Hollowed out

SITKA spruce trees of Alaska are proving to be no more than sitting ducks for the budworm caterpillars that are munching their way through the forests. In the past five years, these caterpillars have affected as much as 40,000 hectares (ha) of sitka spruce. Ecologists are now blaming higher temperatures and, after a warm start to the summer this year, are preparing themselves to face the most destructive year yet.

Although native to Alaska, the western black-headed budworm, Aderis gloverana, has only recently become so lethal to the trees. After hatching, it burrows its way into the buds of sitka spruce and ties them shut with silk. The larvae feed on the buds through the spring, and if conditions are favourable, they emerge en masse as caterpillars in the summer, swarming over the trees and eating their needles, which hampers the tree's growth and often leads to its death.

The budworm is not the only threat to the Alaskan coniferous forests. In recent decades, the spruce bark beetle, Dendrotonus rufipennis, has killed an estimated 1.3 million ha of sitka and white spruce hybrids. The beetle uses its antennae to detect subtle chemical signals from the spruce trees and lays its eggs in those that are slightly stressed or weakened. It them emits pheromones that attract more beetles.

Several researchers believe the plagues of bark beetles and budworms in Alaska are related to the changing climate. Ed Holstein and Roger Eurnside, both ecologists with the us Forest Service in Portland, USA, note that "there has been a significant warming trend throughout the south-central and interior Alaska for at least 60 years" (Western Forester, Vol 42, p 8).

Meanwhile Glenn Juday, a forest ecologist at the University of Alaska at Fairbanks, USA, points out how recent summers have allowed the bark beetle to halve the length of its life cycle from two years to one. At the same time, argues Juday, warmer, drier conditions are stressing and weakening trees, making them more vulnerable to insect attacks.

The most recent outbreaks of bud-worms and bark beetles began after an exceptionally dry summer in 1993. Numbers declined after a relatively cool, moist year in 1995, but are now rising again after a warm 1997. The United Nations' Inter-governmental Panel on Climate Change has predicted earlier springs and warmer, drier summers in the Arctic and sub-Arctic regions in coming years. It says a combination of insect damage, fires and other climate-induced changes could eventually lead to a loss of up to 65 per cent of the world's northern forests.