“None Dwell in Their Tents”
Let their habitation be desolate; and let none dwell in their tents.
Psalm 69:25 (KJV)
Eastern tent caterpillars (Malacosoma americanum) are a common sight this time of year, especially in their last instar, after they abandon their tents. They stray onto the porches and wander over the furniture, looking for protected places in which to pupate. On a visit this past Monday, my three-year-old niece Elanor decided that they were cute — not typically my own reaction — and began petting them, prompting the caterpillars to arch their heads back like housecats. Yesterday afternoon, I watched one crawling up the neck and across the face of a box turtle, which merely shut one eye while the caterpillar took its measure.
The tents began appearing at the end of April in unusual numbers, especially on black cherry trees, which are the favorite food source for tent caterpillars and occur in unnatural abundance across Pennsylvania due to 150 years of clearcut logging practices and the reversion of old fields and pastures. Black cherry is a common first-succession tree species in many forest types, and thanks in part to its relative unpalatability to white-tailed deer, it can form almost pure stands in many areas that would have formerly hosted oak-hickory, beech-hemlock, or mixed deciduous forests. Here in Plummer’s Hollow, many of our southeast-facing slopes are dominated by black cherry stands, and I figured they’d be completely defoliated by this time.
Instead, Sapsucker Ridge is white with blossoms, filling the air with an ambrosial scent. One finds only a few black cherries as badly defoliated as the one in the second photo; the tree above is more typical. Though dotted with tents, only scattered branches have actually been stripped of their leaves.
A closer look reveals that most of these tents are filled with dead caterpillars. The few still alive twitch spasmodically. What happened? I’d guess that the unusually cold, wet weather over the past few weeks is at fault. Nighttime temperature routinely dropped into the low 40s this month, and sometimes even into the high 30s; daytime temperatures rarely exceeded the mid-50s; and rain was almost constant for the first three weeks of the month. Not only would the cold have shut down their temperature-sensitive digestive systems for prolonged periods, but the rain would have kept them confined to their silken tents, and the two together would’ve made them much more susceptible to starvation and disease. According to the Wikipedia article on tent caterpillars,
The tent is constructed at a site that intercepts the early morning sun. The position of the tent is critical because the caterpillars must bask in the sun to elevate their temperatures above the cool ambient temperatures that occur in the early spring. Studies have shown that when the body temperature of a caterpillar is less than about 15 °C [59 °F], digestion cannot occur. The tent consists of discrete layers of silk separated by gaps and the temperature in these compartments varies markedly. Caterpillars can adjust their body temperatures by moving from one compartment to another. On cool mornings they typically rest in a tight aggregate just under a sunlit surface of the tent. … Later on in the spring, temperatures may become excessive at mid day and the caterpillars may retreat to the shaded outside surface of the tent to cool down.
Entymologist Vincent G. Dethier’s wonderful and evocative classic, The World of the Tent-Makers: A Natural History of the Eastern Tent Caterpillar (University of Massachusetts Press, 1980) describes in Chaper 13 (appropriately enough) the battery of predators and diseases that keep this native insect in check. He details the spread of a deadly virus from colony to colony, then adds:
As if that were not enough the unusually wet late spring had been kind to molds, mildews, smuts, blasts, and bacteria. A particularly virulent spore-forming species of bacterium struck many of the colonies. … The enormous population of tent caterpillars had been cropped by weather, starvation, ants, bugs, parasites, fungi, viruses, bacteria, and misadventure in general. It had been a particularly trying year. Summer had hardly begun and the die had already been cast for the year to come. There would be fewer moths, fewer egg masses, and fewer colonies of the next generation.
So a spring that spells bad news for many farmers is good news for the wood products industry, which relies heavily on black cherry in Pennsylvania. Unlike fall webworms, which come too late in the season to have much of an impact on the trees they defoliate, tent caterpillars can greatly stress the trees they defoliate during their periodic outbreaks.
As global climate change plays hob with our weather patterns, it will be interesting to watch the effects on insect outbreaks, and over the long term, on forest succession. For example, if black cherries continue to predominate as many foresters would like, more-frequent icestorms would have a much greater impact than they would if less brittle species such as red oaks and tulip poplars took their place. Warm winters are said to promote the spread of pest insects, but what about warm winters followed by cold springs? I’d heard that the state was due for some pretty large gypsy moth outbreaks this summer as well, but here in Plummer’s Hollow, at least, their caterpillars are few and far between.
Cross-posted to Via Negativa.