A striking feature of the pine processionary’s life cycle is that the insect spends the winter in the caterpillar stage. Since insects are ectothermic, producing no body heat, one would expect that they would lie immobilized in their nests on cold winter days. To determine how the caterpillars fared during winter, I set up activity monitors near nests in mid-February. The monitors projected invisible infrared beams across pathways the caterpillars used when they foraged. When the beam was interrupted by a passing caterpillar, the signal was sent to a data logger, time-stamped, and later uploaded to a computer for analysis.
During the week of my study, temperatures in the afternoon reached average highs of 63 degrees Fahrenheit, but plummeted rapidly to below freezing after sunset. Yet the monitors revealed that the caterpillars only left their nests after dark and returned before dawn. One morning I observed that water had turned to ice in a pot left outside overnight, and I was sure I would find that the caterpillars had spent the previous night snug in their nests. But the data loggers revealed that they had been active outside their nests until the temperature dropped below 28 degrees, at which point they became immobilized. As soon as the first rays of the morning sun struck them, their bodies warmed, allowing them to make their way back to the nest, and by 9 a.m. all were home.
The study revealed that the caterpillar has one of the lowest “chill-coma” temperatures (the temperature when all activity ceases) ever documented for an insect. It is likely that selection pressure from daytime predators accounts for that seemingly strange behavior. For not all predators are put off by the pine processionary’s toxicity. Remarkably, the hoopoe, the great tit, and the great spotted cuckoo are all able to feed on the caterpillars without ill effect.
Although the nest serves as a secure retreat, it has another equally essential function. During sunny days, it traps solar radiation. Thermo-sensitive probes I placed in nests in February registered as high as 100 degrees during the day. Heat trapping warms the caterpillars enough during even the coldest days to enable them to efficiently metabolize the food they’ve collected during their nightly forays.
In early March the caterpillars take their last meal, an event followed by a grand procession. Up to now they have confined their forays to the host tree, but on this occasion they march down to the base of the tree and set off over the ground in a snaking procession. The leader advances, goaded on by the mass of caterpillars that push from behind. The caterpillars are in search of a place to spin their cocoons. Along the way they test the ground, seeking loose soil, and eventually they bury themselves side by side several inches underground to undergo metamorphosis. The timing is right because the nests would be much too hot to occupy in the torrid summers that characterize much of their range. The pupating processionaries stay underground until August, when, as moths, they burrow up through the soil [see illustration below]. Children who uncover one of the communal crypts while digging in sand in the summer become inadvertent victims of the caterpillars’ hairs. The abandoned nests, too, continue to spew toxic particles into the air well into the summer.
Pine processionary moths lay eggs on the needles of pine trees every August. When the young larvae hatch, they construct and abandon a succession of small nests at different spots on their home pine tree. After two molts they build a large, permanent nest (center) that is formidable to intruders, thanks to toxic hairs strewn throughout, and that provides good shelter through the winter. In March the caterpillars leave their nest in search of a site where they can bury themselves underground, spin cocoons, and metamorphose.