Processionary caterpillars move in single file by following a silk thread containing a pheromone laid down by the lead caterpillar. This behavior allows them to stay grouped to protect themselves from predators and to facilitate their movement toward suitable locations for their development.
Processionary caterpillars live together from birth: they stay in groups and always move sticking close to each other, forming those famous single-file lines. This gregarious behavior (living in groups, basically!) gives them several advantages: they retain moisture better and avoid the cold by staying tightly packed against each other. It also helps them detect food better and protect themselves against certain predators, as they appear more imposing together. So, it’s a very clever instinctive reflex that really boosts their chances of survival.
In processionary caterpillars, pheromones are the key to movement. These are chemical substances produced by glands located at the rear of the caterpillars' bodies. As they move forward, they deposit these scent signals on the ground, creating a true chemical path that the ones behind can easily detect and follow. Each uses its small sensitive antennae to pinpoint exactly where the previous caterpillar has passed, resulting in that famous perfectly straight procession line. This mode of operation allows the entire group to stay coordinated without any conscious effort: each caterpillar only needs to focus on the pheromones left in front of it. The higher the number of caterpillars depositing pheromones, the stronger the chemical intensity becomes, thereby reinforcing the established route and reducing the risk of straying from the group.
Moving in a line allows processionary caterpillars to improve their chances of survival. They save energy by following the trail already created by the first caterpillar. This behavior also prevents the ones behind from having to find their own way, reducing the time spent exposed, where they are more likely to be spotted by predators. Moving in a group also makes them appear larger and more intimidating, thus discouraging some attacks from birds or small mammals. Furthermore, staying together helps maintain an optimal body temperature, especially when it is cold. This promotes their growth and increases their chance of reaching the adult stage.
This mode of movement in single file sometimes exposes the caterpillars to several serious dangers. For example, they move slowly, which makes them particularly visible and vulnerable to certain predators, such as birds or some parasitic insects. Moreover, if the lead caterpillar takes the wrong direction or makes unnecessary detours, the whole group blindly follows, potentially wasting a lot of energy or risking death by exhaustion. And if part of the group is cut off or separated, these isolated caterpillars are often unable to find their way back to the right direction or their colony on their own, greatly reducing their chances of survival. Finally, their strong dependence on pheromones means that the slightest disturbance (for example, an obstacle or heavy rainfall) can completely disrupt the procession, causing disorientation and increased mortality.
The processionary caterpillars get their name from their typical movement behavior: they always follow a very precise line, guided by pheromones left by the lead caterpillar.
The processionary caterpillars are primarily nocturnal: they typically leave their nest at night to feed on pine or cedar needles, thereby limiting their exposure to daytime predators.
The urticating hairs of processionary caterpillars can cause severe allergic reactions in both humans and pets. Therefore, it is advisable not to touch them or get too close.
The procession of caterpillars is an anti-predation strategy: by forming a long line, they can resemble a single snake or other dangerous animal, thus deterring some potential predators.
Yes, their stinging hairs can cause significant allergic reactions in humans, such as itching, skin lesions, or respiratory issues. It is important to never touch them directly or approach them without protection.
Yes, processionary caterpillars have various natural predators, such as certain birds (titmice, hoopoes, cuckoos), parasitoid insects (parasitic hymenoptera), and various invertebrate predators. Thus, promoting biodiversity helps to naturally limit the populations of these caterpillars.
The processionary caterpillars primarily feed on pine needles (for the pine processionary, Thaumetopoea pityocampa) or oak leaves (for the oak processionary, Thaumetopoea processionea). They can cause significant damage and weaken infested trees.
Dogs and cats can be particularly exposed to the risks posed by processionary caterpillars. Contact with the irritating hairs can lead to severe inflammations, primarily affecting the mouth, tongue, and respiratory tract of the animals. In the event of contact, it is essential to consult a veterinarian promptly.
The removal of processionary caterpillars can be achieved through several methods, including the installation of pheromone traps to capture adult butterflies, the use of eco-traps around the trunks of infested trees, or the intervention of companies specialized in biological treatment, particularly with Bacillus thuringiensis.
The procession of caterpillars is usually observed in the spring when they leave their collective nest to reach the ground in preparation for their metamorphosis into chrysalises. In Western Europe, this period often extends from March to May, depending on the regions and climatic conditions.
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