Treating Japanese Beetles

We occasionally publish an answer to one of the many e-mails we regularly receive from tree owners looking for advice from a board certified arborist. This question arrived in our inbox in mid-August:

Dear Vineland,
My birch in the front yard is shedding its leaves. They are being eaten, but I can’t see any critters. The leaves look like lace, then turn brown and fall off. This has been happening for about 2-3 weeks. Do you know what might be going on? About 1/2 the leaves have fallen.

Help!
Sara
Minneapolis

Hi Sara,
You are describing Japanese beetle damage to a “T.” Your birch is not the only tree Japanese beetles are attracted to; they feed on hundreds of species. I was near your house two weeks ago at the U of MN golf course, looking at trees of course, and noticed most of the linden trees have completely turned brown from Japanese beetle feeding damage. The number of Japanese beetles has been growing across the Twin Cities for the past few years. Adults feed on leaves from early July through mid-August. They will emerge from the soil next June and likely return to feed on your tree.

Japanese beetles are party beetles; they are attracted to each other by the scents given off while feeding. Therefore, early detection is key to prevent attracting more beetles. I usually see the first on grape and other vines in early July, at this stage handpicking is a viable option. Trapping them with pheromone traps is another option, but keep in mind the pheromones may attract more beetles. If you try this keep the trap away from the target plants. Spraying options include spraying the lawn to kill them while they feed on grass roots before they emerge from the soil. This may work, but they can fly into your yard from a couple miles away. We could also put an insecticide inside the tree that will kill them when they feed next year.

Many of our customers are riding this wave of Japanese beetles out, waiting to see if it’s as bad next year as this year before committing to an insecticide treatment plan. Also, keep in mind Japanese beetle feeding does not kill your tree.