Japanese beetles on Plants

Symptoms

Adult Japanese beetles chew leaf tissue between the veins and leave a lacelike skeleton. That pattern, leaves chewed between the veins with the veins left standing, is the clearest sign on the foliage. The adults also chew blossoms. Fruit may be gouged in irregular, shallow patches or completely devoured. Damage is worse on leaves and fruit higher on the plant or cane than lower down. A rose, grapevine or fruit tree that looks more ragged at the top than at the base fits that pattern. Feeding on leaves, flowers and fruit is the work of the adult beetles.

Trouble can also show in the lawn. Turf becomes spongy as grub feeding destroys the grass roots. You may also see patches of dead or dying turf. The grubs start feeding on roots as soon as they hatch. Skeletonized leaves are the adults' damage, and spongy turf is the grubs' damage. Roses, grapes, apples, peaches, plums, apricots and wisteria are all listed as hosts, as are corn and soybeans.

Causes

Two life stages cause the damage you see. Adult Japanese beetles, Popillia japonica, feed on leaves, flowers and fruit. Their larvae, called grubs, feed on grass and plant roots in the soil. As soon as the grubs hatch, they start feeding on the roots.

Adults chew leaf tissue between the veins and leave a lacelike skeleton. Adult beetles also release an aggregation pheromone that attracts more beetles. The UC Statewide Integrated Pest Management Program says lure traps can actually increase the number of beetles feeding on leaves and grass roots.

Turf damage comes from the grubs, and leaf, flower and fruit damage comes from the adults. The grubs feed in the soil, while the adults feed on the plant itself. Plants reported as hosts include roses, grapes, corn, soybeans, apples, peaches, plums, apricots and wisteria. The EPPO Global Database lists Rosa and Vitis vinifera among them.

Diagnosis

The facts on file name no look-alike problems, so this section sticks to what separates this problem from a general care fault. A lacelike skeleton, with tissue chewed between the veins, points to adult Japanese beetle feeding. Check the upper parts of the plant, since damage is worse on leaves and fruit higher up than lower down.

Look at the fruit for irregular, shallow gouges or fruit that has been completely devoured. Look at the blossoms, which the adults chew. Damage to leaves, flowers and fruit together fits adult feeding. Check the plant for the beetles themselves, because adult beetles feeding on leaves, flowers and fruit are the cause named. Check whether the plant is a listed host, such as a rose, grapevine, apple or peach.

If the turf feels spongy and has patches of dead or dying grass, the cause named is grubs feeding on the roots. Skeletonized leaves are described as adult damage, and spongy turf as grub damage.

Treatments

The treatments on file run from cultural to organic to chemical, and the order below follows that.

Inspect plants weekly from the time beetles are first noticed, and remove them by hand. Knock the beetles into a bag or a bucket of soapy water. Penn State Extension recommends weekly inspections and beetle removal from the time beetles are first noticed until they are gone.

Avoid placing Japanese beetle lure traps near susceptible plants. The traps' lure can attract more beetles than they capture and increase feeding damage. The UC Statewide Integrated Pest Management Program says traps can actually increase the number of beetles feeding on leaves and grass roots. Keep turf on the dry side in midsummer so beetle eggs laid in the soil dry out and die. Choose plant species or varieties that are less attractive to or more resistant to feeding by Japanese beetle, and remove fallen or injured fruit.

Apply a milky spore bacterial product to lawn soil to suppress grubs. It can take several years to become established. A microbial insecticide containing Bacillus thuringiensis galleriae is another organic option, and Heterorhabditis nematodes can be used to treat soil to target grubs.

A broad-spectrum insecticide can be applied to foliage and fruit, starting when beetles first cause injury and continuing as needed. The product label is the legal instruction and must be followed for the crop and site being treated.

Prevention

Keep midsummer turf on the dry side, which dries out and kills beetle eggs laid in the soil. Choose plant species or varieties that are less attractive to Japanese beetle or more resistant to its feeding. Remove fallen or injured fruit.

Keep lure traps away from susceptible plants, because their lure can attract more beetles than they capture. Adult beetles release an aggregation pheromone that attracts more beetles, which is a reason to avoid drawing them in with lures. Weekly inspections from the time beetles are first noticed are recommended.

Plants such as roses, grapes, apples, peaches, plums, apricots and wisteria are listed hosts. A milky spore product applied to lawn soil can suppress grubs, though it can take several years to become established.

Plants most often affected

The hosts on file for Japanese beetle are roses, grapes, corn, soybeans, apples, peaches, plums, apricots and wisteria. The EPPO Global Database lists Rosa and Vitis vinifera among the hosts. Adults feed on leaves, flowers and fruit, and grubs feed on grass and plant roots in soil. Lawn grass is also affected, through its roots.

• American Basswood
• Blackeyed Susan
• Cherry Plum
• Common Hop
• Confederate Jasmine
• Eastern Redbud
• European Beech
• Garden Cosmos
• Glossy Buckthorn
• Littleleaf Linden
• Purple Coneflower
• Purple Foxglove
• Sweet Basil
• Virginia Creeper