how to kill Japanese beetle in large trees

Asked August 10, 2018, 3:54 PM EDT

We have Ash trees and Japanese beetles have infested the trees. The ground around the trees are full every morning with dead leaves. These trees are full grown, about 30 feet tall. We just put up beetle bags about 30 feet away from each tree but your article says this may attract more beetles. So, what do we do to save our full grown trees? Thanks, jim


1 Response

Thank you for the question. University Extension does not recommend beetle traps because the beetles are very strong flyers and will be attracted from miles around to your traps. You aren't just trapping the beetles on your property, you actually worsen the insect problem by drawing more in.

Control of the beetles can be attempted via foliar sprays to kill feeding adults. These must be applied every few weeks to remain effective. There are systemic soil drenches and tree injections that offer more long term control but must be applied before the beetles are active and take awhile to become absorbed by the tree. There is also control of the Japanese beetle grubs in the lawn. The grubs feed on lawn grass roots in late summer/fall, burrow deep to overwinter, and emerge next summer to start the cycle over. Even if you kill every grub in your lawn, the beetles will fly in from other places to feed on their favorite food sources like your tree. As you can see, there is no easy way to battle this problem. With a tree the size of yours, I would recommend you hire a certified arborist to come out and evaluate the current health of your tree and to help you develop a plan of control for next year. Here is our publication on how to hire such an arborist:

Read more about Japanese beetles and their control in our publication and the excellent one from the University of North Dakota Extension:

For now, take good care of your tree. Water deeply each week when you don't get 1 inch of rain, and protect the trunk from damage from such things as weed whips and lawn mowers. The beetles will disappear soon and your tree will do what it can to recover for the winter. One year of severe damage probably won't harm it, but repeated attacks can weaken the tree and leave it susceptible to other insects and disease.

Thank you for contacting Extension.