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 half of an inch long with a shinny 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.
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 a bit 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 probably 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.
In the Beetles grub stages (from mid-late summer to late spring) these beetles 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, etc).
Treatment options: 1. 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. 2. 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. Japanese Beetle ‘Traps’ found commonly in hardware stores are not effective at treating populations that reach extraordinary numbers. If anything they help draw more beetles into a yard.
If you have any questions about controlling Japanese Beetles in your landscape talk to an arborist today.
For a pdf copy of this factsheet: Japanese Beetles