The Japanese beetle (Popillia japonica) is a destructive invasive pest, recognizable by its metallic green body and bronze wing covers. Introduced to the United States in 1916, it has since become established across much of the country, attacking over 300 plant species in both its adult and larval stages. Adult beetles cause visible damage by skeletonizing leaves, chewing tissue between the veins and leaving a lace-like pattern. Since they feed in large groups and release pheromones that attract more beetles, infestations can quickly overwhelm and defoliate plants. Effective control requires a multi-faceted approach, combining immediate physical removal with long-term preventative soil treatments and targeted insecticide applications.
Immediate Physical Removal and Exclusion
The simplest and most environmentally friendly method for managing small populations is hand-picking. This action is most effective in the early morning or evening when the beetles are less active and more sluggish due to cooler temperatures.
To remove them, knock the beetles directly off the plant and into a container of soapy water, where they will drown. Consistent daily removal is important because feeding damage releases chemicals that attract more beetles to the same plant, signaling it as a good food source. For high-value plants like roses or small fruit bushes, physical exclusion with fine mesh netting or row covers can prevent feeding entirely during the beetles’ active six to eight-week period.
Pheromone-based Japanese beetle traps should be approached with caution, as they often attract far more beetles to the area than they actually capture. The traps use a combination of a sex pheromone and a floral lure, which pulls beetles in from a wide radius. Research indicates that plants located near the traps often suffer increased damage from beetles that are attracted but not caught. If traps are used, they should be placed at least 30 feet away from the plants you are trying to protect, along the property perimeter or near less desirable plants.
Targeting the Next Generation in the Soil
A crucial part of long-term control involves managing the larval stage, known as grubs. Japanese beetle larvae are C-shaped, creamy white grubs that feed on the roots of grasses, causing patches of brown, dying turf. The timing of grub control is paramount, as treatments are most effective against young, newly hatched larvae.
The optimal time to apply grub control products is in late summer or early fall, when young grubs are actively feeding near the soil surface. Two biological options are available for this stage. Beneficial nematodes, specifically Heterorhabditis bacteriophora, are microscopic worms that seek out and infect the grubs in the soil.
Another biological control is Paenibacillus popilliae, commonly known as Milky Spore. This bacterium is ingested by the grubs as they feed, causing a fatal disease that releases billions of new spores into the soil to infect future generations. While beneficial nematodes provide immediate seasonal control, Milky Spore takes time to establish—sometimes two to four years in colder climates—but can offer lasting effectiveness for 10 to 20 years once inoculated. Proper lawn care, such as maintaining healthy soil moisture, can also reduce the attractiveness of the area for egg-laying and minimize the impact of grub feeding.
Utilizing Insecticide Treatments
When infestations are severe and physical or biological methods are insufficient, targeted insecticide treatments can provide necessary relief. These treatments are categorized as either contact sprays or systemic options. Contact sprays, such as those containing pyrethrins, kill adult beetles quickly upon direct application but offer short residual protection, often requiring frequent reapplication.
Systemic insecticides, like certain neonicotinoids, are applied to the soil or trunk and are absorbed by the plant, making the foliage toxic to the beetles as they feed. These treatments offer longer-lasting protection, sometimes for weeks or a full season, but must be applied several weeks before the beetles emerge to allow the chemical to travel through the plant. A newer option, Bacillus thuringiensis galleriae (BtG), is a bacterial insecticide that targets adult beetles and is considered a lower-risk alternative to synthetic chemicals.
Regardless of the chemical chosen, always read the product label carefully to ensure proper application and to protect pollinators. Avoid applying any broad-spectrum insecticide to plants that are currently blooming or attracting bees, as the chemicals can transfer to the nectar and pollen. Applying sprays in the late evening, after pollinator activity has subsided, can reduce the risk to beneficial insects.