Carol O'Meara - Colorado State University Extension

Carol O’Meara – Colorado State University Extension

Heads up, gardeners, we’ve entered the season of Japanese beetles.  They began appearing at the end of June, so it’s time to be on the lookout for them.

Like a villain in a summer blockbuster, the beetles are compellingly gorgeous. But their beauty hides a destructive heart; they feed voraciously on over 300 types of plants, including grapes, raspberries, beans, apples, and roses. Ganging up by the dozens on plants, they can cause serious injury.

The irritating pest popped up in south Denver, spreading outward until we now have established populations that are expanding out of the metro area. In 2015, the bug savaged Linden trees and there is the anticipation that they will do so again this year.

The threat from Japanese Beetles has gardeners scrambling for solutions, because they eat so darned many of our treasured plants—even eggplant is being attacked.  Louisville, Lafayette, and Boulder residents are reporting more and more of the beetles; all the moisture we’ve had this season means a high number of beetles are surviving to reproduce.

They love moist soil, which is one reason why the Front Range isn’t an easy place to combat it. Here, grim faced gardeners are putting trowels to use in thwacking the bugs from plants into soapy water. Obsession is taking hold and productive time is now used on endless scouting for the insect.

Adult Japanese beetles are lovely scarabs with a shiny, metallic-green body and bronze colored wings. Six tufts of white hair adorn each side of their body. At a half-inch long, they are easily spotted in early to mid-summer as they chomp down garden plants.

They’re easy to knock off plants; in fact, they drop easily to the ground as a defense. Just be sure you catch them in your soapy water so they don’t survive the fall. Traps aren’t as effective here because they’re established in so tmany places along the Front Range; in some trials, they actually lured the beetle into the area with the scent.

Sprays of Neem products that contain azadirachtin can stave off the beetles for three or four days, but not all Neem products contain this, so make sure it’s listed on the label as an active ingredient. Pyrethrins also can help control the bug for a few days as well.

The larvae are white grubs that live in soil and feed on grass roots, and are often transported in soil from place to place. Target the grubs with parasitic nematodes in the genus Heterorhabditis, which are applied as a drench to the soil in late summer during cool, overcast days. Milky spore powder can also be applied to the soil and takes several years to develop into an effective deterrent.

For more information on Japanese beetles and their control, see

Colorado State University Extension, together with Boulder County Parks and Open Space, provides unbiased, research-based information about consumer and family issues, horticulture, natural resources, agriculture and 4-H youth development. For more information contact Extension at the Boulder County Fairgrounds, 9595 Nelson Rd., Box B, Longmont, 303.678.6238, or visit the web site at