanese Beetle Damage to Corn and Soybean Plants

Japanese beetle feeding damage on corn and soybean plants can be eye catching. Their appetite for green leaf tissue can result in leaves appearing skeletonized or lacy as they feed on tissue between leaf veins (Figure 1). Though leaf skeletonization can be alarming, often times the feeding is non-economic. On corn plants, fresh silks are especially attractive and the damage can become economic if feeding results in poorly filled ears because kernel ovules are not fertilized (Figure 2).¹

Scouting and Economic Injury Levels for Corn and Soybean

Though economic levels for Japanese beetles are rarely reached, thresholds for both corn and soybean have been established.^1,2

Corn – Treatment may be warranted if there are three actively feeding Japanese beetles per ear, the silks are less than 0.5 inches long, and pollination is less than 50% complete. For threshold assessment, evaluate silk clipping on five random plants in five field locations during the first five days of silking. Scouting should continue throughout pollination.²

Soybean – Treatment may be warranted if leaf defoliation is 20% (Figure 3) for plants that have bloomed and 30% (Figure 4) before bloom.^1,2

For threshold assessment, scout the entire field for Japanese beetle presence and defoliation. The percent defoliation should be estimated on randomly selected leaves in at least five different field areas. Look for beetles throughout the plant’s canopy as they feed from the top and proceed downward. Be aware that defoliation may be caused by other insect species which should be evaluated at the same time.²

Identification and Life Cycle

Adult Japanese beetles (Popillia japonica) are about 5/16 inch long, have a metallic green head and neck region, reddish to bronze wing covers, and a row of six white bristle bunches along each side of their abdomen (Figure 5).

The adults emerge from the soil starting in late May and early June with peak emergence occurring four to five weeks later and live about 30 to 60 days. After mating, which occurs soon after emergence, the females burrow two to four inches into the soil every three to four days for several weeks to lay four to six eggs each time. In about two weeks, small grubs (larvae) emerge from the eggs.²

Japanese beetle larva are 0.5 to 1.2 inches in length, typically white, C-shaped, and similar in appearance to the larval stage of June and masked chafer beetles. Differentiation between the species can be made by observing their anal raster or hair arrangement. Japanese beetle raster arrangement is characteristically V-shaped (Figure 6). Japanese beetle grubs may heavily infest an area; however, a high grub infestation is not always an indicator of potential severe injury from adult beetles, as the beetles are very mobile and can move large distances to locate a preferred food source.²

Japanese Beetle Management

Integrated pest management (IPM) techniques, including cultural and chemical approaches, should be followed for beetle management in corn and soybean fields. Bait traps are not recommended because they can attract large numbers of beetles to a field.¹ Damage from Japanese beetles can add to other stresses the crop is experiencing, and economic thresholds may need to be adjusted if plants are under moisture stress.² This, along with commodity prices, should be taken into consideration when using thresholds to determine if insecticide treatment is needed.

Cultural practices may include controlling weedy hosts such as smartweed and evening primrose that beetles feed on before moving to corn and soybean. Where Japanese beetle presence has been historically high, cover crop selection may be altered. Studies have shown beetle favoritism to buckwheat and egg laying in perennial ryegrass and clover.³

Other influential factors for larval densities include soil moisture, soil organic matter, and tillage practices. Female beetles prefer to lay eggs in moist soil. If irrigation is a standard practice, not irrigating during egg laying may help reduce beetle populations. However, where irrigation is practiced and needed, beetles may lay eggs during the corn silking stage—a primary time for irrigation. Therefore, both factors should be evaluated closely when deciding when to irrigate. Egg laying has also been noted to be higher in no-till and reduced tillage systems because these systems usually have higher soil organic matter.³

Natural beetle predators may include grackles, starlings, and skunks.¹ Moles and shrews may consume the larvae.¹ Early season fields that are dry may increase the potential for larval mortality, as they require moisture for survival.¹

Insecticides are available to help control threshold level beetle populations. However, insecticides can also harm beneficial insects, so they should only be used when necessary. Additionally, since Japanese beetles are very mobile, reinfestation can occur after insecticide efficacy wanes. A recommended Bayer Crop Science foliar insecticide for Japanese beetle control in corn and soybean is Baythroid^® XL Insecticide (Restricted Use Pesticide). Leverage^® 360 Insecticide, a Restricted Use Pesticide, is labeled for foliar applications in soybean to help control Japanese beetle. The type of insecticide approved and the rate of application for the insecticide can be different for corn or soybean. Refer to the product label for use rates and directions.

Compared to other white grub species (June and annual chafer beetle grubs), the grub or larval stage of the Japanese beetle is rarely an economic concern for corn and soybean.² Additionally, as noted, Japanese beetle adults are very mobile and can migrate into fields quickly from nearby fields, so labeled soil applied treatments or seed treatments to help manage the larval stage may not be economical. If larval management is deemed necessary in corn, soil-applied Baythroid^® XL Insecticide, a Restricted Use Pesticide, is labeled for suppression. Certain pre-applied seed treatments by the seed supplier are labeled for control of white grub.

Sources

¹Bick, E., Hooper, B., and Flandermeyer, L. Managing Japanese beetles in Wisconsin corn and soybean fields. University of Wisconsin-Madison, Crops and Soils Division of Extension. https://cropsandsoils.extension.wisc.edu/articles/managing-japanese-beetles-in-wisconsin-corn-and-soybean-fields/
²Dean, A. and Hodgson, E. 2022. Japanese beetle in corn and soybean. Iowa State University Extension and Outreach, Integrated Crop Management. https://crops.extension.iastate.edu/encyclopedia/japanese-beetle-corn-and-soybean
³Shanovich, H.N., Dean, A.N., Koch, R.L., and Hodgson, E.W. 2019. Biology and management of Japanese beetle (Coleoptera: Scarabaeidae) in corn and soybean. Journal of Integrated Pest Management 10(1):1–14. https://academic.oup.com/jipm/article/10/1/9/5454734?login=false
Web sources verified 02/11/25. 1115_149162