Larch Sawfly Larvae Make a Meal of Alaska Tamaracks
While flying in a small plane recently, Ed Holsten had an easy time spotting tamaracks, which are delicate-looking trees that grow in moist Interior valleys. The tamaracks he saw were pinkish-red instead of green because most of their needle-like leaves were gone.
Holsten, a research entomologist for the U.S. Forest Service in Anchorage, said the leaves were eaten by thousands of tiny green caterpillars, the larvae of larch sawflies. The population of these insects exploded this summer.
Holsten estimated that larch sawflies are feeding on at least 1 million acres of Alaska tamarack, an attack he called absolutely phenomenal.
"The outbreak almost appears to cover the whole distribution of larch in the state," he said.
Tamarack trees, also known as Alaska larch and eastern larch, are often mistaken for sickly spruce trees. Like spruce, tamaracks have cones, but deciduous tamaracks have needles that turn golden and drop every fall. The trees replace them each spring.
Mature larch sawflies, which resemble skinny houseflies, lay eggs on tamarack limbs in spring. After tamarack needles pop out like curving green spider legs, the sawfly eggs hatch into a crop of caterpillars. Then, the hungry babies start dining.
Pat Werner, of Fairbanks, called me after she learned I was writing a column on larch sawflies. She broke off tamarack and Siberian larch branches from trees in her yard that were infested with larch sawfly larva. The small, worm-like creatures de-greened the branches in a plastic tub that sat a few inches from my keyboard as I wrote this column.
The caterpillars, the biggest of which are an inch long, are the same bluish-green color of tamarack needles. Their heads are black, as if they're wearing wool caps. They devour tamarack by bear hugging the needles with their six front legs and munching from top to bottom.
The tiny green caterpillars gorge on needles for about 20 days, Holsten said. Then, they either fall to the forest floor, or rappel down like spiders. In the "duff," which is a dry layer of old leaves, grasses and other organic stuff, the caterpillars spin a papery brown cocoon. They tough out the winter in this homemade sleeping bag.
Researchers predicted that lack of insulation due to last fall and winter's meager snowfall probably would kill a large number of larch sawflies. But Holsten has never seen so many sawflies in Alaska, where he's worked for more than 20 years.
In 1993, he mapped about 12,000 reddish acres of trees where larch sawfly larva were dining. In 1994, the infestation dropped to just 300 acres. In 1995, he noticed a sharp increase--about 116,000 acres were defoliated by larch sawflies.
In the summer of 1996, it's hard to find a tamarack that hasn't been feasted upon. Larch species imported from elsewhere, such as Siberian larch, also have been hard hit, although the sawflies seem to prefer tamaracks.
Although the sawfly larvae can denude a tamarack tree in a few weeks, it takes several years of such rude treatment to kill a tree. Young trees growing in poorly drained, thin soil may die in four or five years of constant attack, but trees growing on better sites can sustain as much as 10 years of de-leafing by the sawflies, Holsten said. Without green leaves to gather the energy of the sun, however, the tree's don't grow much while under attack.
Why is 1996 "The Year of the Sawfly?" Holsten suspects that a series of warm, dry summers in the 1990s has provided ideal conditions for the cold-blooded critters. Insect populations are cyclical, he added, and it might be hard to find a single sawfly in a couple of years. It's not hard now, though. Just shake an infested tamarack tree and watch the rain of the caterpillars.
