Japanese beetles are common in gardens and landscapes, often raising questions about their impact and potential harm to people. Understanding their biology and behavior clarifies their role and addresses safety concerns.
Understanding Japanese Beetles
Japanese beetles are easily recognized by their shiny, metallic-green bodies and copper-colored wing covers. Adults are about 0.5 inches long and have six small tufts of white hair along the sides and back of their bodies, just beneath the wing edges. These beetles originated in Japan and were first detected in the United States in 1916. Their life cycle spans one year.
Adult females lay up to 60 eggs in the soil during summer, which hatch into C-shaped larvae known as grubs. These grubs feed on grass roots underground throughout the fall and spring, then pupate, and emerge as adult beetles in late spring to mid-summer. Adults are most active on warm, sunny days.
Are They Harmful to Humans?
Japanese beetles are not considered harmful to humans. They do not bite, sting, or transmit diseases. Their mouthparts, known as mandibles, are designed for chewing plant material, such as leaves and flowers, not for piercing human skin. Therefore, they pose no health threat to people or pets.
While some individuals might report a sensation upon contact, this is a mild prickle caused by the spiny legs of the beetle as it crawls or attempts to cling. This sensation is not a true bite and does not result in injury or allergic reactions. Any perceived “bite” is a reflexive defensive action if the beetle feels threatened or is handled.
Damage to Plants and Landscapes
While harmless to humans, Japanese beetles are pests to plants and landscapes. Adult beetles feed on the leaves, flowers, and fruits of over 300 plant species, including roses, grapes, lindens, and fruit trees. They consume the tissue between leaf veins, creating a characteristic “skeletonized” appearance, where only the veins remain. Heavily damaged leaves may turn brown and drop prematurely.
This feeding causes aesthetic damage and can weaken plants, particularly young or unhealthy ones. However, healthy, mature trees and shrubs can tolerate substantial feeding without long-term injury. Beyond adults, the grubs feed on grass roots underground, which can lead to brown patches of dead turf that can be easily pulled back like a carpet.
Control and Prevention Strategies
Managing Japanese beetle populations involves a combination of methods. For small garden areas, hand-picking beetles is an effective and environmentally friendly approach. Beetles are slower and easier to remove in the early morning or evening; they can be dropped into a bucket of soapy water to drown. This method is particularly effective because their defense mechanism is to drop when disturbed.
Other strategies include:
Using physical barriers like row covers to protect vulnerable plants.
Introducing natural enemies, such as beneficial nematodes or milky spore bacteria, to control grub populations in the soil.
Using chemical controls cautiously and according to product instructions, as some can harm beneficial insects or aquatic life.
Avoiding Japanese beetle traps, as they can attract more beetles to an area than they capture, potentially worsening the problem for nearby plants.