If you had problems last summer with Japanese beetles on your property, the odds are very good they will be pests again this year. In fact, some people are already seeing them in high numbers. So what should you do?
Japanese beetles are largely a cosmetic issue and will not kill a tree unless the tree is extremely stressed and already dying. Healthy established trees can tolerate leaf feeding by these insects. Newly planted trees, or those impacted by construction or other stressors could be at risk. If you have edibles or ornamentals like grape or roses, the beetle damage will be more destructive, and you may opt to treat these plants. Whether or not you decide to act, ensure you provide deep watering to affected trees and shrubs during times of drought to help buffer the stress of leaf damage.
- If you find Japanese beetles on a healthy established tree, there is no need to treat it.
- For smaller Japanese beetle populations on young trees or plants, consider physical removal (hand picking into a bucket of soapy water).
- If you can't tolerate the way your trees or shrubs look, you could use a an insecticidal spray, but keep in mind it only remains on the tree for about two weeks and may need to be reapplied, and you will also kill beneficial insects and/or pollinators that visit flowers. To protect bees, apply insecticides during the late evening after bees are no longer active. Another treatment option is a systemic insecticide which will make the leaves and other plant parts (except rose petals) toxic to all insects (including pollinators) for a season. You should not use a systemic like imidicloprid on linden/basswood trees.
- The grub is the immature stage of the beetle, which can cause large dead spots in your lawn. If you have grub damage, you could apply a granular insecticide through early September to keep Japanese beetles from emerging next summer. Keep in mind the beetle often flies to other areas to look for preferred food sources, so even if you have leaf feeding, it doesn’t mean they are laying eggs and hatching out as grubs in your yard.
- Although they will make you feel like you are making progress, research has indicated the pheromone traps/bags are not the answer, since they just attract more beetles to your yard.
- Japanese beetles are only active for six to eight weeks, so leaf feeding typically ends around early August.
- How to manage Japanese beetles without harming the environment (PDF) - from the Minnesota Landscape Arboretum
- U of M Extension Yard and Garden News - "Don't fall into the Japanese beetle trapping trap"
- U of M Extension Yard and Garden News - "Do four o'clock flowers help?"
- U of M Extension - "Japanese beetles in yards and gardens"