The Japanese beetle, Popillia japonica, is an invasive insect that has spread across much of the United States, becoming a widespread pest. They rarely cause the immediate death of a mature, healthy tree. The threat they pose is conditional, making them a serious danger primarily to young, newly planted, or already stressed trees, as well as to particularly susceptible species. Understanding the nature of their feeding and the secondary effects of their damage is necessary to assess the risk to trees.
How Japanese Beetles Damage Plants
The destructive potential of the Japanese beetle results from the feeding habits of its two life stages: the adult beetle and the larva (grub). Adult beetles emerge in late spring or early summer to feed on the foliage of hundreds of plant species for about 4 to 6 weeks. They use their chewing mouthparts to consume the soft tissue between the veins, a characteristic damage known as skeletonizing.
This feeding pattern leaves behind a lace-like leaf that significantly reduces the tree’s ability to perform photosynthesis. When defoliation is severe, the tree must expend stored energy to produce a second flush of leaves, diverting resources from growth and defense. This attack on the canopy weakens the tree’s overall vigor throughout the summer.
The larval stage (white grub) lives underground for approximately 10 months, feeding primarily on the fine roots of turfgrass. While this root-feeding is often noticed through damage to lawns, it also affects the root systems of nearby trees. Grubs reduce a plant’s ability to absorb water and nutrients from the soil, compounding the stress caused by the adult beetles feeding on the leaves. This dual attack creates a significant energy deficit for the affected tree.
Factors Determining Tree Mortality
The direct damage from Japanese beetles seldom kills a large, established tree in a single season; mortality is often secondary and cumulative. Young trees, especially those planted recently, lack the extensive root and energy reserves needed to withstand severe defoliation. A newly planted tree that loses many leaves will struggle to establish its root system and may die from resource exhaustion.
The length and severity of the infestation are the most significant factors elevating the risk of tree death. Repeated, heavy defoliation over two or more consecutive years depletes the tree’s stored carbohydrates, which are necessary for root growth and winter survival. This weakened state makes the tree highly susceptible to opportunistic pests, such as bark beetles and wood-boring insects, or to various plant diseases that are the ultimate cause of death.
Certain tree species possess a higher inherent susceptibility, making them preferred targets for the adult beetles. Highly favored species include:
- Linden
- Birch
- Crabapple
- Purple-leaf plum
- Norway maple
Conversely, species like red maple, ash, northern red oak, and dogwood are generally avoided. When a preferred host is repeatedly targeted, the combined stress from feeding, secondary pests, and environmental factors like drought can ultimately lead to the tree’s demise.
Controlling and Preventing Infestations
Effective management of Japanese beetles requires an integrated approach that targets both the adult feeding stage and the underground grubs. Maintaining the tree’s health through proper cultural controls is a fundamental preventative measure. This includes ensuring adequate soil moisture during dry periods and applying appropriate fertilizer to help the tree recover from feeding damage.
Physical removal can be an effective control for smaller, accessible plants or minor infestations. Adult beetles are sluggish in the cool early morning and can be easily hand-picked or shaken off foliage directly into a bucket of soapy water. While pheromone-based traps are commercially available, research indicates they often attract more beetles to the yard than they capture, so they should be placed far away from preferred plants or avoided.
Chemical controls are categorized by the target life stage. Systemic insecticides, applied to the soil and absorbed by the tree’s roots, provide long-lasting protection against feeding adults. For managing the grubs, granular insecticides or biological controls, such as the bacterium Paenibacillus popilliae (milky spore) or parasitic nematodes, can be applied to the surrounding turfgrass in late summer or early fall. Milky spore is a long-term solution that may take several years to build up in the soil and provide meaningful control.