Japanese beetles do twice the damage of ordinary insect pests. The larvae, called grubs, live in the soil and feed on the roots of grasses and other plants. The adult beetles feed on the leaves and flowers of over 300 trees, shrubs, and herbs. Control of Japanese beetles requires an understanding of their life cycle and a two-pronged attack – one strategy for the grubs, and one for the beetles.
The Japanese Beetle Life Cycle
Young grubs hatch in late summer, and feed on grass roots through the fall. Mature grubs become inactive as cold weather approaches, and spend the winter in the soil. In spring, these mature grubs become active again, and continue feeding on grass roots until early summer.
Adult beetles begin to emerge in late June and throughout the summer. Adults feed and mate into September. Mated females lay their eggs in excavated soil cavities by late summer.
How to Control Japanese Beetle Grubs
Biological Control: Lawn areas can be treated with an application of milky disease spores, spores of the bacterium Bacillus popillae. The grubs ingest these bacterial spores, which germinate and reproduce within the grub's body and ultimately kill it. Over several years time, the milky spore bacteria builds up in the soil and acts to suppress grub infestations. No chemical pesticides should be used on the lawn simultaneously, as this can affect the milky spore's efficacy.
Another naturally-occuring bacteria, Bacillus thuringiensis japonensis (Btj) may also be used to control Japanese beetle grubs. Btj is applied to the soil, and grubs ingest it. Btj destroys the grub's digestive system and ultimately kills the larva.
A beneficial nematode, Heterorhabditis bacteriophora, also works to control Japanese beetle grubs. Nematodes are microscopic parasitic roundworms that transport and feed on bacteria. When they find a grub, the nematodes penetrate the larva and inoculate it with bacteria, which quickly multiply within the grub's body. The nematode then feeds on the bacteria.
Chemical Control: Some chemical pesticides are registered for control of Japanese beetle grubs. These pesticides should be applied in July or August, when young grubs are feeding. Consult a pest control expert or your local agricultural extension office for specific information on selecting and using pesticides for grub control.
How to Control Japanese Beetle Adults
Physical Control: Where there is one Japanese beetle, there will soon be ten, so hand picking the earliest arrivals can help keep numbers down significantly. In the early morning, beetles are sluggish and can be shaken from branches into a bucket of soapy water.
If Japanese beetle populations are high in your area, beetle control may include making smart decisions about what to plant in your yard. Japanese beetles love roses, grapes, lindens, sassafras, Japanese maple, and purple-leaf plums, so these plants should be avoided if Japanese beetle damage is a concern.
Garden centers and hardware stores sell pheromone traps for Japanese beetles. Research shows these traps are generally ineffective for use in the home garden, and may actually attract more beetles to your plants.
Chemical Control: Some chemical pesticides are registered for control of Japanese beetle adults. These pesticides are applied to the foliage of susceptible plants. Consult a pest control expert or your local agricultural extension office for specific information on selecting and using pesticides for Japanese beetle adult control.