The Japanese beetle, Popillia japonica, has spread throughout much of North America since its introduction. This insect causes significant economic damage to a wide range of plants, including turfgrass, ornamental shrubs, fruit trees, and agricultural crops. Managing this pest is challenging because it attacks plants in two distinct life stages: as a soil-dwelling grub and as a foliage-feeding adult. Effective control requires a targeted strategy that interrupts the beetle’s life cycle.
Recognizing the Pest and Its Lifecycle
The adult Japanese beetle is approximately half an inch long, easily identifiable by its metallic green head and thorax, and its bronze or copper-colored outer wings (elytra). A distinct feature is the presence of five small tufts of white hair along each side of its abdomen. This adult stage is active for about 4 to 6 weeks, typically emerging from the soil in late May or early June, with peak feeding occurring in July.
The larval stage, commonly known as a white grub, is equally damaging beneath the soil surface. These grubs are creamy white, C-shaped, and can grow up to one inch long, feeding primarily on the fine roots of turfgrass and other plants. Most of the beetle’s life cycle, about 10 months, is spent in this grub stage. Grubs burrow deeper to survive winter before moving back up to feed and pupate in the spring. The adult female prefers to lay 40 to 60 eggs in moist, well-irrigated turf in mid-summer, which hatch into young grubs about two weeks later.
Non-Chemical and Physical Removal Strategies
For immediate and localized control on high-value plants, physical removal is a highly effective, non-chemical approach. The simplest method is hand-picking the adult beetles, which can be done most efficiently in the cooler parts of the day, such as early morning or late evening, when the insects are less active. When disturbed, the beetles instinctively drop straight down, a behavior that can be used to collect them.
To capitalize on this defense mechanism, hold a bucket or jar of soapy water directly beneath the beetles before tapping or shaking the branch they are on. The soap breaks the water’s surface tension, causing the beetles to sink and drown. Consistent removal is important because feeding beetles release a scent that attracts more beetles to the same plant.
Cultural controls can reduce the beetle’s presence by making the area less hospitable or by protecting vulnerable plants. Planting species that Japanese beetles avoid, such as magnolias, oaks, and dogwoods, limits available food sources. For small to medium-sized plants, tightly covering them with floating row covers or fine netting during the six to eight-week feeding period provides a physical barrier. Reducing lawn irrigation during the peak egg-laying period in mid-summer makes the soil less moist and attractive for females.
Pheromone traps are generally not recommended for residential use. These traps use a floral lure or pheromone to attract beetles, and while they can catch thousands, they often attract many more beetles to the area than they capture. This influx can lead to increased feeding damage on nearby plants as beetles land on foliage before finding the trap. If traps are used, they must be placed at least 30 to 40 feet away from susceptible plants to minimize this unintended concentration of the pest.
Targeted Biological and Chemical Management
More aggressive management options involve targeting the beetle’s life stages with biological or chemical products, which requires careful timing. Management of the grub stage is a long-term approach aimed at reducing the population that will emerge as adults the following year.
Beneficial nematodes, specifically Heterorhabditis bacteriophora, are microscopic roundworms applied as a soil drench in late summer or early fall when the young grubs are actively feeding near the surface. These nematodes seek out and parasitize the grubs, offering an environmentally conscious form of control, though they may require annual reapplication.
Another biological option for grub control is Paenibacillus popilliae, a bacterium that causes “milky spore disease.” The grubs ingest the spores from the soil, which then multiply inside them, leading to a fatal infection. Milky spore can take several years to establish and provide maximum control, but once established, it offers long-lasting suppression. Chemical options for grubs include systemic insecticides like imidacloprid or trichlorfon, applied to the soil in late spring or early summer to prevent damage, or in late summer for curative control of newly hatched grubs.
Controlling the adult beetles focuses on protecting foliage from immediate feeding damage. Contact sprays like neem oil or pyrethrins offer quick knockdown and are considered less toxic than synthetic alternatives. Neem oil also acts as a deterrent by making the plant less appealing to feeding beetles. These sprays should be applied in the late evening, after pollinator activity has ended. Systemic insecticides can also be applied as a soil drench in the spring, which the plant absorbs, making the foliage toxic to beetles that feed on it in the summer.