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While on holiday in north-west France in March 2010 I came across the line of caterpillars shown in the photograph below. They were on the seafront road in Carnac, heading towards the sea, maintaining close head-to-tail contact. The only vegetation between them and the sea was some raised flower beds. Can anybody identify these caterpillars and explain their behaviour?
鈥� These are unmistakably the caterpillars of the , Thaumetopoea pityocampa. In France this species was only found south of the Loire valley until quite recently, but it is extending its range northwards as the photo confirms. It has not been seen in the UK so far, although its close relative the oak processionary moth, Thaumetopoea processionea, has recently established itself there.
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I don鈥檛 think anyone knows exactly why these caterpillars move in columns, but it is probably because there is safety in numbers, especially when the caterpillars possess quite a nasty weapon. Each segment of the catepillar鈥檚 body has a dorsal pouch bearing microscopic hair-like barbs containing a toxic protein, thaumetopoein. If you handle the caterpillars and get some of the barbs stuck in your skin, you are very likely to scratch, the action of which releases the toxin. Normally this is no more than an irritant, but in sufficient quantity it can produce dizziness and even anaphylactic shock. It can also cause widespread cell death in the affected area, making it effectively fatal for a dog to try licking the caterpillars. Rather than have to remove the dog鈥檚 tongue, a vet will usually put the dog down.
鈥淚t is effectively fatal for a dog to lick the caterpillars, as it will usually have to be put down鈥�
In my part of south-west France the moth starts laying eggs in July, and the caterpillars can reach maturity by Christmas. It seems that in Carnac it takes them a few months longer. When fully grown the caterpillars form columns of the kind depicted and march off to find a suitable spot to burrow into the ground and pupate. There doesn鈥檛 seem to be a natural leader 鈥� whichever caterpillar finds itself at the head of the column determines where they will go. If you join the head of the column to the tail, the caterpillars will quite happily march around in circles.
It seems the caterpillars are preprogrammed to march for a certain time. The ones I have bred march around their containers over a period of several days, stopping periodically before finally pupating. In the wild this is clearly a form of migratory behaviour, ensuring that the population does not remain confined to the tree where they hatched.
Each brood of caterpillars lives communally in a silk nest that is usually attached to the new growth on pine trees. They feed by night, not just on pines but also on other conifers such as firs and cedars, retreating to the safety of their nest during the day.
In France the natural range of pine trees is restricted to mountainous areas, with the exception of the on the country鈥檚 Mediterranean coast. Wild pines grow slowly and their needles produce defensive tannins, making it hard for processionary moths to develop. In mountainous areas it may therefore take several years for a processionary moth caterpillar to grow to maturity.
Where pine trees are cultivated commercially, such as in the vast pine forest that is the Landes de Gascogne park, the trees grow rapidly and the fresh growth is easily digested by the caterpillars. It is not unusual to see pines completely defoliated and even killed by caterpillar infestations. Living close to such an outbreak is uncomfortable. The air is filled with their barbs, which glint in the sunshine, and life becomes one perpetual itch.
The oak processionary moth can likewise defoliate oak woodland. This does not happen in southern France, where I believe the natural diversity of species keeps them in check, but there have been infestations on an epidemic scale in northern France and Belgium. Foresters in the UK fear that similar outbreaks could devastate the country鈥檚 oak forests and that trees already menaced by diseases such as sudden oak death might be especially vulnerable.
Processionary moths are not immune from predators. Some birds, such as crested tits, cuckoos and hoopoes, will eat the caterpillars; the eggs, larvae and pupae are attacked by parasitic insects; and the adults are taken by bats.
Terence Hollingworth, Blagnac, France