Forest insect may be wiping out an Alaska tree

Alaskans may be witnessing the end of the tamarack.

Trees with needles that turn golden each fall before dropping, tamaracks grow in wet, boggy areas north of the Alaska Range. People often mistake tamarack for spruce, but tamarack branches are flexible enough to be tied in a knot without breaking, and the branches are bare in winter. Most of Alaska’s tamaracks are either naked or have few needles this summer. The trees are apparently giving up the fight against the larch sawfly.

Sawfly larva—green caterpillars with black heads—have attacked almost every tamarack tree in Alaska. Research entomologist Ed Holsten of the U.S. Forest Service remembered flying over the Interior in a small plane when the invasion was heaviest, in 1996.

“At the peak of the outbreak, it was mind-boggling,” Holsten said from his Anchorage office recently. “Every pocket of tamaracks we looked at was pink (because the trees had no needles).”

Repeated attacks from the larva of the sawfly have doomed many, if not all, the tamarack in Alaska, Holsten said. In 1993, sawflies infested about 10,000 acres. The next year, the area of affected trees dropped to about 300 acres. In 1995, the insects exploded, covering about 116,000 acres. One year later, sawfly larva attacked almost every tamarack in Alaska.

“It’s a losing battle for the tree when you have that many consecutive years of being hammered like that,” Holsten said. “Combine that with the trees growing at the extreme end of their range, and it’s hard to recover.”

Not that the Alaska tamarack hasn’t battled. The trees respond to caterpillars munching all their greenery by putting out a second set of needles in August. By then, the sawfly larva have rappelled down to the forest floor to overwinter, but Holsten said the energy used to produce those new leaves probably left tamaracks unable to survive attacks the next spring and summer. Part of the problem is the sketchy toeholds tamaracks occupy in Alaska.

“They grow on rotten sites, muskeggy and full of black spruce,” Holsten said. “If this happened back east or in Canada, where the trees grow on better ground, you probably wouldn’t see much mortality.”

Though tamaracks don’t grow south of the Alaska Range, the sawfly has traveled south to feed on Siberian larch, Holsten said. Siberian larch is a species not native to Alaska, but people have planted them in Anchorage and the Mat-Su Valley. Holsten said the sawfly larva now eating the needles of Siberian larch in Southcentral might have hitched a ride over the Alaska Range on larch seedlings grown in the Interior and transported south.

Since sawflies have attacked tamarack of all ages—unlike spruce bark beetles that seem to prefer only mature trees—the insects may have knocked out a tree species in many areas of the Interior.

“I wouldn’t be surprised if (tamarack) was going out of the forest system,” Holsten said.

Holsten said a warmer Alaska might be to blame for the demise of the tamarack. Wetter, colder summers that may have kept sawfly numbers low before have given way to weather that is ideal for them to flourish. Tamaracks are taking the hit, making them possibly the first casualty in a changing Alaska.