DARRINGTON — It’s an inchworm with an appetite that takes its toll in board feet.
Put a bunch of western hemlock looper caterpillars together and they can munch through enough conifer needles to kill a tree, and many more trees after that.
This summer, an outbreak of the caterpillars has defoliated hemlock and other conifers about four miles south of Darrington on the Mountain Loop Highway, and to a greater degree around Baker Lake in the north Cascades, state officials say.
Officials aren’t sure how many trees or acres of forest have been destroyed, said Glenn Kohler, a forest entomologist for the state Department of Natural Resources.
“It’s patchy,” he said, adding that most of the damage has been in the forest understory — shrubs, young hemlocks and Douglas firs. The caterpillars prefer hemlocks, which don’t take defoliation very well, Kohler said.
While the insects can make a big dent in small areas of forest, their outbreaks usually fade after two or three years before they can do widespread damage, Kohler said.
Sometimes, even if the caterpillars don’t eat enough of a tree to kill it, the tree will turn brown.
“The whole forest will appear scorched,” he said.
State officials want to keep track of the loopers so they can educate the public about what’s happening, including private landowners whose property may be affected.
Western hemlock looper caterpillars turn into moths in late summer. Recently, the moths have been seen near the Darrington Ranger Station, said Matt Riggen, a visitor information assistant at the station.
“If someone sees more than 25 of these adult moths, that’s an indication there’s probably an outbreak nearby,” Kohler said.
When an outbreak of loopers starts, it’s likely they’ll be around for a couple of more years, Kohler said.
“We’re assuming the defoliation is going to become more severe next year and spread out,” he said.
Within three years, though, nature usually levels out the loopers’ numbers with predators, parasites, extreme weather and a naturally occurring virus, Kohler said.
Fortunately, testing for that virus, called nuclear polyhedrosis, shows it’s around this year in fairly high levels, he said.
Humans can kill the insects by spraying trees with a pesticide made with bacillus thuringiensis, or BT, a bacteria that’s toxic to the larvae. It’s rarely used, though, because of the insects’ typical natural decline after an outbreak, Kohler said.
“It is killing trees but it’s not completely decimating the whole forest,” he said.
The last outbreak in Snohomish County was in 2001-2003, near Lake Chaplain and Arlington, and around Baker Lake. That time, the outbreak affected 17,000 acres in all three areas in 2001, 35,000 acres in 2002 and 1,400 acres in 2003.
An outbreak near Raymond on the Washington coast in 1963 killed 20 percent of the trees in areas sampled over a 70,000-acre section of forest, Kohler said. It took the forest 10 years to completely recover, he said.
The insects, native to the Pacific Northwest, surge when the conditions are right. The moths lay eggs in lichen and moss on the conifers in late summer or early fall, and the eggs hatch the following June or July.
A dry September is one indicator of a strong population of loopers the following year, but the other factors, including predators and the virus, are in play as well, Kohler said.
The caterpillars are wasteful eaters, often chewing off needles at their bases and letting the rest fall to the ground, according to a U.S. Forest Service management guide for employees.
Hemlock loopers can eat through all of a tree’s needles in a single year, stressing the tree, slowing its growth and sometimes killing the top of the tree, known as top kill, according to the Department of Natural Resources. Trees that lose more than 75 percent of their needles may not survive.
“Late in summer, larvae are very mobile, crawling over tree trunks and shrubs, and dropping by silken threads from the trees to the ground,” according to the Forest Service guide.
The caterpillars pupate for 10 to 14 days in bark crevices, moss, lichen, or under debris. The moths usually only live a few weeks before mating, laying eggs and dying, Kohler said.
“By fall, the ground may be littered with parts of needles, insect frass (excrement), and later by thousands of dead moths,” the Forest Service guide read.
Anyone who has seen more than 25 hemlock looper moths at one site is asked to send the location, time and date of the observation to Kohler at glenn.kohler@dnr.wa.gov.
Bill Sheets: 425-339-3439; sheets@heraldnet.com.
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