The Japanese Beetle is an invasive pest that feeds on the leaves, flowers, or fruit of more than 300 plants. Hosts include Rosaceae, little-leaf linden, birch, elm, pin oak, grapes, and other vines. They are one of the major pests in the United States, causing monumental crop damage each year.
Japanese beetles were first discovered in Minnesota in 1968. Before coming to the United States in 1916, they were only found in Japan, hence the name. It is now found throughout the eastern U.S., except for Florida, and continues to move westward.
In Minnesota, infestations happen in the Twin Cities metropolitan area and southeast region. The Minnesota Department of Agriculture maintains a distribution map of Japanese beetles.
How to identify Japanese Beetles
Before becoming adults, Japanese Beetle grubs live in the soil and feed on plant roots. These grubs are 1/8 to one inch long, c-shaped, and cream-colored with tan heads.
Adult beetles are approximately 1/3 to 1/2 inches long and have metallic green heads. They have copper-colored backs, tan wings, and tiny white hairs lining each side of the abdomen. The sides of the stomach have five white patches of hair, and the tip has two patches of white hair. Adults emerge from the soil at 950 growing degree days (GDD) and begin feeding.
Once adults, they don’t live long, but they are voracious. They attack plants in groups, which is why damage is so severe. Although the lifecycle of the adult Japanese beetle is only 40 days, it can cover a lot of ground.
A closeup of Japanese beetle (Popillia japonica) on green leaf, an invasive species to North America.
Signs of Japanese Beetle damage
Japanese beetles can cause widespread damage. They often travel long distances as they’re attracted to pheromones from other beetles.
While mature trees can tolerate feeding, young trees experience stress, injury, or death.
Skeletonized Leaves
Often, it’s easy to identify the damage. Adult beetles feed on leaves on the tops of trees. They don’t eat the veins of the leaves, leaving a lacy, skeleton-like appearance. They then work their way down the tree as they defoliate the upper canopy. Damaged leaves turn brown and may fall off.
Japanese beetles are not usually far from damaged leaves, so inspect the plant if you spot this. Also, keep an eye on the ground beneath the plant; the beetles may drop off if disturbed.
Brown Lawn Patches
Females lay eggs in the soil throughout the adult active period. Eggs hatch soon after, and grubs feed on roots in the ground before digging deeper to overwinter.
Japanese beetle grubs damage grass as they feast on the roots of lawn grasses and garden plants. This can cause brown patches in the lawn as the grass dies.
Treating Japanese Beetle infestations
Non-chemical and insecticide options exist for managing Japanese beetle adults and grubs.
Soap and Water
Japanese beetles are easy to see and slow-moving. So, they’re relatively easy to shake off plants and into a cup of soapy water. This is a time-consuming yet effective way to kill them.
This works well on small plants if caught early and checked regularly.
A selective focus shot of Japanese beetles Popillia japonica
Insecticides
Insecticides are used to either prevent or cure damage from Japanese beetles. If caught early, insecticide sprays are very effective in managing feeding. Spray applications should repeat on 7-14 day intervals until feeding ends. Contact us to have our technicians administer a treatment if you’re seeing activity.
Preventing Japanese Beetles
Unfortunately, you can’t get rid of Japanese beetles. For general prevention, keep your landscape healthy. Remove diseased trees and any premature fruits, which can attract Japanese beetles.
Beetles are also attracted to new growth, so avoid pruning that triggers sprouting. To keep trees healthy, mulch and water to help them recover from heavy feeding.
Eliminate Grubs
Turf treatments can kill grubs feeding on the lawn, reducing the number of beetles in your yard. But beetles will travel far to feed, which won’t completely solve the problem.
In the late spring and fall, spray the lawn with diluted liquid dishwashing soap. Add two tablespoons of dishwashing soap to one gallon of water per 1,000 square feet to make this mixture. The grubs will surface from the soil. Continue to spray once a week until no more grubs surface.
Companion Planting
Companion planting can be valuable in preventing pests. This involves planting species near your trees that will deter Japanese beetles. Herbs and aromatic plants, such as garlic, rue, or tansy, make excellent companions.
Japanese beetles are particularly attracted to geraniums. They eat the blossoms, get dizzy from the natural chemicals, and fall off the plant. To keep the beetles away from trees, plant geraniums nearby.
