Largest recorded outbreak of pine butterflies takes a toll on Oregon's forests, stripping needles from 250,000 acres of trees

LOS ANGELES , March 14, 2012 () – Oregon’s Malheur National Forest is in year three of a massive outbreak of pine butterflies, a natural phenomenon that occurs for two to three years every three decades—and it’s taking a toll on the state’s pine trees, PBS reported on March 12.

Oregon Public Broadcasting in its Oregon Field Guide series earlier this month explored the population surge of pine beetle caterpillars that has stripped needles from 250,000 acres of trees.

The phenomenon is so infrequent—occurring only every 20-30 years—that few people have seen it. And probably even fewer would imagine caterpillars could cause such damage to trees.

The butterfly’s eggs are laid directly on the pine needles and, when they hatch, they become a hungry horde that devours the very needles that sheltered them, said Lia Spiegel who studies insects for the U.S. Forest Service. Pine butterflies die shortly after laying eggs, she added.

The very hungry caterpillars may not kill many trees outright, but they can stunt their growth for years and do leave the pines extremely vulnerable. According to Rob Progar, an entomologist with the Pacific Northwest Research Station, a tree of weakened vigor is more susceptible to attack by bark beetles.

Spiegel said the defoliated area was much bigger this year than last year, though she noted: “If the buds are still alive, they're not dead. They don't look very healthy, but most of them will probably survive.”

Speigel said that, historically, she had never seen an outbreak like this one. "They've never lasted more than three to four years,” she told PBS.

Although the damage caused by the caterpillars is more cosmetic than anything, Spiegel said local people can get a little upset about it, because the trees look as though are dead once they are robbed of most of their pine needles.

Aerial surveys show the affected trees, which are left with very few needles, as gray instead of green, PBS noted. There is no way to forecast how far the eruption will spread, though the Oregon Department of Forestry estimates the needles have been eaten off trees across nearly 400 square miles.

Meanwhile, the flight of so many butterflies provides a spectacular image for those fortunate enough to witness it. Progar expects there to be “so many butterflies in the air that it will look like a snowstorm in August.”

The primary source of this article is Oregon Public Broadcasting (via PBS), Portland, Oregon, March 12, 2012.

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