Foresters Keep an Eye on Idaho Pine White Outbreak

by Deb Courson, Ag Weekly
Monday, August 30, 2010 10:07 AM CDT

CASCADE, Idaho – Millions of small, white butterflies have been fluttering around forests and yards in many areas of the state this summer, a phenomenon not often seen in Idaho. Central and southern Idaho, and central Oregon, are enduring a pine white butterfly outbreak.

It only happens every 30 to 50 years, according to USDA Forest Service entomologist Dayle Bennett. The butterflies themselves are harmless, he says, but the eggs they’re laying inside pine and fir tree needles will hatch caterpillars next spring that can defoliate trees. The affected pines may look a bit odd, he adds, but most will survive.

“Pines will be putting out a new complement of needles in the spring, and the caterpillars mostly feed on the older needles. So, that’s a bit of a saving grace.”

Pine white butterflies are always present, Bennett says, although usually not in such large numbers. Even when an outbreak occurs, it may not guarantee trouble. Bennett witnessed swarms of pine whites north of Boise in the mid-1990s, too, he recalls.

“We saw them one year, and then we didn’t see them again after that. That could be the case with this outbreak, but it could also be that it will last another couple of years.”

Pine white butterflies are native insects that have co-evolved with pines, Bennett explains, and they naturally die out after a couple of years. The Forest Service has no plans to take action, since effective insecticides would do more harm than good, he says. If property owners are concerned about the health of their landscape pines, Bennett advises them to contact a local extension office for information about possible control methods.

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