Colorado’s unusually warm, dry winter has created challenges for ski resorts and water managers, but it may also bring a modest benefit for gardeners frustrated by pesky Japanese beetles.
Lisa Mason, a horticulture specialist with Colorado State University Extension in Arapahoe County, said this winter has been “exceptionally dry,” a condition that can affect the pest long before it appears on roses and grape vines in early summer.
“If they don’t live in completely moist soil, they are susceptible to drying out, which is an excellent thing,” Mason said.
Japanese beetles are an invasive species in Colorado. The metallic green and copper insects feed on more than 300 plant species as adults, including roses, grapes, raspberries, hollyhocks and several vegetables. In their larval stage, they live underground as white grubs, feeding on the roots of turf grass.
“They will only be found in turf grass,” Mason said of the grubs.
Females lay eggs in July and August during the heat of summer. Those eggs hatch into grubs that burrow into lawns, where consistent soil moisture is critical for survival through the winter months. When soils are dry, the grubs are more vulnerable.
In parts of the Front Range, soil conditions are noticeably parched.
“The grass is very dry,” Mason said. “Usually, the ground is frozen.”
Even so, Mason cautioned against expecting a dramatic drop in beetle populations this summer. Grubs can burrow deeper — sometimes 6 to 8 inches below the surface — to find moisture.
“It’s really going to depend on weather patterns,” Mason said. “They’re very adaptable.”
Japanese beetles were first detected in Colorado decades ago and are now firmly established along the Front Range, particularly in well-irrigated turf areas such as lawns, parks and golf courses. Mason said she would not expect a mass die-off, even in a dry year.
“Since they’re an invasive species and pretty adaptable, I would not expect like a mass die-off or anything,” she said. “Maybe that would happen and we’d be very lucky, but probably not.”
While the dry winter could reduce their numbers slightly, Mason said gardeners should not assume the problem will disappear. The beetles are likely here to stay, even if this year brings a small reprieve.
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