Japanese Beetles Are Back: How to Deal With Them

Back without popular demand: Here comes the annual showing of Japanese beetles, the embodiment of beauty and the beast rolled into one. The four- to six-week period of intense activity by the gleaming, copper-colored adult Popillia japonica is underway.

These beetles may seem to have it in specifically for your roses, raspberries, crab apples or grapes, but those are just a few of the 300-plus plant species they are known to feed on in North America.

The expert advice might sound counterintuitive: Stop trapping them. (Farewell, beetle bags, despite the marketing promises.) And maybe hold back on watering lawns in the July heat, as female beetles will be seeking a moist spot to lay eggs.

Yes, those are steps toward making peace with this here-to-stay invasive pest, which scientists have sought to subdue since shortly after it was identified in New Jersey in 1916.

Nearly a century later, a 2015 U.S.D.A. homeowners’ guide to Japanese beetle management put the cost of control in the United States — including the removal and replacement of damaged turf — at $460 million annually. Half of that damage is caused not by the adults, but during the beetles’ larval stage, by the grubs.

From their Northeastern start, the beetles have managed to establish populations throughout the Midwest and are making headway into Nebraska and neighboring states, and even Colorado, said Dr. Potter, who described them as “not new in the Great Plains, but on the move.”

A worrisome detail there: The beetles eat flowers of milkweed (Asclepias), biting into their nectaries and draining them. The plants then fail to set seed — yet another potential threat to the shrinking milkweed populations, which, in turn, threatens the monarch butterfly.

And onward the beetles march. The Sacramento area is now trying to eradicate them before they become established, as are locations in the Pacific Northwest, including Portland, Ore., and British Columbia.

Europe is also under pressure. This most recent fight will be especially challenging, Dr. Potter said, in the same way that it is in the United States for organic grape or blackberry growers. Europe does not have a chemical lawn-care industry like we do, and chemical pesticides are often the go-to for tackling grub infestations here. Overall, European restrictions on synthetic insecticides are far more stringent.

“It is a major concern — imagine what it would do in French vineyards,” Dr. Potter said. “They can’t spray their way out of this problem.”

Ninety percent of insect species are specialists, focusing their diets on no more than two or three plant families, Dr. Potter said. Not this one.

The Japanese beetle is not just a generalist, but “an extreme one,” he said. “Even gypsy moths don’t feed on as many plants as the Japanese beetle. It’s probably the champion insect in North America — remarkable for a nonnative.”

A Japanese beetle’s gut has very strong enzymes that allow it to tolerate a wide range of chemicals in plants, and those enzymes are revved up by an extra step — an appetizer, you might say: The beetles take a test bite that elevates their stomach enzymes.

And they prefer not to dine alone. You’ll rarely see a single beetle feeding on a plant. Instead, they aggregate.

Dr. Potter’s 1990s research demonstrated what draws additional beetles once the feeding begins: They are attracted to the volatile compounds released by damaged foliage, he said, “like sharks to a blood trail.”

Tissue between the leaf veins is chewed out, and the foliage skeletonized. Flowers are appealing targets as well, as are overripe fruit like blackberries — “a sugary beetle-energy drink,” Dr. Potter said.

There is sex involved, too, which he described as “lots of beetle orgies on the plants.” Between meals, females fly off, burrowing several inches into the soil to lay five or six eggs at a time. They return to feed and mate again after a day or several, a process they’ll repeat maybe a dozen or so times in their lives.

In her short adulthood, a female can lay 40 to 60 eggs. Those eggs quickly become inch-long grubs that live below ground until pupating the following spring and emerging as adults in summer.

With two scent lures — an intense floral one and a synthetic pheromone to lure males — beetle bags do a great job. But they work too well in most settings, attracting far more beetles than they trap, from neighboring yards and beyond.

“We typically see more damage where traps are used,” Dr. Potter said, an insight derived from research as far back as the 1980s.

Traps are powerful tools, however, for surveillance and interception in places where the beetles are not yet established — at airports, for instance, and in nursery areas in northern Italy, Germany and Switzerland.

Gardeners can make lawns more resistant to the effects of the grubs’ root-chewing by elevating the mower’s cutting height. “It helps the turf grow deeper roots, so the lawn can tolerate more grubs before you see damage,” Dr. Potter said.

For a more grub-resistant lawn in the Northeast, fertilize cool-season grasses in the fall, not spring.

And again: Don’t overwater during July. Let lawns go dormant, or something close to it; in dry summers, beetles avoid laying eggs in nonirrigated lawns. “If you are the only lawn on the block that’s irrigated, the beetles will fly to your property and lay eggs,” Dr. Potter said.

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