New England Longhorns - Summer World: A Season of Bounty - Bernd Heinrich

Summer World: A Season of Bounty - Bernd Heinrich (2009)

Chapter 12. New England Longhorns

23 July 2005. TODAY I FOUND SEVERAL FOOT-LONG LIVE oak twigs on the ground under the tree I had planted about fifteen years ago next to the cabin. How could they possibly have broken off? Looking at the breaks, I see a circular groove; they have been girdled, as though by a sharp knife. There is nobody else here, and they came off from near the top of the tree. I wonder what happened, and I sketch what I see. Is it a sign of a longhorn?

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 at the girdle where the larva could intercept them. But, no, I learned that girdling is the work of an adult female. In this case the female had worked long and hard for many days, chewing through solid wood, to girdle the branch. Then she had deposited an egg in the dying twig. The twig would then break off. That is, she had neutralized the tree’s ability to defend its limb and provided food for her offspring. I had witnessed a week’s summer work of the oak girdler, Oncideres quercus.

There are exceptions even to the generalization that the larvae are restricted to dead trees or parts thereof. One of them, the sugar maple borer, Glycobius speciosus, is a large, conspicuous beetle with bold yellow markings that mimics a yellow jacket wasp. (Not to be confused with the large black white-spotted Asian longhorn beetle, Anoplephora glabripennis, that is currently infecting sugar maples in New York and Chicago.) This native sugar maple borer would be hard to miss if it were to fly by or land near you, and the evidence of its presence in our New England woods is even more prominent. This beetle deposits its eggs under sugar maple bark crevices in the summer. The larva then chews into the bark and continues to chew, making a burrow in the inner bark and sapwood. Sugar maple, this longhorn’s food plant, is not called rock maple for nothing, but the soft, flabby white grub manages to chew through the solid live wood with its pair of small but apparently iron jaws (mandibles), to make a chamber in the wood where it stays for the winter. It resumes feeding under the bark during the following summer—unlike most insects, for whom one summer per lifetime is enough. A baby warbler eating caterpillars can reach full size in six days, but the diet of wood eaten by the longhorn larva necessitates slow growth—it reaches full size only at the end of its second summer. The next spring, it burrows deep into the solid wood, where it excavates a cavity and leaves an exit hole for the adult to finally escape during the third summer, to complete its brief life as an adult.

Unlike the young pine sawyer grubs that feed in the inner bark of a dead pine by making random burrows, the young sugar borers often burrow horizontally in the inner bark of the upright tree. This inner bark is phloem, a live tissue that transports the photosynthetic products of the tree, principally sucrose, downward. The larva’s feeding interrupts this nutrient flow and by girdling the tree, produces maximum damage. The girdling kills the wood above and below the feeding furrow of a single larva, and that furrow later leaves a huge scar on the tree, one that becomes more visible as the tree continues to grow.

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Fig. 25. Feeding damage of the native sugar borer.

The destructive girdling feeding pattern could have a practical advantage for the larva. Perhaps the tree can cut off nutrient flow around a larva, but by girdling nearly all the way around the whole tree the larva is assured of a fresh food supply. Even a single larva could easily kill a tree, if it went just a little bit farther to complete a full circle all around the tree, as the oak borer always does around a twig. And now we have an irony and a puzzle: the beetles, despite their deadly power, damage but do not devastate the population of sugar maple trees.

As I have indicated, the pine sawyers converge and come a-flying, probably from miles around, to attack a single injured or dying tree, and that tree is then soon riddled with hundreds of larvae within days. In the woods of New England there are sugar maple trees of all ages, and they are one of the most dominant of forest trees. Since the sugar borers attack only healthy trees, they have a practically infinite food supply that stretches from southern Canada south to North Carolina and west to Minnesota. Any adult beetle emerging from one tree has other food trees directly adjacent to it, and it could presumably lay hundreds of eggs and summarily kill them all. Yet by far the majority of sugar maples, although this tree is the exclusive host of this beetle, are uninjured. The beetle’s typical mark is found in only one of perhaps hundreds of trees. The beetle defies the predictions or extrapolations of what would almost qualify as one of the many unsavory “laws” of nature: to multiply until resources are exhausted, and then to “crash” in a massive die-off that then starts the whole process all over again.

Why does the sugar borer’s population not skyrocket? Why does the borer not eat all that it can until its main resource, sugar maple trees, has been devastated? What prevents the familiar, often frightening scenario that is generally avoided only because of parasites, diseases, and predators that multiply as soon as the population increases above a critical level? Nobody knows. This is not rocket science, but it too is complicated.

The sugar borers have achieved, or are held to, something enviable. They are in a world of plenty, so none go hungry, destroy their habitat, or jostle and interfere with each other. Somewhere there is a check on their natural rate of increase, and you can be sure of one thing—that if they could tell us what they wanted at any one time, they would vote to obliterate the forces that hold them in check, the forces that ensure their long-term benefits. And so, probably, would we, if we voted merely on the basis of our individual interests.