The big white balls of silk you see decorating the sunny side of pine trees, from the south of France to Paris, are not leftover Christmas baubles. They are the communal 'nests' of caterpillars who nightly devour the pine needles, following each other in single file along a silk track. You need to be careful of them and observe from afar, as they are very irritant. The older caterpillars are capable of releasing their irritant hairs into the air around them, and the hairs persist a long time in abandoned nests.
Each hair is a tiny harpoon which contains an extremely irritant protein called thaumetopoeine. If you scratch or rub the hairs into your skin they release the substance. Often the effect is mild and passes quickly (especially if you take a hot shower) but it can be serious if you get the hairs in your eye, breath them in, are allergic or are repeatedly exposed. For serious exposure you should consult a doctor. Horses and dogs are particularly sensitive to thaumetopoeine and you need to be on the alert for any signs of distress in your pets. Also be careful handling your pets, as they can collect the caterpillar hairs in their own coats, bringing them inside and transferring them to your hands and arms if you pick the animals up or pat them.
tree with its silk nests...
They have been present for a long time in the south of France (except for higher altitude locations such as Ventoux) but for some decades now they have been moving north and gaining height. Now they can be found in the south of Brittany and the north of Burgundy, although there is a mysterious lacuna around Orléans. The progression has been rapid - about 50 km every decade - seemingly associated with climate change and the practice of preferentially planting pine trees along autoroutes. In this way, it is predicted that Belgium and England could be reached, up to a latitude where the days are too short to allow the caterpillar to develop.
crossing a farm track in April last year.
The newly hatched caterpillars immediately spin themselves a lightweight little silk 'pre-nest'. They leave this shelter every night (unless it is very cold) and feast until the early morning. Fabre describes them following their silken trail, and we now know that they impregnate the trail with a pheromone which tells them and their brethren how old the trail is, how often it is used and so on.
separated they can follow the pheromone trail.
From February to May they start coming down from the trees and forming the processions they are known for. A caterpillar destined to become a female moth will descend, followed by all her brothers and sisters, nose to tail, to search the ground for a suitable spot to pupate. At the end of an expedition, which can take 5 days, she will stop, the troop will regroup around her and they will all dig themselves into the earth 5-20 cm deep. Underground, each caterpillar spins its cocoon and goes into diapause. This state of suspended animation can last from anywhere between a few days and 5 years.
up a cat which was carrying the caterpillar hairs in its fur.
At the moment there is a lot of research going on to develop survey methods, improve control and find out more about less problematic biological controls such as the role certain birds can play (e.g. tits).
*The bacteria is Bacillus thuringiensis. To be affected the caterpillar must eat the bacteria, which produces a poison on exposure to the gut fluids. This bacteria can kill many types of caterpillar, not just the target species, so must be used in an informed and careful manner.