Know Your Enemy: Japanese Beetles

Last year I wrote a series of posts about critters we love to hate: mosquitoes, ticks, horseflies, black widows and carpenter bees. With the arrival of spring, I decided to pick up where I left off. First up: Japanese beetles or, as rose-growers call them, #@!*ing Japanese beetles.

As the name suggests, Japanese beetles – or Popillia japonica – originated in Japan. But they’ve been in the United States for a while now, having first been sighted in New Jersey in 1916. Since their debut in the Garden State (fittingly enough), they’ve spread across much of the continental U.S., though they have yet to find a foothold west of the Rockies.

For East Coast folks, like myself, they are a familiar feature of summer life. As a kid, a family friend would give me glass Pepsi bottles, with instructions to stuff them full of the Japanese beetles in her yard. I got a quarter for every bottle I filled with the shiny, green scarabs (a quarter went further in the early 80s).

Why did my family friend put a bounty on Japanese beetles? Because she was a gardener.

Japanese beetles emerge each year between late May and early June, and spend the next six weeks eating, mating and laying eggs. Mostly eating.

Adult Japanese beetles will eat the leaves – and flowers – of a wide variety of plant species (over 300, according to the USDA). But they have a particular fondness for Crape myrtles, flowering crabapples and roses (which are what I was paid to protect as a child).

Japanese beetles are drawn to floral scents, to the volatile compounds released when leaves are damaged (e.g., when beetles are chewing on them) and to the pheromones released by other Japanese beetles. In other words, if you see one Japanese beetle, you’ll probably see a lot more in the very near future. This tendency to aggregate means that Japanese beetles can strip every leaf off a tree during their short adulthood.

And Japanese beetles are also bad news for lawn enthusiasts.

After mating, females lay their eggs in turf, often targeting the well-manicured lawns where they are least welcome. In part, this is because they like grass that is moist, like an irrigated yard, to make sure their eggs don’t dry out. They also like short (i.e., mowed) grass, because it is easier for the females to reach the top layer of soil where they lay their eggs.

After about two weeks, the eggs hatch into larvae and burrow into the ground where they will spend months feeding on the grass roots. This damages the grass, often resulting in large, brown spots where the turf has died off.

So, what is a home owner to do?

First off, let’s address two fairly common myths about Japanese beetles.

Myth one: If you treat your lawn for beetle larvae, your trees and flowers will be safe. False. Adult Japanese beetles can travel miles from their birthplace to feed.

Myth two: Japanese beetle traps are an effective way to protect my plants. False. In fact, worse than false. While these traps will catch a lot of beetles, they won’t catch enough to protect your plants – and they may actually attract more beetles to your property.

If you live in an area that’s likely to have Japanese beetles, one option is to plant trees that are resistant to them, such as oaks or other hardwoods. You may also consider the use of pesticides (more information on that here).

And, because they tend to aggregate, if you shake all the beetles off your rose bushes in the morning, they may just wander off to someone else’s bushes (that are still covered in beetles).

It is important to remember that Japanese beetles usually don’t kill the plants they feed on. They just make them ugly. So you can always decide to simply wait them out.

Note: Many thanks to Dr. Steve Frank, assistant professor of entomology at NC State, for taking the time to talk to me about Japanese beetles. To keep track of pest species as they emerge each year, follow Steve on Twitter @ornapests. For other insect observations, follow him @ecoipm. Any errors in the above post are mine and mine alone.