Pine Processionary Caterpillar Thaumetopoea pityocampa

Shelter Building
Midwinter Foraging and Thermoregulation
Trail Making and Processionary Behavior
Anti-Predator Defense


Overview and Life History

The pine processionary caterpillar is the best known of all the processionaries, 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.  Of interest here, however, is the fact that is among the most social of caterpillars.  Sibling groups stay together throughout the larvae stage, often pupating side by side at sites they reach by forming long, over-the-gound, 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 and the caterpillars typically eclose from them four or more weeks after they are laid.  The eggs are completely covered with scales that detach from the abdomen of the female.
egg mass of the pine processionary
Newly eclosed caterpillars on egg mass

Despite their small size, the newly hatched caterpillars have remarkably strong mandibles and are able to penetrate the tough needles of the host from the start. 
First instar caterpillars feeding on pine needles


Shelter Building

They 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 in the third instar 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 frass that is produced as the caterpillars process their meals accumulates at the bottom of the shelter.
Temporary shleter of early instar caterpillars built over egg mass
Permanent shelter of caterpillars


Midwinter Foraging and 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 bran ches 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 locomote, albeit very slowly, at sub-zero temperatures.  The permanent nests are typically sited to intercept solar radiation 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 caterpillara are in the fifth instar and are fully 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.
Caterpillars moving to feeding sites at night
Infrared activity monitor and caterpillars moving to feeding sites at night

Caterpillars feeding at night
Caterpillars feedin at night


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 c aterpillars 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.  Most likely, silk serves to increase steadfastness on the smooth plant surfaces. Pine processionary caterpillars can distinguish old from new trails. They 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 marker to find their way.


While the caterpillars may form single file processions as the advance over the branches of the host tree in search of food,  the most spectacular processions are formed when the caterpillars are fully grown and abandon the host tree in search of pupation sites.  Over-the-ground processions of as many as three hundred caterpillars have been observed.  The caterpillars may travel long distances from the natal tree looking for soft soil in which they bury themselves an d form their cocoons.
caterpillar in line

Studies show that during these head to tail processions, stimuli associated with setae found on the tip of the abdomen of the precedent caterpillar serve to hold processions together and such stimuli take priority over those associated with either the trail pheromone or silk.  A caterpillar can be readily induced to follow a model made by attaching the integument of the abdomen of a killed caterpillar to a wooden dowel .  processionary folloing model


Anti-Predator Defense

The caterpillars of the pine processionary are highly urticating in the third and subsequent  instars.  Contact with the hairs causes skin rashes and eye irritations. Susceptible individuals may also develop an allergic response to a protein associated with the hairs of the caterpillar.  larva



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Dajoz, R. 2000. Insects and Forests. The Role and Diversity of Insects in the Forest Environment.  Lavoisier publishing, Paris.

Demolin, G. 1962.  Comportement des chenilles de Thaumetopoea pityocampa au cours des processions de nymphose. C. R. Acad. Sci. 254:733-744.

Demolin, G. 1971.   Incidences de quelques facteurs agissant sur le comportment social des chenilles de Thaumetopoea pityocampa Schiff. (Lepidoptera) pendant la période des processions de nymphose. Répercussion sur l’efficacité des parasites. Ann. Zool. –Ecol. anim., no hors série: 33-56.

Ducombs, G., Lamy, M., Mollard, S., Guillard, J. M., and Maleville, J. 1981. Contact dermatitis from processional pine caterpillar (Thaumetopoea pityocampa Schiff., Lepidoptera). Contact Dermatitis 7:287-288.

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Fabre, J. H. 1916. The Life of the Caterpillar. Dodd, Mead and Co. Inc., New York.

Fitzgerald, T. D. 2003. The role of a trail pheromone in the foraging and processionary behavior of Thaumetopoea pityocampa. J. Chem. Ecol.  in press.

Fitzgerald, T. D. 2008. Lethal Fuzz. Natural History Magazine 117:28-33 (September) also: http://www.naturalhistorymag.com/master.html?http://www.naturalhistorymag.com/0908/0908_feature.html

Halperin. J. 1990. Life history of Thaumetopoea spp. (Lep., Thaumetopoeidae) in Israel. J. Appl. Ent. 110:1-6.

Lamy, M., Pastureaud, M-H., Novak, F., Ducombs, G., Vincendeau, P., Maleville, J., and Texier, L. 1986.  Thaumetopoein: an urticating protein from the hairs an integument of  the pine procession catepillar (Thaumetopoea pityocampa Schi ff., Lepidoptera,Thaumetopoeidae).  Toxicon  24:347-356.

Mallmann, R. J. 1962. Observations sur les réactions tactiles de la chenille processionnaire du pin, Thaumetopoea pityocampa Schiff.  Insectes sociaux 9:335-345.

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Vega, J. M., Mo neo, I., Armentia, A., Fernández, J., Vega, J., De La Fuente, R., Sanchez, P., and Sánchís, E. 1999.  Allergy to the pine processionary caterpillar (Thaumetopoea pityocampa).  Clinical and Experimental Allergy 29:1418-1423.

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