Japanese beetle damage may not appear until later in the summer, but they begin laying their eggs in July. Now is the time to prevent them. Photo courtesy of David Cappaert, Bugwood.org

by Courtney Llewellyn

If you’ve seen the movie “The Mummy,” you know scarab beetles can cause a lot of damage. Japanese beetles fall into this category, and July is when they start to become active.

The invasive pests were first found in the U.S. in New Jersey in 1916, and since then have spread to most states east of the Mississippi River. They continue to spread westward, being found in Texas and the states north of it today. The adult beetles are known for their metallic green bodies and coppery wings. They have a one year lifecycle and their eggs are laid in soil in July and August – so now is the time to get ahead of them.

Jeff Fowler, a senior Extension educator in horticulture with Penn State Extension, notes that Japanese beetle are among the worst pests of turfgrass and woody and ornamental plants in the U.S. The adult beetles feed on a wide variety of plants, trees and shrubs; their large grubs feed on the roots of turfgrass.

“We cannot eliminate Japanese beetles; we need to focus on controlling their population while minimizing their impact on the environment,” Fowler said.

To effectively manage these pests, you need to address both adult and larval populations. Insecticides are the most effective method of managing infestations, but you might want to consider non-chemical controls as well. Non-chemical controls include commercial traps (which are good for attracting beetles but not controlling infestations). The traps use two types of bait – a sweet-smelling food-type lure and a synthetic sex pheromone. You can also try physical controls and biological controls (such as predatory flies and parasitic wasps). You’ll want to place traps away from plantings so you’re not luring the beetles to a food source. Fowler noted research has shown non-chemical controls aren’t as effective as insecticides, but they may help to control a localized population of beetles.

The beetles lay their eggs beginning in late July, but rainfall and soil moisture can affect beetle density. Irrigated areas will have higher densities of grubs, and grub damage can be seen in patches of dead or dying grass. The impacted areas can also become spongy. The damage usually appears in late August to early October. This type of injury is less evident in spring because turf is more vigorous then. Additionally, later growing season rainfall will encourage roots to grow, so damage will be less noticeable.

While the grubs tend to impact turfgrass, adult Japanese beetles attack more than 300 species of plants (including ornamentals like roses; fruit trees; linden, birch and elm trees; and canna, zinnia and marigolds). They cause damage to both flowers and foliage. They usually feed on upper leaf surfaces, leaving lace-like leaves. They prefer hot weather and plants growing with full exposure to the sun, according to Fowler.

In addition to the aforementioned traps and control methods, you can help limit damage by planting trees and shrubs that have proven resistant to the pests near your more valuable crops to deter them. These include American elder, American sweetgum, oaks (black, red, scarlet and white), boxelder, boxwood, butternut, common lilac, Euonymus, flowering dogwood, green and white ash, holly, magnolia, pear, persimmon, red and silver maple, red mulberry, shagbark hickory, tuliptree and white poplar.

As their territory continues to expand, it’s critical to scout for and control Japanese beetles as best you can.