Summer World_ A Season of Bounty - Bernd Heinrich [57]
LONGHORNS ARE AN IMPRESSIVELY LARGE AND DIVERSE group, and they are not exclusive to Texas. They are certainly more interesting than those notorious longhorns that are sliced and served at restaurants. I’m talking about longhorn beetles, of course—those that go by the family name of Cerambycidae. You need a field guide to identify most of them, and my guide with color plates, by Douglas Yanega, which is restricted only to those of New England, lists 344 species. All have long curved “horns” (antennae). Body markings vary from pastel browns, grays, and black to garish yellows, blues, and orange-red. The colors are arranged in all sorts of intricate stripes and patches. I don’t think I’ve seen more than a dozen or two dozen species, although some can’t be missed. The adults in one group of species feed on flowers in the clearing around my cabin in the Maine woods. In most other species the adults don’t feed at all. The larvae of many species eat bark and wood, and make themselves conspicuous by their feeding “tracks,” which you see inscribed (along with those of some other insects) on the surface of logs when you peel off loose bark.
Although most longhorn beetles don’t feed as adults, their larvae can be a nuisance or even a menace to trees. However, this situation could be worse—most trees have evolved defenses against longhorns, which are one of their main enemies. For example, balsam fir trees exude sticky resin when their bark is injured, and any beetle grub attempting to enter the body of the tree will be immediately challenged by a chemical counterattack of sticky, unsavory petrochemical-scented resin. In turn, the beetles have evolved a more nuanced attack. They wait until a tree is dying or very recently dead and defenseless before they lay eggs on it, and their larvae can start to eat the still moist and as yet unspoiled carcass. Indeed, longhorn beetles have an uncanny ability to detect the smell of death and injury on a tree, because invariably in the summer when I chop down a pine, fir, or spruce, one group of these beetles, the sawyers, Monochamus, come flying in—within minutes! Undoubtedly the beetles’ chemical sensors, arrayed on their “horns” (which are a little longer than body length in females or more than twice body length in males), are attuned specifically to chemicals in pitch, and in the case of the males, presumably also to the females’ scent.
The larvae that hatch from the sawyers’ eggs burrow under the bark and later bore into and through the wood. Within weeks you hear their loud chomping—a common summer sound in the Maine woods, resembling that of a crosscut saw. Piles of “sawdust” (dried digested wood) accumulate under most logs. However, as far as I know, the sawyer beetle grubs are never successful in attacking a healthy tree. As a rule, longhorn beetles attack only dead or dying trees, and when they do, it’s in droves. Like bark beetles attacking a fir tree, wolves attacking a moose, or male wood frogs attracting females, they succeed by cooperation even though they are proximally opportunistic competitors.
Although very few longhorns can successfully handle a whole live tree by direct attack on the trunk, some can take trees limb by limb. I was surprised, for example, when I found the one-third-inch-thick twig of a red oak tree on the ground next to my cabin. How did this get here? I wondered. It looked as if it had snapped off, but oak twigs don’t snap off as neatly as the break seemed to indicate. Looking closer, I saw that the twig had been girdled. I immediately suspected a longhorn beetle larva. It seemed logical to suppose that in the summer the nutrients traveling down from the leaves through the branch were accumulating