Please mention The Gers Local when responding to adverts Yes they’re back! Processionary Caterpillars


You may have noticed the pine trees at the side of the road with their sad evidence of infestation—or be unlucky enough to have an infested tree in your garden—here are some of the facts!


Overview and Life History


The pine processionary caterpillar was studied as early as 1736 by Raumier and later by Fabre (1898) whose essay “ The life of the caterpillar” is among the classics of popular entomological literature. The insect is found in the warmer regions of southern Europe, the Near East, and North Africa. It is the habit of the caterpillars to move over the ground in long head-to-tail processions and to sting with urticating hairs anyone who attempts to molest them that has brought the caterpillars to the attention of the public. It is also one of the most destructive of forest insects, capable of defoliating vast tracts of pines during its episodic population surges. Sibling groups stay together throughout the larvae stage, often pupating side-by-side at sites they reach by forming long, over-the-ground, head-to-tail processions.


The insect is active only during the colder times of the year, spending the warm summer months as a pupa buried in the ground. The moths begin to emerge from the soil in August and shortly thereafter mate and seek out pine trees where they place their eggs. Each female produces a single egg mass which it fastens to a needle of a suitable host trees. Egg masses contain up to 300 or so eggs. The eggs are completely covered with scales that detach from the abdomen of the female.


Shelter Building


These caterpillars are highly social. At first they are nomadic, spinning and abandoning a series of flimsy shelters constructed by enveloping a few needles in silk but later they initiate the construction of a permanent nest and settle down to become central place foragers. There are no definitive opening in the shelter that allow the caterpillars to enter and exit. Rather, the caterpillars force their way through the layers of the shelter as they move in and out. The mess that is produced as the caterpillars process their meals accumulates at the bottom of the shelter.


Most likely, silk serves to increase steadfastness on the smooth plant surfaces. Pine processionary caterpillars can distinguish old from new trails. The caterpillars also differentiate between trails made by different numbers of caterpillars and follow preferentially those marked by the most caterpillars. Trail marking enables the caterpillars to aggregate at feeding sites and allows them to find their way back to nest after feeding. When they move over the branches, the caterpillars may move head to tail in small groups or they may move completely independently of others. In either case, they rely on the trail maker to find their way.


12 Midwinter Foraging & Thermoregulation


Colonies are active throughout the winter months. Activity records of colonies foraging in pine forests in mountainous regions near Barcelona, Catalonia, obtained with infrared activity monitors, show that the caterpillars leave their nests soon after sunset and travel to distant feeding sites located on the branches of the host tree.


Here they feed overnight then return to the nest at dawn. Observations in mid-winter show that the caterpillars forage on the coldest of nights and are able to move, albeit very slowly, at sub-zero temperatures. The permanent nests are typically sited to benefit from the suns rays and during sunny days warm to well above ambient temperatures.


The caterpillars rest in the nest during the day and at the elevated body temperatures they experience due to heating of the nest by the sun are able to efficiently digest the food they collect overnight.


By March the caterpillars are full grown. At this time the leave their nest, following each other in long, head to tail processions and seek out pupation sites in the soil.


Trail Marking and Processionary Behavior


Recent studies show that the caterpillars lay down a trail pheromone as they advance over the branches of the host tree. The unidentified marker is secreted from the ventral surface of the abdomen. The caterpillars mark when the tips of their abdomens brush against the substrate. Although the caterpillars also secrete silk and mark their pathways with the material,studies show that it plays little or no role in the elicitation or maintenance of trail following.


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