Thermal properties of the tent of early instar colonies of the eastern tent caterpillar, Malacosoma americanum (Lepidoptera: Lasiocampidae)

Abstract

The extent to which the small tents of the early instars of Malacosoma americanum warmed when irradiated by sunlight was investigated by continuous electronic monitoring of field colonies for 137 colony-days. Tent temperatures exceeded the threshold for larval growth of 15 °C on 88–96% of the study days, exceeding in degree-minutes the ambient excess-over-threshold by a factor of approximately five. In still air, tents constructed by third instar caterpillars achieved greater temperature excesses over ambient than did a model of a tightly sealed glass house, but smaller tents and tents exposed to simulated wind did not perform as well. Tents of M. americanum warmed more rapidly and to a high temperature than silk nests of markedly different design constructed by other species.

Introduction

The communal silk nests of social caterpillars are multifunctional structures. They serve to shield caterpillars from predators and parasitoids, enclose moist internal microclimates conducive to molting, provide resting areas from which intermittent forays in search of food are launched, act in some species as communication centers from which nest mates are recruited to food finds, and facilitate en masse thermoregulation (Fitzgerald, 1993 and references therein; Fitzgerald, 1995, Rehnberg, 2002, Rehnberg, 2006, Ruf and Fiedler, 2002). It is the latter function that has drawn the attention of most researchers. The nests of different species of caterpillars vary markedly in size, shape, and in the density of their walls, but all show marked temperature gains when exposed to solar radiation (Knapp and Casey, 1986, Joos et al., 1988, Casey et al., 1988, Fitzgerald and Underwood, 2000, Ruf and Fiedler, 2002, Fitzgerald et al., 2003, Rehnberg, 2002, Rehnberg, 2006). The best studied of these structures are those of the lasiocampid caterpillars, the nests of which are referred to as tents.

Lasiocampids in the genera Malacosoma and Eriogaster construct their tents by laying down new strands of silk over existing tent walls during intermittent en masse spinning bouts. The strands are stretched slightly before they are attached and their subsequent axial retraction causes sheets of newly spun silk to tighten and lift from subjacent silk surfaces (Fitzgerald and Willer, 1983, Fitzgerald et al., 1991). Successive episodes of spaced spinning bouts result in the creation of a structure that consists of multiple layers of silk separated by air-filled gaps. When exposed to solar radiation, the layered structure of the tent creates a thermally heterogeneous environment within which the caterpillars thermo regulate by moving between warmer and cooler layers (Casey et al., 1988, Ruf and Fiedler, 2002).

Eastern tent caterpillars (M. americanum) are widely distributed throughout eastern North America (Stehr and Cook, 1968). They typically eclose from their eggs in mid-April and initiate the construction of a tent in the branches of the host tree shortly thereafter (Fitzgerald, 1995). Basking on or in the tent is essential to the growth of the caterpillars because ambient air temperatures in the early spring are often too low to allow the caterpillars to digest their meals. Both the forest tent caterpillar M. disstria (Hodson, 1941, Ives, 1973) and M. americanum (Knapp and Casey, 1986, Casey et al., 1988) grow little if at all when their T_b's (body temperatures) fall below approximately 15 °C. On cold mornings, eastern tent caterpillars are commonly found clustered just under the surface of their tent on the side that faces the morning sun. Their dark bodies absorb solar radiation and the walls of the tent limit convective heat loss, resulting in significant gains in T_b over T_a (air temperature). This enables the caterpillars to process food collected during their overnight forays (Knapp and Casey, 1986, Casey et al., 1988, Joos et al., 1988, Fitzgerald, 1995).

Although spot measurements of clusters of early instar eastern tent caterpillars by Knapp and Casey (1986) showed that the small caterpillars had T_b's in excess of T_a, all previous studies of the ability of lasiocampid tents to gain heat when irradiated have focused on the large, many-layered structures of the maturing caterpillars (Casey et al., 1988, Joos et al., 1988, Ruf and Fiedler, 2002). Heat trapping by the relatively thin and airy tents inhabited by the caterpillars during their first several stadia, however, is likely to be even more important to the growth and survival of the caterpillars. This is the case because the caterpillars are among the earliest to emerge in the spring and during the first few weeks of their lives ambient temperatures commonly fall below freezing. During early spring, basking is essential not only to growth but to survival as well and whole populations of M. americanum have perished during their first stadium when persistent cloudy and cold weather prevented the caterpillars from processing food (Blackman, 1918; personal observation). Ruf and Fiedler (2005) also determined that the inability of the caterpillars of Eriogaster lanestris to build an initial tent or their failure to maintain an existing tent were the most important factors affecting colony survival. We report here the results of a study of the small tents of first to third instar eastern tent caterpillars undertaken to determine their effectiveness in creating a thermal environment which fosters the growth and development of the caterpillars.