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The war within the aspen leaves

On one of the friendliest platforms imaginable, a ferocious battle rages.

While mowing its way through the surface of a trembling leaf, an aspen leaf miner meets one of its kind. Instead of offering a nuzzle of recognition, the tiny caterpillar tears into the other with its sickle-like mouthparts, while trying to avoid a fatal gash from the other.

Diane Wagner, Pat Doak and students who work with them are among the few humans who have seen this war within the aspen leaves. They think the fierce, silent conflict has probably benefited both the insect population and a tree that’s been under siege in Interior Alaska for a long time.

Leaf miners are moth larvae that stencil the surfaces of aspen leaves with their transparent tracks. After infestation, the leaves appear silvery from a distance.

Interior aspen trees have shimmered with these pale leaves in late summers since at least 2003, when Wagner and Doak, both researchers with the �IJ�ͼ�� Institute of Arctic Biology, first began studying them.

Driven by their curiosity, the biologists have become world experts on the handsome white moths smaller than a grain of rice, and the even smaller yellow caterpillars that hatch within aspen leaves.

“Aspen’s a weedy species,” Wagner said. “There’s no industry use of aspen pulp in Alaska. For us, the outbreak is purely a vehicle to study ecology, evolution and natural history.”

In their detailed look at the aspen leaf miner moth and larvae during the past few decades, Wagner and Doak have found that the creature has not wiped out stands of Interior Alaska aspens, a possible outcome scientists wondered about early in the outbreak.

“There’s no question they cause important tree- and stand-level effects, like less CO2 uptake and lower growth rates,” Wagner said. “But they haven’t killed many outright.”

Over the years, the researchers have found out many things about the little white moths and their offspring. The larvae within the leaves emerge in mid-July as white moths. They overwinter beneath spruce trees and can survive temperatures colder than 25 below zero Fahrenheit.

When the moths emerge in springtime, they mate and lay eggs on upper and lower surfaces of aspen leaves.

When those eggs hatch and the caterpillar tracks meet, sparks fly. The scientists discovered this when looking at leaves under a microscope.

“They fight!” Wagner said. “Sometimes, both die in the end.”

Each aspen leaf’s top and bottom surface can support only one or two caterpillars, but leaves often hold many more eggs in springtime. This self-regulation through mortal combat could benefit both the tree and the insect.

“They kill each other so effectively that on average the leaf damage is only about 65 percent; that preserves some leaf tissue that can function normally,” Wagner said. “That also probably helps the outbreak to continue.

“If the leaf miners didn’t kill one another, during a high-density year they could consume all the leaf tissue available,” Wagner continued. “And they’d still all starve.”

During this 20-year plus journey of studying a tiny organism that surrounds everyone who has lived in or visited Interior Alaska, Wagner and her colleagues have looked at everything from leaf genetics to the physiology of both aspens and a specialist that defaces their solar panels.

“It’s such a rich system,” Wagner said. “I’m a generalist, so I like to ask lots of different questions. It’s been really fun.”

Since the late 1970s, the �IJ�ͼ�� Geophysical Institute has provided this column free in cooperation with the UAF research community. Ned Rozell ned.rozell@alaska.edu is a science writer for the Geophysical Institute.