Infestation Spread Throughout Subdivision

LAWSON HILL – The most widespread and destructive forest defoliator in western North America has been identified in Lawson Hill.

Yvette Henson, Colorado State University Extension Director for San Miguel County, identified samples she collected in early July as the Western spruce budworm after being contacted by Lawson Hill resident Jody Van Stratt. Van Stratt had noticed the worms, which have brown heads, brownish-olive bodies, two pairs of white dots on each body segment and can grow up to three centimeters long during late larval stages, dangling from trees in the neighborhood, according to Henson.

Henson said she was so curious about the insects, which move by drawing their hind legs toward their front legs like inchworms, that she rushed to identify them that same night. “I was kind of like, ‘oh no,’ just because it has made an impact before in the state,” she said.

Tree ring records indicate that there have been at least 14 outbreaks of the pest, known by its scientific name as Choristoneura occidentalis Freeman, in the San Juan Mountains over the past 350 years, according to a study co-authored by University of Arizona and U.S. Forest Service researchers that appeared in the Canadian Journal of Forest Research in 2003.

But the name Western spruce budworm is misleading. In Colorado they primarily infest Douglas and white fir, but will also attack Engelmann spruce, blue spruce and subalpine fir, according to a fact sheet published by the CSU Extension Cooperative. The largest outbreak exceeded two million acres.

Henson returned to Lawson Hill the following week to determine whether other trees had been infested. She noticed the telltale pinkish-brown tree branch tips throughout the subdivision.

“In that whole neighborhood there are lots of infested trees,” she said. “I drove all through Lawson Hill and they were all up and down the roads.

“Their preferred host is Douglas fir, but these are on spruce,” Henson continued. “It probably means there are more spruce available.”

Henson said she doesn’t know if this is the first year of the infestation, or if they were simply not noticed until now.

“I’d be curious to know if someone else noticed them,” she said.

Adult budworm moths, a rusty, mottled mix of orange, brown, black, and white, emerge in Colorado from late June to early August. After mating, the females lay between 25 and 40 green eggs along the underside of needles. Just shy of two weeks later, the eggs hatch into larvae.

Instead of feeding, however, the larvae inch their ways into crevices beneath bark scales or among lichen. They then spin loose, silky structures called hibernaculae, in which they remain dormant throughout the winter.

The larvae become active again around May, emerging at the same time as the new spring growth. They feed on old needles or host tree flowers until a week or two later when they enter developing buds. As the new needles grow, the larvae again spin loose shelters around the developing foliage.

Like Noah in his ark, the budworms remain inside their shelters for around 40 days. There they continue to feed and grow, somewhat protected from their predators. Most of the damage to trees is done during this period, by the end of which the clumps of damaged new growth have turned pinkish-brown.

Following this feeding period, the larvae take about a week to morph into their adult selves. They emerge, mate and then die in short order, and the cycle begins all over again.

And as with so many of the ailments to befall Colorado’s trees and forests of late, drought may play a part in this infestation.

“Anything that is stressed is more susceptible to infestation,” said Henson, pointing back to the 2002 drought. “We’re still seeing so many things we think we can link to that.”

According to the CJFR study, the researchers found little budworm activity during dry periods. Instead, outbreaks tended to occur during periods of increased moisture.

The Lawson Hill budworms turned to moths last week.

“Now they’ll mate and lay eggs, which they should be doing soon if they haven’t started already,” said Henson.

There is little to be done about them for the moment. The application of control chemicals must be timed to coincide with the new growth – to nip the larvae in the bud – so to speak.

“They would have to be treated next year,” said Henson. “In a way that’s good because it gives [Lawson Hill residents] a whole year to look at their options.”