The Japanese beetle (Popillia japonica) is a highly destructive pest that causes significant damage to landscapes across North America. While this invasive insect is a formidable presence, the answer to whether it kills trees is nuanced. Large, established, and healthy trees rarely die from a Japanese beetle infestation alone, but the defoliation can cause severe stress. However, young trees, new plantings, and trees already weakened by other factors are highly susceptible to fatal injury from this pest.
Damage: The Two Phases of Attack
Japanese beetles inflict damage in two distinct phases corresponding to their life cycle stages, both of which can compromise tree health. The adult beetles emerge from the soil typically between late June and early July and begin feeding immediately. They prefer to feed on the upper surfaces of leaves, consuming the soft tissue between the veins, a pattern known as skeletonization. This severe feeding damage reduces the tree’s ability to photosynthesize and can make the canopy appear scorched or brown.
The second, more subtle phase of attack involves the larval stage, or white grubs. After hatching in mid-summer, these grubs feed on the fine roots of grass and other plants, including trees. Their feeding on the root system of a young tree can severely limit its ability to absorb water and nutrients, weakening the tree’s foundation and making it less resilient to stress like drought.
Assessing the Threat: When Japanese Beetles Become Lethal
Damage from Japanese beetles transitions from cosmetic to lethal when certain conditions align, primarily revolving around the tree’s health and age. Healthy, mature trees can generally tolerate a significant loss of foliage during the six to eight weeks of peak adult feeding and will typically recover the following spring. This is because the damage occurs late in the season after the leaves have already produced most of the tree’s necessary food reserves for the year.
The threat becomes much greater for vulnerable hosts, including newly planted trees, saplings, and those already stressed by disease, drought, or prior injury. Trees highly preferred by the beetles are also at higher risk due to the sheer number of pests they attract, including:
- Japanese maple
- Linden
- Crabapple
- Cherry
- Birch
Repeated, heavy defoliation across multiple years depletes the tree’s stored energy reserves, a cumulative stress that can lead to secondary issues. This severe damage can attract wood-boring insects, which can then deliver a final, fatal blow to a severely weakened tree.
Effective Management Strategies
An effective control plan requires targeting the Japanese beetle at both its adult and grub stages to minimize immediate damage and prevent future infestations. For managing the adult beetles, physical removal is a simple, non-chemical option for smaller trees or low infestations. Homeowners can hand-pick or gently shake the sluggish beetles from the foliage early in the morning and drop them into a bucket of soapy water. While pheromone traps exist, research indicates they often attract more beetles to the area than they capture, potentially increasing the damage to nearby plants.
For severe infestations on larger trees, contact insecticides may be necessary. These should be applied cautiously, often in the late evening, to reduce the risk to beneficial pollinators.
The most critical preventative step involves treating the soil to control the grubs, which is best done in late summer or early fall when young grubs are actively feeding. Biological controls, such as beneficial nematodes or milky spore disease, offer long-term, low-impact solutions by attacking the grubs. Systemic insecticides can also be applied to the soil to target the larvae, providing protection for the following year. Proper cultural maintenance, including adequate watering and mulching, is important for increasing a tree’s natural resilience.