The Japanese beetle, Popillia japonica, is a highly destructive pest across North America, causing widespread damage to lawns and ornamental plants. This non-native species was accidentally introduced to the United States in 1916. In Japan, the beetle is not a significant pest because local predators and pathogens effectively keep its population in check. Upon arrival in North America, the beetle found abundant food sources but lacked the specialized biological checks that regulate its numbers. This imbalance necessitates specialized controls to manage the insect’s population.
Generalist Predators of the Adult Beetle
Many common local animals opportunistically feed on adult Japanese beetles, though their impact on a large infestation is minor. Various birds, including starlings, robins, and crows, readily consume both the adults and the subterranean larvae. Small mammals (moles, skunks, raccoons, and shrews) also find the grubs to be a readily available food source when feeding near the soil surface.
A range of native invertebrates also preys upon the beetles, including spiders, assassin bugs, and generalist ground beetles. These animals are generalist hunters, meaning they do not exclusively rely on Japanese beetles for sustenance. While they contribute to population reduction, their feeding habits alone cannot control the rapid reproduction and aggregation behavior of the beetle. Effective management requires specialized agents that target the beetle’s most vulnerable life stages.
Targeting the Grub Stage: Specialized Biological Controls
The most promising natural controls are specialized biological agents imported or cultivated to target the beetle’s life stages. These agents focus largely on the larval, or grub, stage, which spends nearly ten months of the year feeding underground. Targeting the grubs prevents them from maturing into the destructive adults that skeletonize plant foliage.
Entomopathogenic Nematodes
Entomopathogenic nematodes (EPNs) are microscopic, parasitic roundworms highly effective against Japanese beetle grubs in the soil. Species like Heterorhabditis bacteriophora and Steinernema glaseri actively seek out the grubs. Once a nematode penetrates a grub, it releases a symbiotic bacterium that multiplies rapidly, causing a fatal blood infection within 24 to 48 hours. The nematode feeds on the liquefied internal tissues, reproduces, and releases new generations of offspring back into the soil.
Milky Spore Disease
Milky Spore disease is caused by the bacterium Paenibacillus popilliae, a naturally occurring pathogen of Japanese beetle grubs. When a grub ingests the bacterial spores, the bacteria multiply in its bloodstream, giving the grub a characteristic opaque, “milky” appearance. The grub dies and decomposes, releasing billions of new spores into the surrounding soil, which builds up the pathogen’s presence over time. Milky Spore provides long-term, self-perpetuating control, though it may take two to four years to achieve widespread effectiveness.
Introduced Parasitoids
Specialized parasitic insects introduced from Asia help manage the beetle population. The Spring Tiphia wasp, Tiphia vernalis, is a solitary wasp that targets the grubs. The female wasp burrows into the soil, paralyzes a grub with a sting, and attaches a single egg to the exterior of its body. The hatching wasp larva slowly consumes the grub, which eventually kills the host.
The Winsome fly, Istocheta aldrichi, targets the adult beetle stage. The female fly glues a small, white egg directly onto the adult beetle’s thorax. When the egg hatches, the maggot burrows into the beetle’s body cavity, feeding internally until the host dies. These parasitoids are highly species-specific, meaning they do not harm native North American insects, making them valuable tools for long-term biological control.
Implementing Natural Control in the Landscape
Successfully deploying these natural controls requires careful attention to timing and habitat management. The most effective window for applying grub-targeting agents like nematodes and Milky Spore is typically late summer to early fall. This timing corresponds with when the Japanese beetle eggs have hatched and the new grubs are actively feeding near the soil surface before winter.
Nematodes are live organisms and must be applied under specific environmental conditions to ensure their survival and effectiveness. They should be applied in the early morning or evening, or on a cool, overcast day, and immediately watered into the soil to prevent desiccation from sunlight. For Tiphia wasps to establish, adult food sources are required. Planting flowering plants that provide nectar and pollen (such as cherry and maple) helps sustain the adult wasps so they can continue to parasitize the grubs.
Supporting natural controls requires avoiding broad-spectrum insecticides, which indiscriminately kill beneficial predators and parasitoids. Utilizing an Integrated Pest Management (IPM) approach combines biological methods with non-chemical tactics, such as hand-picking adult beetles, providing the most sustainable defense. Natural controls are not instantaneous solutions and may take multiple seasons to reduce the beetle population, but they offer long-lasting, environmentally sound management.