The Japanese beetle, Popillia japonica, is an invasive and highly destructive pest across much of North America. Introduced accidentally to the United States in the early 1900s, it has spread widely, causing extensive damage to turf, garden plants, and trees. Adult beetles possess a remarkably broad diet, feeding on the foliage, flowers, and fruit of over 300 different plant species. Homeowners in affected areas must be vigilant in protecting their ornamental trees and shrubs from its feeding habits.
Identifying Japanese Beetle Damage on Trees
The most defining characteristic of adult Japanese beetle feeding is called skeletonization. This occurs when the insects consume the soft leaf tissue between the veins, leaving behind a distinctive lace-like pattern. This damage can give the foliage a scorched appearance.
Adult Japanese beetles are most active during the summer, typically from late June through August. They tend to aggregate in large numbers, beginning feeding at the top of the canopy before moving downward. Severely skeletonized leaves often turn brown and drop prematurely, causing significant defoliation. The metallic green and coppery-brown adult beetles are easy to spot, often clustered on the upper surfaces of leaves where they prefer to feed in the sun.
Trees Most Susceptible to Japanese Beetle Damage
Japanese beetles are highly attracted to certain plants that release volatile chemicals. These chemical cues, often associated with fermentation or damaged tissue, signal a preferred feeding site and draw other beetles to aggregate. This tendency to mass together on a few host plants quickly leads to severe defoliation.
Ornamental trees like Littleleaf Linden (Tilia cordata) and Japanese Maple (Acer palmatum) are frequently attacked. The beetles are also strongly drawn to fruit trees, including Apple (Malus species), and various Ornamental Cherry (Prunus species) and Plum cultivars. Trees with tender, soft foliage are preferred targets, explaining the high susceptibility of River Birch (Betula nigra). Other common targets include roses, Crape Myrtle (Lagerstroemia indica), and Pin Oak (Quercus palustris). Understanding these preferences is helpful for landscape planning and prioritizing which trees need protection during the summer feeding period.
Trees Resistant to Japanese Beetles
Many common tree species possess traits that discourage Japanese beetle feeding, making them suitable choices for planting in high-population areas. Resistant species either lack the volatile compounds that attract the insects or have leaf structures too tough for the beetles to consume. The insects avoid leaves that are thick, waxy, or leathery, as well as those with chemical defenses.
Conifers, such as Spruce (Picea species) and Fir (Abies species), are rarely attacked, offering reliable evergreen options. Deciduous trees like Red Maple (Acer rubrum) and White Oak (Quercus alba) are also avoided by the adults. Other resistant woody plants include Magnolia (Magnolia species), Boxwood (Buxus species), and Holly (Ilex species).
Immediate Steps for Tree Protection
When adult Japanese beetles are actively feeding, immediate action is necessary to limit defoliation. Physical removal is the most direct and chemical-free method, effective on smaller trees or shrubs. Beetles are sluggish in the cool, early morning hours, making this the best time to shake or hand-pick them off the foliage.
The collected beetles should be dropped into a container of soapy water, which breaks the surface tension and causes them to drown quickly. For small, highly-valued ornamental trees, fine-mesh netting or row covers can be draped over the canopy during the peak feeding season. This physical barrier prevents beetles from accessing the leaves while still allowing light and air circulation.
For larger infestations where physical removal is impractical, targeted contact sprays offer a temporary solution. Products containing insecticidal soap or botanical insecticides like neem oil can be applied directly to the feeding beetles and foliage. These treatments provide a quick knockdown effect, but they must be reapplied frequently, especially after rain. Label directions must be followed precisely to ensure safety and effectiveness.