The Japanese beetle, Popillia japonica, is native to Japan, where its population is naturally kept in check by predators and climate factors. Since its accidental introduction to the United States near Riverton, New Jersey, in 1916, it has become a pervasive and destructive pest throughout North America. Without native biological controls, the population exploded, causing significant damage to over 300 plant species in both its adult and larval (grub) stages. Gardeners and agricultural professionals now seek organisms that consume or parasitize this pest, often focusing on biological agents that have been intentionally introduced.
General Avian and Mammalian Predators
A wide array of common animals will opportunistically feed on adult Japanese beetles and their subterranean grubs, though this predation rarely provides reliable, large-scale population control. Many birds, including starlings, robins, blue jays, cardinals, and crows, readily consume the adult beetles they find on foliage. These birds may also dig for the grubs, particularly in lawns, though their consumption is usually incidental to their general diet.
Small, ground-dwelling mammals contribute to grub control, especially when the larvae are close to the soil surface. Animals such as raccoons, skunks, moles, and shrews will actively forage for the grubs, which are a concentrated source of protein. However, the feeding behavior of these mammals often results in secondary damage to turfgrass as they rip up lawns to access the grubs. Amphibians like toads and general insect predators like spiders also consume the beetles, but their impact on the overall population is usually minimal.
Specialized Parasitic Insects
More effective biological control often involves specialized insects, known as parasitoids, that specifically target the Japanese beetle. Two key species, introduced from Asia, are now established in North America and have a notable impact on the beetle’s life cycle. These insects are considered beneficial because they are highly host-specific, meaning they do not attack native North American beetle species.
Tiphia Wasp
The Tiphia Wasp (Tiphia vernalis) is a small, solitary, black wasp that targets Japanese beetle grubs. After emerging in the spring, the female wasp burrows into the soil to locate a fully-grown grub. She paralyzes the larva with a sting and attaches a single egg to the grub’s body. The wasp larva hatches and feeds externally on the grub, ultimately killing it before spinning a cocoon in the soil to overwinter. This wasp can parasitize up to 60% of Japanese beetle larvae in an area, providing significant localized suppression.
Tachinid Fly
The Tachinid Fly (Istocheta aldrichi) is a small, gray fly that attacks the adult beetle. The female fly glues a small, pearly white egg directly onto the thorax, just behind the head, of an adult Japanese beetle. The fly larva hatches within a few days, bores into the beetle’s body cavity, and begins to feed internally. The adult beetle becomes lethargic and dies completely within five to ten days. Since this fly targets the adult stage before it can lay its full complement of eggs, it prevents the next generation of pests from developing.
Subterranean Biological Controls
The most actionable and widely recommended biological controls focus on the Japanese beetle’s larval stage, which spends most of its life feeding on grass roots beneath the soil surface. These subterranean controls include microscopic organisms that are harmless to humans, pets, and plants.
Entomopathogenic Nematodes (EPNs)
Entomopathogenic Nematodes (EPNs) are microscopic, soil-dwelling roundworms that actively seek out and infect Japanese beetle grubs. Species such as Heterorhabditis bacteriophora are highly effective because they actively hunt for their host deep within the soil. Upon locating a grub, the nematode enters through a natural opening, such as the mouth or spiracles, and releases symbiotic bacteria into the host’s bloodstream. The bacteria multiply rapidly, causing septicemia and killing the grub within 48 hours. The nematodes then reproduce inside the carcass and emerge to seek new hosts. For maximum effect, EPNs should be applied in the late summer or early fall when the grubs are young and feeding near the soil surface.
Milky Spore Disease
Another soil-based control is the bacterium Paenibacillus popilliae, commonly known as Milky Spore Disease. This microbial insecticide is highly specific to Japanese beetle grubs, which must ingest the bacterial spores as they feed on roots. Once inside the grub, the spores germinate and multiply, causing the insect’s hemolymph, or internal fluid, to turn a characteristic milky white color. The infected grub eventually dies, releasing billions of new spores into the soil to infect future generations. While it can take one to three years to achieve widespread control, the spores persist and remain effective in the soil for years, offering a self-sustaining, long-term method of population suppression.