Summer World_ A Season of Bounty - Bernd Heinrich [58]
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.
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,