Japanese beetles are highly invasive pests that have become as destructive as they are difficult to control. They are known to feed on many species of grasses and over 300 ornamental plants and agricultural crops. Although Japanese Beetles have been around since 1916 in the Eastern United States they have only recently become a pest in the St. Louis metropolitan area.
In their adult stage these beetles are about 1/2 inch long with a shiny metallic green and bronze body. They are usually found in large numbers. When in their larva, or grub, stage they are about one-inch-long white worms.
Significance in the Landscape
Adult beetles emerge from the ground to feed in late spring and early summer. Infested plants appear to turn a light shade of brown from a distance but up close you can see where the beetles have chewed apart foliage leaving it highly skeletonized. By late summer Japanese beetles have finished their breeding season and adults are soon gone. Weakened or defoliated trees will be stressed as they approach the peak of summer. However, by this time of the year most trees have completed the majority of their growth and are directing their energy into storing reserves for dormancy. Trees that haven’t suffered as much damage will likely be okay for the rest of the year; although it will be a good idea to watch them closely for signs of stress or other problems.
Susceptible Plant Species
In the beetle’s grub stages (from mid-late summer to late spring) they live in turf grass as well as forested areas. When they reach their adult phases, they prefer to dine on linden and birch trees as well as plants in the rose family (cherry, plum, crabapples, roses, serviceberry, etc).
Prevention and Treatment
- Spot spray them as they converge on a particular tree. This type of contact treatment may last up to seven to ten days depending on weather conditions. Multiple treatments may be needed throughout the season for optimal control.
- Control with a systemic treatment. The timing of this application is critical so that the product moves into the system when the beetle populations peak and out of the system when they subside. For linden trees the application must be made in June just as they begin to flower.
- Do not use Japanese beetle traps. They are not effective at treating populations, as studies have shown that they actually draw more beetles into a yard.
If you have any questions about controlling Japanese beetles in your landscape, contact one of our Arborists today.
For a pdf copy of this factsheet: Japanese Beetles