Why did the Pine Processionary Caterpillar cross the road?
My first encounter with the pine processionary caterpillar
I was on a bike ride when I saw them.
Peddling down the road, I was enjoying some early spring sunshine before the oppressive summer heat divided to make its appearance when I spotted this brown line on the road. At first, I thought it was a stick. And then I noticed the line moving. Not a stick. Then I thought maybe a snake, though its shape wasn’t like any snake I’d ever seen, the line too segmented, the movement too fragmentary.
It wasn’t until I was right on top of it that I realised that my moving stick, my fragmented snake was a chain of caterpillars.
Dozens and dozens of caterpillars — 37 of them to be exact (yes, I counted) — following each other in an orderly fashion, one after another, as their designated leader slowly guided them across the road.
37 orange and brown caterpillars with small blue-grey bands and copious amounts of hair covering their bodies.
I’d never seen anything like it despite having lived in this area of France most of my life.
Of course, I had to stop and take a closer look. I squeezed the breaks of my bike, laid the bike on the grassy verge, and then proceeded to spend the next 15 minutes crouched down in the middle of the countryside road, looking at these little wonders of nature making their way across the asphalt.
These little guys are called pine processionary caterpillars, named after the distinctive way they move, single-file and head-to-tail.
These insects will start out their lives in pine trees. The caterpillars will hatch from eggs high up in the trees in autumn, at which point they will immediately start eating every pine needle in sight.
They will create nests, webs of dense white silk that the caterpillars themselves have trouble penetrating, having to force their way out every time they need to feed. These nests are both a safe spot to hide away from predators and a warm place to shelter in during the winter months, the interior of the nest a few degrees higher than the surrounding environment.
The caterpillars will stay in these nests during the day, only coming out of the nests’ protective threads at night to eat their way up and down the pine tree. They will spend the colder months in those trees, feeding on the pine needles, growing and growing. They will moult three times before reaching their last caterpillar stage, the stage that I saw them at.
In spring, as the days lengthen and the landscape transitions into various shades of green, the caterpillar will march down the trunk of their pine tree in that single-file manner, descending to the ground one by one.
The caterpillars will follow each other close enough to feel stimuli from the setae (or hairs) of the individuals directly in front of them. This stimulus, this point of contact, will allow them to stay together throughout their journey, therefore creating a procession that can be up to 300 caterpillars long.
These caterpillars will forge across fields and roads, through prairies of tall grass or meadows of colourful flowers, sometimes crossing the path of a bike rider along the way. They will press on with the sole purpose of finding some soft ground in a bright area where the soil can regularly soak up the heat of the sun’s rays.
Once that soil is found, they will burrow underground, digging anywhere from 5 to 20 centimetres deep. Surrounded by the dirt, each caterpillar will weave a cocoon in which the transformation from caterpillar to moth will take place. This is what is called the pupa, the stage in an insect’s life where the immature larva turns into a mature imago (or adult). For butterflies and moths, this pupal stage is named the chrysalis. If the weather is right, this stage can only take a few weeks, but if it’s too hot or too cold, it could take years for that capillarity to reach adulthood.
At the end of the pupal period, on one summer’s eve, the moths will dig their way out of the soil, emerging into the open air, having reached they final stage of life.
The pine processionary moths are a drab grey-brown with darker markings and white spots adorning the wings.
These moths won’t survive long. A night, maybe two. Just long enough to mate and for the female to find a pine tree similar to the one she grew up in where she will lay her eggs on the very needles that will constitute the caterpillar’s diet. Once that is accomplished, the moths will die, the next generation of their species insured.
I didn’t know at the time that these insects were considered pests in my country. These tiny creatures have such voracious appetites that caterpillar colonies are capable of defoliating whole trees, making those pine trees more susceptible to diseases and parasites. The pine processionary caterpillars have provoked the deaths of so many pine trees to have caused serious economic damage in Southern Europe.
The caterpillars are also known for their urticating hairs, which can cause irritations, pain, rashes, and, in rare cases, serious allergic reactions. Thankfully, I didn’t touch them when I came across them (never touch nature unless you know it’s safe).
I stood on the roadside, my bike leaning against me as I waited for all 37 caterpillars to cross the road just in case a car came by, just in case these 37 larvae needed saving from an unfortunate tire.
Cars are a rare sight in this part of the middle-of-nowhere countryside. None come to interrupt the caterpillars’ crossing. My caterpillar-saving services were not needed.
I looked on as they disappeared one after another into the grass of the verge, their small size and understated colours granting them the power to vanish from sight.
Sources:
- Pine processionary moth
- La chenille processionnaire du pin
- Role of Trail Pheromone in Foraging and Processionary Behavior of Pine Processionary Caterpillars Thaumetopoea pityocampa by Fitzgerald, T.D.
- Periodicity and synchrony of pine processionary moth outbreaks in France by S. Li, J.J. Daudin, D. Piou, C. Robinet and H. Jactel
