- Japanese beetles feed on the leaves of over 300 species of plants.
- In Minnesota, they are mostly found in the Twin Cities metropolitan area and in southeast Minnesota.
- Adults feed on the leaves of plants between the veins, leaving a skeleton of brown fibers where the leaves used to be.
- Grubs chew off grass roots and large dead patches of grass develop in grub infested areas.
How Japanese beetles got here
The Japanese beetle is a serious pest of turf and ornamental plants. Grubs feed on the roots of grass and adults feed on the leaves of more than 300 plant species.
Japanese beetles were first found in United States in 1916, after being accidentally introduced into New Jersey. Until that time, this insect was known to occur only in Japan where it is not a major pest. It is controlled in the eastern United States by soil-inhabiting protozoans that are not present in Minnesota.
There are a number of related beetles that also feed on the roots of grasses. In Minnesota, the Japanese beetle is the worst pest, so it's important to identify which species a grub belongs to as management is not the same for all species.
How to identify Japanese beetles
- Approximately one-third to one-half inch long.
- Metallic green head with copper-brown wing covers.
- Sides of abdomen have five white patches of hairs, and tip of abdomen has two patches of white hair.
Eggs are laid in the soil and generally not seen.
- C-shaped, white to cream-colored grubs.
- Up to about one inch long.
- Japanese beetles (Popillia japonica) spend the winter as grubs in the soil of turf grass areas.
- In spring, they move near the soil surface, where they finish feeding and pupate.
- They emerge as adults in late June or early July and can fly a long distance to suitable feeding sites.
- When an adult starts feeding on a plant, it emits a pheromone that attracts other beetles, so that beetles will come and join it on that plant. This behavior results in large numbers of beetles feeding on one plant while neighboring plants are left alone.
- The female releases pheromones to attract males. After mating, adult females lay up to 60 eggs in turf grass.
- The eggs hatch in about one to two weeks, and the grubs feed on the roots of grasses and other plants.
- As the soil starts to cool in the fall, the grubs dig deeper in the soil, where they overwinter.
- Adults primarily feed in July and August, although you can find some activity into the fall.
- Japanese beetles have one generation per year.
Damage caused by Japanese beetles
Adult Japanese beetle damage
Adult Japanese beetles feed on the leaves of many different plants including birches, basswood, apples, raspberries and soybeans. They feed on the leaves between the veins, so when they are finished, they leave a skeleton of brown fibers where the leaves used to be, a type of damage referred to as skeletonizing.
Damage to fruit, such as raspberries, can occur where the beetles eat flowers along with the fruit or if the beetles kill so many leaves that the plant is weakened and cannot support fruit.
Grubs chew off grass roots and reduce the ability of grass to take up enough water to withstand stresses of hot, dry weather. As a result, large dead patches of grass develop in grub infested areas.
These dead patches can be rolled back like a carpet to expose the lack of turf roots. Grubs can be found in adjacent green areas. Early recognition of the problem can prevent this destruction.
Starlings and crows, as well as moles, shrews, and skunks may be seen digging up grubs, also damaging the turf.
Grub populations between 7 and 15 per square foot can cause significant damage to non-irrigated turf. Irrigated turf can withstand a higher grub count because the increase in water compensates for the roots chewed off by the grub.
Managing Japanese beetles
For fruit growers, Japanese beetles are best controlled as adults.
Physical removal is a viable option for small fruit patches or a few trees or shrubs.
- Remove the beetles by hand and put them in soapy water.
- Hand picking is most effective as the beetles first arrive, before they release their aggregation pheromones to attract others.
- The best time to handpick beetles is in the evening and early morning, when they are less active.
- Don't use Japanese beetle traps. Research has shown that traps attract more Japanese beetles than they catch, and will typically cause more damage to plants in a garden.
Insecticides can help manage adults.
- Neem extracts like Azadractin have been shown to provide short term protection, especially if only small to moderate numbers of Japanese beetles are present.
- There are several contact, residual insecticides including permethrin, bifenthrin, malathion or carbaryl.
Timing is critical to control grub infestations. The best time to apply insecticides for grubs is from mid-July until early September. Granular applied insecticides distributed on soil with a spreader are usually the best insecticides for Japanese beetles.
There are conventional insecticides that kill grubs (imidacloprid) and biorational insecticides that conserve beneficial insects in turf (halofenozide and Acelepryn).
CAUTION: Mention of a pesticide or use of a pesticide label is for educational purposes only. Always follow the pesticide label directions attached to the pesticide container you are using. Remember, the label is the law.
When treating fruits or vegetables, be sure that the plant you wish to treat is listed on the label of the pesticide you intend to use. Also be sure to observe the number of days between pesticide application and when you can harvest your crop.
Reviewed in 2018