The Japanese beetle (Popillia japonica) is an invasive species native to Japan, first discovered in the United States in New Jersey in 1916. In its native environment, natural enemies keep its population in check, but in its introduced range, it thrives without these restraints. The insect causes extensive damage in two life stages: as a larva and as an adult. The larval stage, known as a white grub, feeds on grass roots, leading to dead patches of turf that can be lifted like a loose carpet. Adult beetles, identifiable by their metallic green bodies and copper-colored wing covers, emerge in summer and consume the foliage and flowers of over 300 plant species, leaving behind skeletonized leaves. Because of this dual-stage destruction, control efforts require a comprehensive approach combining several strategies.
Cultural and Physical Management Strategies
The simplest management strategies involve direct action and habitat manipulation. One highly effective, non-chemical method is hand-picking or removing the adult beetles from plants. Beetles are often sluggish in the cool hours of the early morning or evening, making it easier to shake or knock them into a container of soapy water, which quickly kills them. Regularly removing beetles is beneficial because feeding damage releases volatile chemicals that attract more beetles to the same plant.
Physical barriers, such as fine mesh or floating row covers, protect vulnerable plants during the peak adult feeding period (late June through August). These covers mechanically prevent the beetles from landing and feeding. Cultural practices that stress the grubs can also reduce populations; for instance, keeping the soil drier during the mid-summer egg-laying period discourages females from depositing eggs in the turf.
Pheromone traps, which use a sex attractant and a floral scent, are available to capture adult beetles. However, research indicates these traps attract many more beetles to the immediate area than they actually catch. This often results in increased damage to nearby plants, so they are not recommended for protecting a home garden or landscape. They are more useful as a monitoring tool to assess the beetle population in a wider area.
Chemical Intervention for Adults and Grubs
Chemical control methods are divided into immediate treatments for adults and long-term, preventative treatments for grubs. For adult beetles feeding on foliage, contact insecticides provide quick suppression. Products containing pyrethroids, such as bifenthrin or cyfluthrin, are commonly applied, but their short residual activity necessitates repeated applications during the four- to six-week feeding season.
An alternative foliar treatment is the application of systemic insecticides, such as chlorantraniliprole, which are absorbed by the plant and move throughout its tissues. This offers longer control with lower toxicity to beneficial insects like bees, provided the product is not applied directly to flowering plants during peak pollinator activity. For grub control in turf, the timing of application is crucial, with preventative treatments applied in early summer (May through early July) before the eggs hatch.
Preventative grub control relies on neonicotinoids like imidacloprid or clothianidin, or the diamide chlorantraniliprole, which provide season-long residual control in the soil. Curative treatments, intended to kill existing, later-stage grubs in late summer or early fall, use fast-acting chemicals like trichlorfon. Controlling grubs in a lawn will not eliminate the adult beetle problem, as adults are highly mobile and fly in from surrounding untreated areas to feed on landscape plants.
Biological Agents Used in Population Suppression
Biological control utilizes living organisms or microbial pathogens to suppress Japanese beetle populations. One recognized biological agent is Paenibacillus popilliae, the bacterium that causes Milky Spore Disease in grubs. When grubs ingest the spores, the bacteria multiply in their blood, giving the grub a characteristic milky appearance before it dies.
The disease is not fast-acting and can take two to four years to build up in the soil for substantial control, as each infected grub releases billions of spores upon death. However, the effectiveness of commercial milky spore products has been inconsistent in research trials.
A more reliable biological option is the use of entomopathogenic nematodes, which are microscopic parasitic roundworms. Species like Heterorhabditis bacteriophora are commercially available. These nematodes are applied as a soil drench and actively seek out and infect the grubs, introducing symbiotic bacteria that kill the host. For best results, nematodes must be applied to moist soil during warm temperatures in late summer or early fall when grubs are actively feeding near the surface.
On a larger scale, introduced parasitic insects have been established to regulate populations. These include the parasitic wasp Tiphia vernalis, which targets the grubs, and the Winsome fly (Istocheta aldrichi), which lays eggs on the adult beetles.