Spring brings with it growing grass, jumping fish and an enduring race between tent caterpillars and their host plants. Seemingly ubiquitous in Central Oregon, the industrious tent caterpillars pitch thousands of tiny silken tents among the upper leaves of awakening bitterbrush throughout the region.
Those, oh, best beloved newly hatched tent caterpillars trying to get their foot in the door before their host plant discovers they are there and outgrows them; a race that goes on every spring—probably ever since tent caterpillars discovered bitterbrush.
Moderately sized moths in the genus Malacosoma, tent caterpillars live throughout North America, including Mexico, and in Eurasia. Twenty-six species have been described, six of which occur in North America. They often are mistakenly and unfortunately considered pests due to their habit of defoliating shrubs and trees (if they win the race). Among the most social of all caterpillars, they often work together like bees.
Tent caterpillars (moth babies) are recognized easily because they are social, colorfully fuzzy, always active in the sunlight—and build those conspicuous silken tents in the upper branches of our bitterbrush.
Tent caterpillars hatch from their eggs in the early spring at the time the leaves of the bitterbrush are just unfolding. The tent is constructed at a site that intercepts the early morning sun — a vital element in tent caterpillar racing.
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. ("Cool" is a relative term in our part of Oregon; it only means the ice on the frog pond is less than 2 inches thick. "Cold" is another matter entirely.)
Studies have shown that when the body temperature of a caterpillar is less than about 15 degrees C, digestion cannot happen. The tent helps boost the temperature: It consists of layers of silk separated by gaps, and the temperature in these compartments varies markedly, so caterpillars can adjust their body temperatures (if it's not snowing) by moving from one compartment to another.
On cool mornings they typically rest in a tight ball of fuzz just under the sunlit surface of the tent. It is not uncommon to find the temperature of the fuzzball is as high as 54 degrees F, warmer than the surrounding air temperature on cold but sunny spring mornings. Later on in the spring, temperatures may become excessive at midday and the caterpillars may retreat to the shaded outside surface of the tent to cool down—but in our part of Oregon that means they might get caught in a snowstorm!
Now about the "race."
Tent caterpillars require young, tender leaves to help them grow. Thus, they must complete their larval development before the leaves of the host plants become too old and tough for them to eat.
At the onset of a foray of foraging, caterpillars leave the tent en masse, moving to distant feeding sites on the host plant as soon as they can move in the morning sun. Immediately after feeding, the caterpillars return to the tent and join others in the warmth and sunlight to facilitate the digestive process.
Our prolonged cool mornings of spring, however, provide bitterbrush with a pole position in the race, giving them a head start on growing tougher leaves that caterpillars can't digest; before the caterpillars warm up.
The observer of the caterpillar-plant contest often will see the insect-losers in their tents, desiccated by the heat and dryness of the High Desert Country. The caterpillars are black, shriveled-up husks of their once fat and happy selves. But when the caterpillar wins, the host plant is almost completely defoliated.
In the long run, however, there really isn't a distinct winner or loser. The caterpillars that made their nests too close to a fast-growing pine that shades the bitterbrush in early summer will lose. In spite of the shade, the plant grows tougher leaves and the larvae can't outrun the growth because they can't get warm soon enough.
In other locations with mini-climates, sometimes the caterpillar wins, and the plant is partially defoliated, but will not die. In the long run, both species will go on-and-on; it is a race that doesn't need outside meddlers. That means it is not necessary to use chemicals to change the balance of nature, which means our native insect pollinators will be healthy to carry on their vital role in the Big Picture—and our honeybees will also live to see another day.