Japanese beetle populations across Colorado remain steady, infected areas spread

Across Colorado, the summer heat is drawing Japanese beetles out of their winter hibernations, albeit a bit later than normal.

While experts say the populations are on track with those of previous years, infested garden material has transported the invasive species to new locations in the state.

“Colorado is the perfect habitat for these beetles, from the turfgrass to plants for the adults to eat,” said Lisa Mason, a horticulture specialist with the Colorado State University Extension office. “They’re probably slowly spreading along the Front Range each year.”

During the winter, the larvae dig deeper into the dirt and are protected from the cold and snow. As the weather warms up in the spring, they crawl closer and closer to the surface until they undergo metamorphosis and adult beetles emerge.

According to Mason, the beetles usually emerge from the end of June through the beginning of September, and females will lay eggs in moist turf grass in late August.

“When those eggs hatch, larvae start feeding on the roots of the grass,” she said. “Japanese beetles are dangerous because they have two damaging parts of their life cycle, which is unusual for an insect. The larvae live in and damage turfgrass ecosystems and the adults feed on more than 300 types of plants.”

Mason said the insects emerged late this year due to colder temperatures in the beginning of the season, but the populations are now on track from previous years.

But those populations are slowly spreading into new areas.

Last summer, for the first time in more than a decade, Japanese beetles were seen in Grand Junction, likely due to the transport of infested landscaping materials by a contractor who violated the Japanese beetle quarantine from Front Range counties, according to Colorado Department of Agriculture spokesperson Olga Robak.

In addition to being transported by a third party, the beetle populations can also spread one to five miles per year, moving from plant to plant in an area.

“This is a big deal for the western slope, and could threaten our agriculture and horticulture industries if we don’t contain it,” Robak said.

The infestation is less than 15 miles southwest of Palisade, the center of Colorado’s fruit-growing region.

Japanese beetles were reported in Palisade by a gardener in 2002, and they weren’t eradicated until 2009 after a long, concentrated effort and significant damage to grape and peach crops in the area.

The beetles were first seen in the Denver area in 2005 and over the years the population became established, marking one of the highest infestation levels in the state.

While there are no concentrated state efforts to remove the beetles from the Denver metro area, Robak said the department is coordinating with Mesa County and Grand Junction officials to develop an eradication plan.

In March, Mesa County commissioners officially declared the beetle a nuisance and planned to take action to control the pest’s population in the county.

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