What to do about Fort Collins' recent moth invasion

Vagabonds have taken over the town, but not the kind that first come to mind.

Residents who celebrated this year's markedly low population of "miller" moths are now being inundated by a different moth species: the vagabond crambus. This species is one of several species known collectively as sod webworms. As caterpillars, they munch on grass blades, sometimes to the detriment of lawns.

Colorado State University professor Whitney Cranshaw, an entomologist who previously provided advice on stories involving miller moths and yellow jackets, said this is the largest number of this particular species he's ever seen in Fort Collins.

However, he said residents shouldn't be overly concerned about the abundance of moths. They did no noticeable damage to lawns as caterpillars and aren't causing damage now as moths. In Colorado,  it's "extremely" rare for an outbreak of damaging caterpillars.

"The fact that we are seeing lots of moths indicates that we just went through an 'outbreak' of the lawn-feeding caterpillars — and apparently they did so little damage that no one noticed," Cranshaw wrote in an email.

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All of the moths fluttering around now were feeding on lawn grasses a month ago, but they produced less leaf clipping than mowing did. And, even though the moths are abundant now, Cranshaw doesn't foresee an impending epidemic of caterpillars next year. He said natural controls that have built up this summer will take care of the next generation.

For residents who want to protect their lawns against the caterpillars next year, Cranshaw said insecticides are an ineffective treatment and a zero-return investment. Instead, he recommended tending to the overall health of a yard with regular watering and fertilization. A well-growing yard has a good ability to tolerate sod webworm larval damage, he said.

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The best way to manage the moths, who will lay their eggs and be gone in the next few weeks, is to limit nighttime sources of light around the house. Turn off porch lights and keep curtains drawn to avoid attracting them.

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