With metallic-green heads and bodies and copper-colored wing covers, Japanese beetles (Popillia japonica) seem like tiny, cute and harmless creatures in a 1960s Godzilla monster flick.
They are not.
Although their flights through any garden are Toho Studio-slow, Japanese beetles can form into destructive, munching platoons. They assault flowers. They attack tree leaves. They feed from the top sides of leaves, gnawing the soft tissue between the veins. In large numbers, they can quickly skeletonize hundreds of leaves.
Although beetle-infested trees can look fire-damaged, experts say that established, healthy trees usually survive. The same goes for garden plants. Studies cited by the University of Kentucky suggest that these bug banquets are sparked by odors released from the damaged leaves.
If you spot one or more of the 1/3- to 1/2-inch beetles on, say, a rose or marigold, flick them off with a finger before others join the feed. Local horticulturist Julie Weisenhorn recommends a bolder response.
“You want to hand-pick the beetles in the morning and evening and drop them into a bucket of soapy water,” Weisenhorn told EPLN in late July. “You’ll cut down on other beetles coming into your yard. The more you let beetles feed, the more beetles you will get.”
Weisenhorn co-authored “Japanese Beetles in Yards and Gardens,” an article on the University of Minnesota (U of M) Extension Service website. Weisenhorn and other Extension experts answer listener questions Saturday mornings on WCCO Radio’s Smart Gardens program.
The soapy water dunks work well in June and July, says Weisenhorn. That’s when adult, leaf-munching Japanese beetles are most active.
Without management, Japanese beetle invasions of your green thumb paradise can be as inevitable as movie monsters rampaging through Tokyo. The Extension Service article reviews various organic, non-chemical and insecticide applications for adults and larva.
The Beetles: their local story
In the Twin Cities, Popillia japonica begin emerging from their underground grub phase in late June and July. In adult form, they feed on some 300-plus plant species, including roses, basil, marigolds, zinnias, grapes, currants, apple and crab apple, birch, maple, linden and elm trees.
They fly around and land on plant and tree leaves and enthusiastically eat and mate, usually in morning and afternoon sunlight. For 10 months of the year, Japanese beetles are in their larval phase.
Egg-bearing, female beetles will land in summer grass then burrow 2 to 3 inches into the soil. Individuals lay from 40 to 60 eggs during their life span. Those eggs hatch into larva; cream-colored, brown-headed grubs that grow up to an inch in length.
The grubs chew on vegetable seedlings and the roots of lawn, park, golf course and cemetery turf. Healthy, green turf can die from concentrations of hungry grubs. Damaged root systems can not supply water and minerals to the plant. Yellowing and browning areas of yard grass in August could indicate grub damage. If these areas lift off the soil as easily as a door mat, look for grubs. If you spot them, don’t despair.
About the Beetles: Getting Better
The Extension article is encouraging. Beetle populations vary from year to year. There are many ways to deal with them. The damage is mostly cosmetic, not fatal.
If grubs are present in August, U of M Extension recommends a fast-acting insecticide treatment in September for partial control. In November, as the grubs burrow deeper in anticipation of the winter freeze, relax. Think about Thanksgiving.
Treatment of grubs in the spring is not recommended. But from mid-June to mid-July, just before the adult beetles emerge from the soil to feed and lay eggs, preventive insecticide treatments can help. The Extension article reviews various applications. Our local experts pay due respect to pollinator safety.
Beetle control fails
Five summers ago, the birch trees in this writer’s backyard were attacked by countless hordes of beetles. To see Popillia japonica chomping on hundreds of birch leaves is upsetting. To watch scores of others (from another planet?) fly in for the feed is disgusting. I became panicky.
Not knowing much, I declared war and fought back. Armed with a garden hose and a nozzle dialed to the powerful “Jet” mode, this first responder began knocking the beetles off the birch leaves with fluoridated, City of Eden Prairie H2O. A few of the bugs fell to the ground, many, many, many more flew off.
I relaxed some with a confident smirk and a lemonade on a patio chair. The beetles returned the next morning.
I headed to Menard’s and bought three beetle traps. I had read about them on the internet. The narrow-necked plastic bags hang on flimsy metal stands. Scented lures are attached to them. Excited beetles crawl in but can’t crawl out.
As the traps attract beetles, the female beetles release pheromones. This appeals to even more beetles. Even when your traps are positioned a seemingly safe distance from your patio pots of lush ornamentals, be alert. University of Kentucky research shows that beetle traps attract more beetles into one’s yard than they capture.
Check out EPLN’s short video of a beetle trap action this August.
“So, your neighbors will love you,” quipped Julie Weisenhorn. The traps, she explained, were designed for farmers to monitor beetles not to manage them. A high beetle count signals farmers to treat their crops.
In Minnesota, picking the occasional Popillia japonica off garden plants in August remains a good thing to do. They do not bite or sting. Says Weisenhorn, “It’s just something we have to do, just like shoveling snow and raking leaves.”
Learn more
For more information on Japanese beetles, consider the following:
“Japanese Beetles in Yards and Gardens,” by U of M Extension entomologist Jeff Hahn, horticulturist Julie Weisenhorn, and educator Shane Bugeja.
“Japanese Beetles in the Urban Landscape,” an article authored by experts at the College of Agriculture, Food & Environment, University of Kentucky.
“Don’t Fall Into the Japanese Beetle Trapping Trap,” by Marissa Schu, U of M Extension educator.
Editors note: Contributor Jeff Strate serves on the Eden Prairie Local News Board of Directors.