The beetles are back
Summertime is munch time for Japanese beetles as they devour just about everything in our gardens
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 sixweek 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.
Nearly a century after they were found in New Jersey in
1916, a 2015 U.S. Department of Agriculture 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.
Still, this is a troublemaker at both stages of life — and its wide-ranging diet doesn’t hurt its chances, either.
Based on decades of tracking the beetles’ seemingly inexorable march westward in North America, Daniel A. Potter, a professor in the department of entomology at University of Kentucky, described the arc: “The first few decades in a new area, the insect goes crazy and builds to high levels before the population starts to stabilize. Then it goes from a plague to a nuisance.”
These bugs really get around
Although the adult Japanese beetle has a life expectancy of only 30-45 days, it is “a highly mobile insect,” Potter said, and that’s one of its strategic edges.
The ability to take flight allows the beetles to infest areas several miles away. And human movements have enabled longer-distance travel: The beetles hitchhike rides on airplanes and vehicles, and the grubs can be transported in soil around plants shipped across the continent.
In her short adulthood, a female Japanese beetle 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.
A worrisome detail: 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.
Gardeners and growers from Colorado to California and the Pacific Northwest are trying to eradicate the beetles before they become established.
Europe is also under pressure. This most recent fight will be especially challenging, Potter said, in the same way that it is in the United States for organic grape or blackberry growers.
“It is a major concern — imagine what it would do in French vineyards,” Potter said. “They can’t spray their way out of this problem.”
So what’s wrong with beetle traps?
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,” 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.
One nontoxic approach that does work in the garden is a tried-and-true USDA recommendation: Drown them.
In the cool of the morning, go out and knock the sluggish beetles into a container of soapy water.
“Our research shows that they can’t really fly below 70 degrees Fahrenheit,” Potter said, findings revealed by such heroic tactics as graduate students tickling the bugs with a long camel-hair brush at 3 a.m.
Shaking beetles off plants in the morning and evening will reduce feeding, which decreases the volatile odors produced and, in turn, limits continued attacks.
While this strategy is not practical in a vineyard, it can minimize damage to treasured plants.
Can you battle grubs without chemicals?
Gardeners seeking nonchemical treatment options for grubdamaged turf may have tried milky spore powder, a biological control containing a live bacterium. But recent research has not shown any benefit from using it to treat individual lawns.
Nematodes — tiny roundworms, specifically the recommended species Heterorhabditis bacteriophora — are another biological tool, and more effective. These living organisms must be mail ordered (from companies like Arbico Organics) and sprayed on a lawn in a solution of water. Calibrating application rates can be tricky, and nematodes are very sensitive to heat and sunlight, requiring application in the early morning or near dusk. Also, the lawn must be irrigated before and after application.
But here’s some disappointing news: There is little evidence that controlling the grub population will reduce damage to your roses or raspberries — or vice versa. Adult beetles can fly from a distance, seeking favored foods or turf that is suitably moist for egg-laying (about 11% soil moisture is required for egg survival and hatching).
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,” Potter said.
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,” Potter said.
Four to six times as many eggs may be present in irrigated turf, he said, explaining why during droughts, a lot of damage is seen on irrigated golf-course turf unless it is treated with insecticides.
Beyond that, what makes for a boom or bust population year? There is no evidence that a cold winter kills the beetles — and, in general, not a lot of hard evidence on why their numbers fluctuate overall.
“Even after 40 years of studying Japanese beetles, I cannot predict a bad or good year,” Potter said. “There are still too many things we don’t understand about them.”