Japanese beetle larvae, Popillia japonica
Japanese beetles have been one of the key insect pests of both turfgrass and landscape plants in the eastern US and now in the Western U.S. These beetles cause significant defoliation and damage to leaves and flowers of many plants. The adult Japanese beetle has an oval body and is about 15 mm (0.6 inches) in length. It is generally metallic green with coppery brown wing covers, which do not quite cover the tip of the abdomen. Along the sides are five patches of white hair tufts. The antennae are clubbed at the end and may spread to a fan-like form. Japanese beetle larvae are white grubs that feed on the roots of grasses. They have a creamy white body with a dark head and the legs on the thorax are well developed. Normally the body curves into a C-shape. These features are also typical of other white grubs found in association with turfgrass in Colorado, such as masked chafers and May/June beetles which are discussed in factsheet: ‘Billbugs and White Grubs’. Japanese beetle larvae are slightly smaller than these other species when full grown, but they are best distinguished by their iridescent green and copper color and the pattern of hairs on the abdomen (‘rastral pattern’), which forms a distinctive V-shape. The eggs of Japanese beetle can be cylindrical or spherical.
- The Japanese beetle is an invasive species in the United States. It is currently established in 28 states and is continuing to expand its range. It was first detected in Colorado in 1997.
- Japanese beetle adults feed on fruit, foliage, and flowers of over 300 plant species. The larvae primarily feed on grass roots.
- Signs of feeding injury in turf include brown or spongy patches.
- Japanese beetle traps can capture many adults but have never been shown to reduce damage to nearby plants.
- Japanese beetle larvae can be controlled with certain biological controls and insecticides.
White grubs (larvae) of the Japanese beetle. Image credit: David Shetlar, Ohio State University
Adult of Japanese beetle. Image credit: USDA ARS Photo Unit, USDA Agricultural Research Service, Bugwood.org
Japanese beetles mating. Image credit: Ada Szczepaniec, Colorado State University, Fort Collins, CO
Adult of T. vernalis, which is a parasitoid wasp that can attack the larvae of Japanese beetle. This wasp was intentionally introduced to the United States in the 1920s to help suppress populations of Japanese beetle. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Turf damaged by feeding of Japanese beetle larvae. Image credit: M.G. Klein, USDA Agricultural Research Service, Bugwood.org
Adults may begin to emerge from the soil in early June and are usually most abundant from late June through early August. As adults, Japanese beetles can be found aggregating on the sunny sides of plant foliage, fruit, and flowers where they feed and mate. Mated females seek areas where soil is suitably moist to lay a small cluster of eggs among plant roots. A total of 40-60 eggs may be laid, singly, by each female beetle about 15-20 cm (5.9-7.9 inches) beneath the surface of soil. Turfgrass with ground cover, moderate to high soil moisture, and loose sandy soils are highly attractive oviposition sites for Japanese beetles. The eggs hatch about two weeks later, and the grubs (larvae) begin feeding on nearby plant roots. Eggs and young larvae are susceptible to desiccation and may die if soil temporarily dries out during this period. The larvae continue to feed and develop through three instars until soil temperatures drop to about 16°C (60°F), at which point the mature larvae overwinter in the upper 5-15 cm (2-5.9 inches) of soil where they can tolerate temperatures as low as -13°C (9°F). During extremely cold periods, larvae tunnel deeper in soil. Activity resumes the following spring once soil temperatures reach 10°C (50°F). The mature larvae begin to pupate after feeding for 4-8 weeks. The pupal stage can last anywhere from one week to 17 days. One generation is produced per year.
Larvae of Japanese beetle feed on the roots of turf grasses, lowering its aesthetic value. The consumption of roots limits the plant’s ability to acquire water, which can lead to brown and spongy turf or irregular patches of turf that can be rolled. However, turf can often compensate for feeding injuries in the absence of other plant stressors. It is likely there will be increasing turfgrass damage in areas where this species becomes established, adding to the damage done by native white grubs present in Colorado turfgrass (e.g., masked chafers, May/June beetles).
To detect eggs and young larvae, samples of turfgrass should be examined after adult emergence. Turf samples should be taken following the emergence of adults. This involves cutting four 6’’ x 6’’ squares with a hand trowel to expose the root zone for inspection. Roots should be carefully examined for the presence of grubs. If no grubs are present, other factors may be responsible for stressed or dying roots such as disease or moisture stress. For a more detailed sampling protocols in turfgrass, visit the Utah State University factsheet.
Certain grass varieties, such as Kentucky bluegrass (Poa pratensis) are less suitable hosts for Japanese beetle larvae than other varieties, and hybrid bermudagrass discourages oviposition by Japanese beetles. It is important to note that turfgrass varieties differ in their susceptibility to insect pests, and selecting the best variety for optimal resistance should consider the assemblage of turf pests in an area. For example, soils with various zoysiagrass cultivars are highly attractive oviposition sites for Japanese beetles, but the same cultivars have shown resistance toward other turf pests such as the western chinch bug (Blissus occiduus) and fall armyworm (Spodoptera frugiperda).
Mowing height can also affect the susceptibility of lawns to grub damage. Generally, allowing grasses to grow taller also increases root mass and the plant’s ability to tolerate some feeding injury. Lawns mowed shorter will have smaller roots and are more susceptible to grub damage. Allowing the top couple inches of soil to partially dry during peak-beetle flights when eggs are being laid and hatching—July and early August—can kill eggs and early-stage larvae. In cases where grubs have already caused some root injury, irrigating in late summer or fall can help promote regrowth of roots. Applying nitrogen-containing fertilizer to the soil is recommended in fall. Anything that can improve growing conditions – watering, fertilization, core aeration, mowing – will allow turfgrass plants to better tolerate root damage white grubs produce.
Two species of parasitoid wasp, Tiphia vernalis and T. popilliavora, and one parasitoid fly, Istocheta aldrichi, have become established in the northeastern United States. However, these parasitoids are not commercially available for purchase and do not provide reliable site-specific control of Japanese beetles.
Soil drench applications of insect parasitic nematodes can provide effective control of Japanese beetle grubs in lawns. (These organisms are discussed in more detail in Extension Fact Sheet 5.573, Insect Parasitic Nematodes). Heterorhabditis spp. are commercially available nematodes that can decrease larval populations of Japanese beetle by up to 90% in one year. Applications of H. bacteriophora are made as a soil drench, preferably during cool, overcast periods, and must be immediately watered into the turfgrass. They should be applied when Japanese beetle larvae are present and active.
Another potential biological control option for Japanese beetle control is milky spore (Paenibacillus popilliae), a bacterium that produces “milky disease” in Japanese beetle grubs. Milky spore can be applied to turfgrass where Japanese beetle grubs are active and may infect some of the grubs, producing a chronic infection that reduces survival and reproduction. While commercial formulations of P. popilliae have not been shown to consistently reduce larval populations of Japanese beetle, the bacterium can provide some long-term suppression of the beetles if it becomes established in the soil. In areas of the eastern United States, where milky spore has long been widespread, it annually infects a small number of grubs, resulting in some reduction of the Japanese beetle populations (less than 5%).
Applications of Bacillus thuringiensis japonensis (Btj) can be effective against immature Japanese beetles, specifically first and second instars. The entomopathogenic fungus Metarhizium brunneum F52 (formerly Metarhizium anisopliae F52) can infect larvae of Japanese beetle, but its efficacy is highly dependent on abiotic conditions such as temperature and soil moisture.
The incorporation of multiple pest management tactics for Japanese beetle control, rather than relying solely on insecticides, is highly recommended. This will help maintain the efficacy of currently available insecticides since resistance has been documented in populations of Japanese beetle larvae.
Several insecticides are available that can provide excellent control of Japanese beetle grubs in lawns. Most commonly available are insecticides that are applied preventively to kill young grub stages. Applications of these types of products are best made just before eggs hatch or shortly after (typically mid-June to early July). There can be some risk to pollinators if insecticides are applied to lawns that have flowering plants attractive to bees. If flowering plants are present, the site should be mowed immediately before treatment to remove the attractive blooms.
Althoff, E., and K. Rice. 2022. Japanese Beetle (Coleoptera: Scarabaeidae) Invasion of North America: History, Ecology, and Management. Journal of Integrated Pest Management. 13(1): 1-11. Available https://academic.oup.com/jipm/article/13/1/2/6503655
Wood et al. 2009. Ovipositional preferences of the Japanese beetle (Coleoptera: Scarabaeidae) among warm- and cool-season turfgrass species. Journal of Economic Entomology. 102(6): 2192-2197. Available https://pubmed.ncbi.nlm.nih.gov/20069848/
Behle et al. 2015. Evaluation of Metarhizium brunneum F52 (Hypocreales: Clavicipitaceae) for Control of Japanese Beetle Larvae in Turfgrass. Journal of Economic Entomology. 108(4): 1587-1595. Available https://academic.oup.com/jee/article/108/4/1587/2380836