Are Japanese beetles feasting on your garden? How to keep them at bay in Pennsylvania

As we head into the dog days of summer, you might be focused on planning a last-minute vacation, spending quality time with family or helping your kids get ready for school. But if you’re in Pennsylvania or other eastern states, you might need to spend some time protecting your garden from pesky critters.

One of the most troubling plant pests observed in the Keystone State is the invasive Japanese beetle. First spotted in the U.S. back in 1916, these insects are known for their metallic green appearance and knack for feasting on the leaves, flowers and fruit of hundreds of plant species.

Knowing how to recognize these invasive insects and limit their damage can make a significant difference when it comes to the health of your yard. Here’s what you need to know about Japanese beetles in Pennsylvania.

What do Japanese beetles look like? When do they grow?

Adult Japanese beetles (Popillia japonica) are known for their striking appearance. You’ll notice a metallic green color on their head, thorax and abdomen, while their legs are dark green. Their wing covers are brown and extend all the way to near the tip of their abdomen, where you’ll see a pair of white tufts of hair. According to Penn State Extension, you can find five similar tufts along the side of both wing covers.

Japanese beetles produce one generation of offspring each year. Adults emerge from the soil in June and early July before mating and laying eggs, which will hatch later in July. Young larvae feed on turfgrass roots and burrow into the ground in the fall once temperatures drop. They resume feeding once temperatures rise in the spring and emerge as adults following a pupation period around May.

These insects’ eggs are elliptical, creamy white in color and roughly 1/16th of an inch in diameter. You can find them in moist, well-drained soil. Keen observers will notice a series of hairs and blank spaces on the back sides of these white grubs, known as a raster pattern.

Japanese beetle larvae are recognizable as soft-bodied, C-shaped grubs with a white-brown head capsule that are approximately 1 inch long at maturity. Grubs have two pairs of legs, pronounced mandibles and reddish dots that run down their sides and help them breathe, Penn State Extension writes.

Japanese beetle pupae are oblong with segmented orange or brown bodies but lack other distinguishing features, such as legs or eyes. Color resembling adult beetles will take shape as the pupae progress toward adulthood as blood enters the outer exoskeleton.

Why are Japanese beetles such a big deal?

Just like many other invasive insects — including the spongy moth and spotted lanternfly — Japanese beetles have feeding habits that threaten plants and local ecosystems. If left uncontrolled, these beetles can tear through gardens and even kill some plants or trees.

Adult Japanese beetles chew large, irregular holes between the veins of many types of leaves, often leaving behind a leaf that resembles a piece of lace, according to the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service. The damage can be extensive and, in severe cases, cause permanent damage or death to smaller trees and plants that cannot survive defoliation.

Rose bushes are a favorite target of adult Japanese beetles. They are also known to enjoy snacking on tree varieties, including birches, elms, cherries, black walnuts and some maple trees, including Norway maples.

Japanese beetles can produce larval damage as they feed while growing through the fall. Large infestations can result in the destruction of connective roots and tissue in turfgrass, possibly leading to large swaths of grass that could be easily pulled up from the ground. This can lead to uneven footing and injuries, especially when turf can easily slide and give away, Penn State Extension writes.

Weakening trees, plants and shrubs can make them more susceptible to secondary pests and diseases. According to the nonprofit Arbor Day Foundation, the estimated damage and cost of controlling Japanese beetles in the U.S. exceeds $460 million each year.

Could drought affect Japanese beetle populations this summer?

Much of Pennsylvania is experiencing abnormally dry conditions as of early August, according to the U.S. Drought Monitor. Drought conditions are most prevalent in southwestern Pennsylvania, where Somerset and Bedford counties are currently in what the agency describes as “extreme drought.”

Japanese beetles are known to give plants and trees a drought-like appearance after feeding, but actual drought conditions can negatively impact insect populations, according to Penn State Extension experts.

“With the dry situation we find ourselves in, many of these eggs may have desiccated, or dried out, and will not hatch,” Penn State Extension educator Danny Kline wrote in an email to the Centre Daily Times. “The female Japanese beetles will seek out moist soil to lay their eggs in. So, I’m not sure how big the grub populations will be this year.”

Ben McGraw, an associate professor of turfgrass science at Penn State, said well-watered home lawns could attract egg-laying Japanese beetles, whose offspring may feed on turfgrass roots.

“Since the prime egg-laying period was extremely drought-y, I would suspect that grub populations will be lower than in a rainy summer,” McGraw wrote in an email. “However, this drought may drive females to seek out the few moist turf areas, like irrigated home lawns, and lay their eggs in these soils, which may lead to greater localized damage.”

How can I control Japanese beetles?

You can scour your yard or garden for adult Japanese beetles and drop them into a bucket of soapy water. Repeating this process for a few evenings in a row can persuade beetles to leave your lawn alone, as they are generally drawn to plants by the stress chemicals they give off while under attack. Limiting damage can lessen the chances of subsequent Japanese beetle feasts.

Lawn traps are a popular control method for Japanese beetles, but they are not without their faults. Most traps are baited with lures that may feature scents of flowers or beetle pheromones that attract them into an inescapable bag. Some traps feature large panels that stun beetles if they make contact with them during flight.

Regardless of the method, bag-like traps can contain only so many beetles, and full traps near your garden may attract more beetles than your trap can accommodate. A full trap may become a target for remaining beetles, who are drawn to the area by pheromones and in search of something to eat.

Proper placement of a bag-like Japanese beetle trap is key. If your yard has room to spare, consider placing your store-bought trap far enough away from the plants you want to protect. A trap too close to your garden could simply invite beetles to feed on your plants.

Adults aside, Japanese beetle larvae can become a nuisance in your yard, too. Kline says the easiest way to assess grub damage is by performing a “tug test” on your lawn.

“Grab some handfuls of grass and pull up. If there are no grubs present, the turf will not pull up. It will still be attached by the roots, even if it has been dry,” Kline wrote in an email. “If grubs have destroyed the roots and connective tissues, the turf will pull right up like a piece of carpet that is not attached to the floor, and you will see the grubs right below the surface.”

“Grub damage will be exacerbated by dry conditions,” Kline added.

Visit extension.psu.edu/japanese-beetle to learn more about Japanese beetles in Pennsylvania. You can report sightings of invasive insects in Pennsylvania by emailing the commonwealth’s Department of Agriculture at badbug@pa.gov.