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A Cottonwood, a Beaver, and a Beetle

This Moment of Science examines the unlikely relationship between the cottonwood tree, the beaver, and one particular leaf-eating beetle. Many trees are equipped with a defense mechanism for use against their herbivore predators. When the predator takes a bite into the tree, the tree retaliates by releasing noxious chemicals into its new leaves. These chemicals steer away most critters that try to feast on the tree.

However, this defense mechanism doesn't quite work in the case of the cottonwood. When a beaver chews on a cottonwood, the tree does release these noxious chemicals as usual, but the bitter chemicals released into the tree's new leaves actually attract a certain leaf-eating beetle.

The leaves that sprout up after a cottonwood has been cut down by a beaver attract fifteen times as many of these adult beetles as the leaves of an untouched cottonwood. And in an experiment, researchers found that the beetles grew faster and larger on these chemical-bearing leaves. Not only that, but the beetles appropriate these chemicals for use in their own defense against predators.

Ants, which are common on cottonwood trees, like to eat beetle larvae. Larvae on these cut-down cottonwoods, however, have a surprise for their ant predators. They store the noxious chemicals from the tree's leaves in their glands, and when an ant disturbs them the larvae turn the glands inside out, exposing drops of a repugnant, bitter fluid that sends the ants running away.

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