The Japanese beetle, Popillia japonica, is an invasive insect that has become a widely destructive pest in North America. This half-inch-long scarab is easily recognized by its metallic green head and thorax, contrasted with coppery-brown wing covers. Both the adult beetle and its larval stage cause extensive damage to turf and ornamental plants. Understanding the specific dietary habits of each life stage is the first step in managing this widespread garden nuisance.
The Adult Beetle’s Primary Host Plants
The adult Japanese beetle is a generalist feeder, consuming the foliage, flowers, or fruit of over 300 plant species. They exhibit a strong preference for certain plants, often aggregating in large numbers to feed communally on these favored hosts. This congregating behavior intensifies the damage, often overwhelming individual plants.
Their feeding pattern is distinctive, characterized by skeletonization. The beetles use their chewing mouthparts to consume the soft tissue of a leaf, leaving behind only the tougher, lace-like network of veins. Preferred woody plants include linden, birch, maples, and crabapples, while roses and grapevines are also highly susceptible.
Adults damage fruit and flowers, sometimes completely destroying blooms. Agricultural crops are also targets, with the beetles feeding heavily on the silks of corn plants, which can interfere with successful pollination. This wide range of feeding activity is concentrated during their adult lifespan, which typically lasts about 30 to 45 days during the summer months.
The Distinct Diet of Japanese Beetle Grubs
The diet of the larval stage, commonly called white grubs, represents a shift from the adult’s above-ground feeding habits. These C-shaped larvae live underground, primarily feeding on the fine roots of grasses, particularly in lawns and turf. The grubs move closer to the surface to feed in the spring and again in late summer, consuming the root system that anchors the grass.
This subterranean root pruning severely limits the turf’s ability to absorb water and nutrients, which is especially noticeable during dry periods. Heavily infested areas develop irregular brown patches of dead grass that feel spongy underfoot. The root loss can be so severe that the turf can be rolled back easily like a loose carpet.
While turf roots are their main food source, the grubs also consume organic matter within the soil profile. They go through three developmental stages, with the larger, mature grubs causing the most significant damage from midsummer through early fall. The presence of these grubs attracts animals like raccoons and skunks, which dig up the lawn to feed on the larvae.
Plants That Deter Japanese Beetles
Many plants are naturally unattractive or unpalatable to the adult Japanese beetle, offering a means to reduce feeding damage. Plants with tough, leathery foliage, strong scents, or bitter compounds are avoided by the pests. Incorporating these species into a landscape helps divert feeding pressure away from more susceptible ornamentals.
Several common landscape shrubs and trees are considered resistant. In flowerbeds, perennial flowers are also left alone by the beetles. Some plants, like the geranium, contain compounds that can temporarily paralyze the beetles after consumption.
Resistant species include:
- Boxwood
- Juniper
- Red maple
- Coreopsis
- Foxglove
- Lilac
Other deterrents rely on strong aromas. For instance, the sulfur compounds in chives, onions, and garlic are highly unappealing to the adult beetles. The compound nepetalactone found in catnip also acts as an effective natural repellent.