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Death from above: Forest hosts caterpillar boom

Two tussock moth caterpillars crawl along a branch.  The current outbreak is the worst Plumas County has seen in 25 years.  Photo by Austin Hagwood
Austin Hagwood
Staff Writer

A constant drone echoes from the forest along La Porte Road, drawing attention to treetops bleached orange and entire hillsides bathed in leafy fire.

Beneath the dead foliage, a battle for survival rages.

Hordes of Douglas-fir tussock moth larvae in caterpillar form ooze from branch to branch, leaving a devastating trail of defoliation in their wake.

A coating of hairy spines guards their black-and-orange bodies and resembles science fiction more than biological defense.

While the caterpillars are native to the Plumas National Forest, an infestation has exploded this summer in a population boom not seen for several decades.

According to Danny Cluck, forest entomologist for Forest Health Protection in northeastern California, tussock caterpillar outbreaks are a common event statewide, although Plumas County has not hosted one since 1988.

“The outbreaks are cyclic anywhere in the state where you have a high concentration of white fir, so that’s one of the reasons we’re seeing it here,” Cluck said. “Even though it has the name Douglas-fir tussock moth, in California it prefers white fir and we see outbreaks every five to 10 years. This outbreak appears to be on almost the exact same footprint as the one in the 1980s, which stretched from the Lake Almanor area to the Little Grass Valley area.”

Similar California tussock moth infestations occurred near Mount Shasta and Yosemite in 2006, with the most recent population explosion found in the San Bernardino National Forest three years ago.

Forest Service entomologists caught early warning signs of an infestation over a year ago when pheromone-baited traps set for the insects returned unusually high yields. When light defoliation spread in forest areas south of Quincy and toward Susanville, they prepared for the worst.

Plumas County Agricultural Commissioner and Sealer of Weights and Measures Tim Gibson said that although caterpillar damage appears “alarming,” the situation is better than advertised.

“This is a native insect, so it’s not like the gypsy moth, which would be even more troublesome,” Gibson said. “It’s primarily feeding on the white fir — there can be red fir in the same area and it will be untouched.”

Cluck and Gibson also noted that long-term damage to the forest appears unlikely, given the fir tree’s regenerative ability. Smaller trees with fewer needles stand a higher chance of mortality than their larger counterparts.

“Even though they look very dramatic — the trees turn a brownish color because all of their needles are being chewed off — unless a tree is completely stripped of its needles, it’s likely to recover,” Cluck said. “It takes about 90 percent of foliage to be consumed for a tree to die. Trees that have lost 50 percent will probably recover. The top of the tree may also die but the rest of the tree can survive.”

Despite stress on smaller trees, Cluck said the most severe threats affect recreation areas.

“In high-use recreation areas like Little Grass Valley Reservoir, it becomes a huge issue,” he said. “If you have high defoliation you lose some of the trees that make those areas what they are. There is also a public health concern with the caterpillars because some people are allergic to their hairs.”

For the entomologist and the ag commissioner, hope remains that the caterpillar calamity will prove short-lived. The needle-eaters face natural threats ranging from parasitic flies, which attack and lay eggs on the larvae, to a natural virus that spreads more easily through the population when it balloons.

With so many indigenous hazards, Cluck and Gibson anticipate a population crash next year. Both reject applying aerial spray to the trees as unnecessary and undesirable for the current outbreak.

“The good news with these outbreaks is they’re very brief. It’s a four-year pattern,” Cluck said. “This is the third year. Based on past history, the fourth year is when the population is likely to collapse, so that’s one reason not to spray. Years ago we would have sprayed the trees with a bacteria, but in California we haven’t sprayed an outbreak since the 1980s.”

Gibson expects to see an improvement next year but said the current drought could impact tree recovery.

“We’re in the middle of the cycle and are seeing the worst of it right now,” he said. “In late July the caterpillars stop feeding and begin looking for a site to pupate, so the damage that we see is about the limit for this year. The wild card in all this is the drought. Trees are stressed, so we don’t know how well they’ll bounce back from all this.”

The Forest Service also plans to conduct aerial surveys of affected areas to determine if tree die-off would merit a timber salvage effort. When asked whether defoliated fir trees are at risk for increased bark beetle attack, Cluck remained optimistic.

“A tree that’s lost a lot of its foliage is going to be a stressed tree, and there’s potential for that tree to be attacked by bark beetles and killed,” he said. “But as far as bark beetles go, you really need a population of beetles to be there already to take advantage of these trees, and we haven’t seen much bark beetle activity in the areas where we have the caterpillars.”

Cluck advised the public to stay calm and reiterated that tree loss will be minimal and limited to smaller white firs. Gibson added that even a stream of water or mild pesticide will protect Christmas tree-sized firs for property owners and encouraged logging companies to contact his office.

“The Forest Service assures me that it would take years for anything to be sprayed even if they wanted to, so we won’t see any spraying with this infestation,” he said. “If a private timber company really felt there was a need to spray, they could do it. They would have to hire a licensed pest control company, and the proper permit would come from our office. We wouldn’t prevent that.”

County residents are urged to stay on watch for the insects and prevent children from touching the caterpillars’ hairs in case of an allergic reaction.

“You might notice some new defoliation next year,” Cluck said. “But they’ll probably die soon after that and we won’t even notice them for another 25 years.”

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