Among the willow trees of California's Sierra Nevada mountains, a good offense has turned into a bad defense.

Like many plant species, willows have evolved the ability to defend themselves by synthesizing chemicals in their leaves that either repel or kill leaf-eating insects. (One reason so many vegetable plants domesticated for human consumption are vulnerable to insects is that these bitter-tasting toxins have been bred out.)

The willows produce salicin, an aspirin-like substance once used in herbal medicine, which acts as an irritant, discouraging insects from eating willow leaves. But, as three ecologists from the University of California, Irvine, have found, at least one species of beetle has outwitted the willow.

The beetle larvae, which eat willow leaves, have evolved the ability to convert salicin into a slightly different chemical, salicylaldehyde, that is toxic to the larvaes' chief predator, a wasp. Not only do willow leaves die to feed beetles, they enable the beetles to chew voraciously, free of their predators.

According to John T. Smiley, Jonathan M. Horn and Nathan E. Rank, who report their findings in the current issue of Science, willows with the highest levels of salicin had the worst amount of leaf damage from the beetle.