Arboreal Self-Defense

Trees know what it takes to relieve gypsy moth headache

Deploying high concentrations of an aspirin-like compound as a chemical defense, some aspen trees are capable of spoiling the voracious appetites of gypsy moth caterpillars, one of the world’s most feared forest insect pests.

Writing in the scientific journal Oecologia, UW–Madison scientists Richard Lindroth and Shaw-Yhi Hwang report that some aspen trees produce in their leaves an effective chemical defense against an insect pest that has so far stymied nearly every effort to slow its destructive advance through North America’s forests.

The discovery, says Lindroth, sprung from the observation that in stands of aspen infested with gypsy moths some trees are largely untouched, while others are stripped of foliage.

“Aspens are one of the most widely distributed trees in North America and are one of the gypsy moth’s major foods in the Great Lakes region,” says Lindroth, an ecologist who studies the interrelationships of plants and insects. “But it’s well known that not all trees in a stand are uniformly susceptible to defoliation.”

The unanswered question was why, and the answer, he says, seems to lie in a genetic tradeoff between growth and defense: “Trees that grow fast are poorly defended. Trees that are well defended grow poorly. It seems you can’t have both guns and butter.”

Using aspen collected in Wisconsin and Colorado, Lindroth and Hwang measured insect performance on eight trees grown from 13 different aspen clones, stands of trees sharing a common root system and comprising an individual organism. Monitoring survival, development, growth and food utilization, the Wisconsin scientists found some of the trees served as little more than leafy salad bars, while others seemed to weather the assault with slight damage from an otherwise pernicious pest.

These experimental results, according to Lindroth, mirror field observations of insect defoliation of aspens. The differences between the appetizing and unappetizing trees was revealed by chemical assay of the trees’ leaves.

The aspen that deter the gypsy moth caterpillars, says Lindroth, have in their leaves higher concentrations of salicylates, compounds closely related to aspirin. In big doses, these compounds not only make leaves unappetizing but toxic, says Lindroth.

“These aspirin-like compounds can provide resistance,” he says, “and there seems to be a strong genetic factor” that governs whether a tree grows efficiently or deploys these chemical defenses.

These new results, says Lindroth, may allow scientists to perform chemical surveys of aspens stands and predict their susceptibility to defoliation prior to any gypsy moth onslaught. In addition, better knowledge of trees’ natural defenses will help scientists devise new control methods, perhaps deploying chemical insecticides or biological controls on top of natural resistance factors.

Importantly, the new results may also spur the development of new lines of aspen better able to resist the rapacious assault of insect pests like gypsy moths. “Identification of natural resistance factors paves the way for development of not only more resistant aspen, but of more resistant hybrid poplars, trees that are close relatives of aspen and that are widely grown for use as biomass fuel and windbreaks,” says Lindroth.