June is a glorious month in the garden, but it also the cue for adult Japanese beetles (Popillia japonica) to emerge from the soil. They’ll hit their peak from late June through at least August. I haven’t spotted the metallic green and copper devils in my garden yet, but if the past is any indication, they’ll visit soon.
Colorado State University offers this comprehensive fact sheet with everything we need to know to defend our plants. It’s an especially valuable reference when considering products which may or may not be beneficial in reducing the pests while doing minimal damage to the ecosystem. If there were best seller lists for gardener information, this would be on the top!
Here are important takeaways from the fact sheet:
Japanese beetles feast on foliage, buds and flowers of their favorite plants. Particularly susceptible are roses, Virginia creeper, Linden trees, Rose of Sharon, Japanese maples, Silver lace vine and Gaura are among their favorite targets. Tell-tale signs of Japanese beetle damage are a skeletonized pattern of mutilation on tender, new foliage and deformed flowers or buds. While unsightly, the damage will not kill the plant.
Traps are not beneficial. That yellow trap you’re tempted to hang in your yard is an open invitation for more Japanese beetles to visit. The trap won’t be able to catch them all and the effect is more, not less damage. Perhaps if your neighbor hangs one…?
Picking does help. Japanese beetles are easy to spot in the cooler parts of the day and can be coaxed into a jar of soapy water with a twig or a shake. Catching is preferable to squishing, as a squashed beetle releases a compound which lures more of their kind. While it may not be fun, catching is oddly satisfying!
Do insecticides work? As always, caution must be used to avoid damage to pollinators, especially when plants are in flower. Products containing pyrethrins, azadirachtin and acetamiprid – used in the early morning or at dusk – when bees and other pollinators are less active – are the safest. See the fact sheet for more details and follow all product recommendations carefully.
Consider removing temptation. Can you replace your Virginia creeper with something less enticing? Do you have roses that have struggled for years? Perhaps it’s time to replace with something less alluring. Conversely, when adding to your landscape avoid plants which are irresistible to this insect.
Late season turf damage. Each female Japanese beetle lays 40-60 eggs in her 4-8 week life span. Eggs are laid deep in moist turf soil. Since eggs and subsequent larvae thrive on moisture, keeping soil on the dry side will inhibit grub development and decrease turf damage. Grubs also munch on turf roots, so mowing at a higher height, which promotes vigorous roots, can help reduce turf damage. More control ideas can be found in this fact sheet.
Have you spotted Japanese beetles in your garden yet?
Submitted by Linda McDonnell, a Denver County Master Gardener