HOLLOWS, PEEPERS, AND HIGHLANDERS: AN APPALACHIAN MOUNTAIN ECOLOGY

28. Trees and Caterpillars

205 28 Trees and Caterpillars M uch of the energy passing through an Appalachian ecosystem follows a single path. Arriving as sunshine, energy then flows through deciduous leaves, moth caterpillars, and fecal rain. This is why, by late summer, it is nearly impossible to find an intact leaf. A forest of holey leaves shows that caterpillars are exerting tremendous feeding pressure on broadleaved trees. Yet the struggle between trees and caterpillars would not have become a dominant act had trees served merely as passive targets. In their own subtle ways, trees fight back. Let us first consider the larva’s role in the relationship, focusing on the eastern tent caterpillar. Residing east of the Mississippi River from Nova Scotia to Florida, this well-known insect overwinters communally in egg masses on its principal host trees, cherry and apple. Deposited near branch tips the previous summer, eggs hatch as leaf buds unfurl. Because a female lays all her eggs, which contain fully formed caterpillars, in a single cluster, the “cats” that develop from one egg mass are siblings. A few days after hatching, the caterpillars construct a tent near their tree’s center, a site offering branches for support and a central position from which to forage. This permanent tent is enlarged several times a day to accommodate the growing larvae. Cats build up the tent one layer at a time and the spaces between partitions provide room for up to three hundred individuals during the morning and afternoon rest periods. Small circular openings in the tent allow entry and exit. 206 During feeding excursions to nearby leaves, the caterpillars secrete strands of silk. Collectively, the strands form a conspicuous silk trail that provides purchase on smooth bark. When a hungry larva leaves the tent in search of food, it lays an exploratory trail chemical signal, or pheromone. Returning from a successful foraging trip, it lays a recruitment trail pheromone by brushing the tip of its abdomen on newly deposited silk trails. Following an old trail, the caterpillar elevates its abdomen, leaving no chemical. Because they are able to discern the age of a pheromone on a silk trail, cats can distinguish between trails to fresh food and paths to exhausted sites. Recruitment via pheromones leads to the efficient forwarding of caterpillars to the most productive foods, as well as the prompt abandonment of areas without young, tasty leaves. As cherry leaves age, they get tougher and drier, and provide less nutrition. As we shall see, from the tree’s point of view these changes may represent an adaptive strategy to protect itself from herbivory. Tent caterpillars create a microenvironment that allows them to be active on cool spring days. Like little greenhouses, tents admit sunshine and retain some of the converted heat within the silk layers , maintaining temperatures up to 40 degrees Fahrenheit warmer than the surrounding air. By adding more silk to the bright side of the tent, cats enlarge their greenhouse in the direction of the sun. Eventually, these sophisticated larvae turn into short-lived, plain brown moths. Albeit social and thereby efficient, the tent caterpillar is just one of thousands of species of Appalachian moths whose larvae eat tree leaves. Damage by insect herbivores decreases tree growth, seed production, and seed viability. Trees seem so vulnerable — they are a large target, rooted to one spot, without a fly-swatting tail. Yet by hook or by crook trees persist, in part through help from birds. Birds lend trees a little relief. At Hubbard Brook, New Hampshire ecologists used netting to exclude birds from patches of striped maple, an understory tree. After several days of protection, the densities of moth larvae were significantly greater inside the cages than outside. The researchers estimated that birds such as warHollows , Peepers, and Highlanders 207 blers, thrushes, and vireos removed an average of 37 percent of the caterpillars from the understory. This effect was most intense from late June to mid-July when adult birds were feeding their demanding nestlings. Bird predation most effectively controls insect numbers when the densities of larvae lie within a normal range. At such endemic population levels, birds reduce and thereby regulate the densities of larval moths on forest vegetation. During epidemic outbreaks, though, birds may take only 0.1 to 1.0 percent of the caterpillar biomass . The situation of caterpillars going amuck describes our current gypsy moth invasion. Trees also get...