They’re so pretty in the sun with their iridescent coppery wings folded over their backs ... and their grabby, rasp-like mouth parts skeletonizing everything in their path. They like what we like—roses, grape arbors, Virginia creeper, tomatoes, berries, linden trees ... over 300 species of plants all together. And there’s not much we can do about them.
The simplest solution, trapping, is not recommended by any horticultural expert. Admittedly a trap will capture thousands of the clumsy flyers, but it will unfortunately attract more beetles to your yard than it traps. Broad spectrum insecticides such as Sevin will kill the beetles all right, along with honeybees, butterflies and other pollinators. Furthermore, the Colorado State University Extension Service says spraying chemical insecticides on flowering plants is not only unwise, it’s illegal.
So, much as we would like to eradicate the stinkers, the best we can hope for is control, says CSU Prof. Whitney Cranshaw, State Entomologist and author of Garden Insects of North America, A Field Guide. In an email, he explained control consists of two parts: control of larvae and control of adults. A plan that does not consider the full life cycle will fail.
“The biggest problem is that [Japanese beetles] are huge feeders on almost all the plants that the pollinators heavily use as well. For the most part we are doing manual removal” (brushing or shaking the beetles into soapy water) “and trying to stay away from planting host plants that they love.”
Japanese beetle larvae hatch and grow under our damp, cool lawns over the winter, and Cranshaw recommends applying Scott’s Grub-ex now. “It is a ‘standard’ insecticide but happens to be very safe for humans/vertebrates and very low risk to honeybees.” He is less enthused about milky spore, a bacterium widely available at garden centers that infects the grubs. “It ... will likely kill at most 5 percent of the grubs,” he says. Still, he considers 5 percent better than none.
Getting rid of the grubs might help us next year—or not. Japanese beetles fly fairly long distances. What do we do about these garden terrorists this year?
“The biggest problem is that [Japanese beetles] are huge feeders on almost all the plants that the pollinators heavily use as well,” says Julie Lehman, Denver Parks and Recreation Greenhouse/Horticulture manager. “For the most part we are doing manual removal” (brushing or shaking the beetles into soapy water) “and trying to stay away from planting host plants that they love.” Lehman recommends essentially the same for homeowners, conceding the beetles are not particularly choosy in their diet and manual removal is labor intensive. She recommends the CSU Extension Service fact sheet found at extension.colostate.edu. Search “Japanese Beetles.”
That said, there are some chemical helpers homeowners can use, though they are far too expensive for the huge Parks and Recreation system. Professor Cranshaw suggests the safest products for bees are those that list pyrethrins, azadirachtin (BioNeem) or the Bacillus thuringiensis var. galleriae (beetleJUS!). Both he and Lehman caution these must not be used on plants that are flowering, and only applied when pollinators are not active, such as evening or early morning. Unfortunately, Cranshaw continues, “All of these [products] have very short persistence, usually only a day. Hand picking is the way to go for most backyard situations.”
A search of home remedies online turned up the usual soap sprays and the same recommendations as Lehman and Cranshaw have offered. However one website by the Old Farmer’s Almanac (almanac.com/pest/japanese-beetles) offered two less common suggestions. One recipe is for a spray using vegetable oil, dish detergent, and rubbing alcohol. A recent short test showed it to be a bit more effective than soap spray alone. However, the Almanac cautions that it could damage leaves, so apply with care. Another suggestion is to plant geraniums around your most beloved flowers. The beetles are attracted to them, “eat the blossoms, promptly get dizzy, fall down, and permit you to dispose of them conveniently with a dustpan and brush.”
If after all this you’re inclined to throw up your hands and do nothing, you’re in good company. During a recent visit to Denver Botanic Gardens, I asked one of the volunteers what the Gardens are doing to control the beetles. “Nothing,” she said. “You should see the rose garden. It’s decimated,” she added sadly.
Update, 8.14.2017: The Profile has learned through Denver Botanic Gardens' Jennifer Riley-Chetwynd that Loddie Dolinski, horticulturalist responsible for the Romantic Gardens, Ellipse Woods, Lainie’s Cutting Garden, the Scripture Garden and the Herb Garden, is in fact combatting the beetles there using a manual removal technique (flicking the beetles into a soapy water bath). The Gardens' primary concern is employing a method that is effective but which does not harm pollinators, and manual removal methods are thus best. According to Riley-Chetwynd, Dolinski reports the best time to combat the beetles is during dawn or dusk hours. Denver Botanic Gardens is offering a Japanese Beetle course in October. For more information visit denverbotanicgardens.org.
