Japanese Beetle

Many experienced gardeners have experienced the ravenous appetite of the Japanese beetle, a medium-sized, metallic green beetle with a bronze shield. It’s an invasive pest that is common in almost all states east of the Mississippi River, including Wisconsin, and can occur in large numbers for several weeks during the summer. Adults are very destructive as they feed on the foliage of a wide variety of plants (over 350 species), while larvae damage the roots of turf and ornamental plants.

 

Life Cycle

Japanese beetles have one generation per year, and transition from egg, to larva, to adult. The larvae and adults both cause damage to plants, with very different habitats and feeding damage. Immature larvae are large, white, C-shaped grubs that live underground fall, winter and spring, near the roots of plants or deeper over the winter (usually down about 6 inches or as deep as 20 inches) and feed on the roots of plants. They need adequate moisture during the summer for eggs and newly-hatched grubs to survive. Adults emerge from the soil about mid-June to feed on flowers and foliage, mate, and then females lay eggs in the soil. Adults are most active in the afternoon in full-sun.

Susceptible Plants and Damage

Japanese beetle adults feed on over 350 different species of plants, but highly prefer grapes, canna lilies, linden trees, birch trees, and plants of the genus Prunus, such as roses, crabapples, and raspberries. They eat flowers or foliage, mostly the upper soft tissue of leaves and avoid leaf veins, leaving behind a leaf skeleton.

Japanese beetle grubs feed on the roots of plants, which is most apparent in turf grass. Damage appears as patches of pale and dying grass that are moisture and nutrient starved due to the loss of roots. Left untreated, patches become bigger and begin to merge, resulting in areas of turf that can be lifted from the soil beneath and actually rolled back. Large, white C-shaped grubs can be observed when turf is lifted.

Plants Susceptible to Japanese Beetles
PerennialsDeciduous and Evergreens
HollyhockApple
HibiscusApricot
RoseBirch
GrapeBlack Walnut
ClematisCherry
LobeliaClethra
PeonyCrabapple
AsparagusHawthorn
RhubarbHorse-chestnut
Red RaspberryIvy
Japanese Maple
Larch
Linden
Norway Maple
Pin Oak
Plum
Willow

Resistant Plants

Plants Resistant to Japanese Beetles
PerennialsDeciduous and Evergreens
ColumbineArborvitae
CoralbellsAsh
CoreopsisBoxwood
DelphiniumDogwood
Forget-me-notEuonymus
FoxgloveForsythia
HostaHemlock
Lily of the ValleyJuniper
PachysandraLilac
PoppyMagnolia
SedumPines
ViolaRed Maple
Red Oak
Redbud
Spruce
Yew

Treatment

The simplest and most effective way to control Japanese Beetle adults is by picking them off and tossing them in a bucket of soapy water. If this isn’t feasible due to large numbers, spraying Bonide Japanese Beetle Killer effectively kills adults, when used according to directions. To prevent damage to known susceptible trees and shrubs, Bonide Tree and Shrub Insect Control can be applied to the soil yearly, in early spring, to treat plants systemically. Always follow instructions on the product label.

Grubs can be treated by applying Bonide Ant, Flea, and Tick Killer Granules to soil. It can also be helpful to keep lawns a little dryer mid-summer, since eggs and young larvae need adequate moisture to survive. However, damaged lawn can recover faster with increase moisture late summer/early fall, after the egg and early larvae period has passed.

Pheromone Traps

We do not carry, nor do we recommend, pheromone traps. Studies have found that pheromone traps actually attract more beetles to your yard from surrounding areas, and they often settle down on their favorite plants before they even reach the trap.