In response to a letter from Vivien Haines-Jessop, KIRSTY TUXFORD unearthed the facts about the processional pine caterpillars that regularly invade the Costa Blanca.

Processional pine caterpillars or Thaumetopoea pityocampa are common to Spain and France. Normally found nesting in the abundance of pine trees on hillsides in this area, they have a terrifying reputation.
So called because they travel nose to tail in a line, processional pine caterpillars are quite small, but they present a major danger to trees and animals, and can cause a severe allergic reaction in humans.
The caterpillar’s ‘fur’ is where the danger lies. It is in fact a layer of poisonous harpoon-like spines that remain toxic even when the caterpillar has died.
The hairs can also become detached and float in the air if the caterpillar is disturbed, creating the risk of inhalation. Care must be taken when walking dogs in areas where the caterpillars may have marched as there is a possibility that some hairs may remain on the ground.
If one should drop on you or your pet from a tree, the advice is not attempt to brush them off with your hands, as even touching them can cause a reaction such as a rash or pain. In rare cases people have been known to suffer from anaphylactic shock, in the same way that people who are allergic to bee and wasp stings can.  If any hairs get into your eyes you could experience some swelling and soreness, similar to the symptoms of conjunctivitis. If you plan to go walking in areas where the caterpillars are rife, you might want to carry antihistamine tablets with you. 
The caterpillars start life in silk cocoon style nests in pine trees. They inflict damage, stripping the trees of their needles, then leave the cocoon and seek out another tree on which to feed.
Most noticeable from January to mid April the caterpillars are at their most dangerous in mid to late February, when they can often be seen in the evenings going from tree to tree.
At the end of the winter months, the caterpillars will make their way down to the warm spots in the soil where they dig themselves into the ground in order to pupate. They may sometimes halt their development during this period, for up to three years, while awaiting favourable climatic conditions.
The moths that lay eggs are particularly attracted to the silhouette of a tree – especially if it stands alone. So, if you have a problem with caterpillars in your garden, it could be resolved by planting clumps of trees to deter the moths from laying eggs.
The damage to the trees themselves can be extensive. Young trees may even die, and saplings that are stripped of needles are more susceptible to secondary pests such as bark beetles and pine weevils.
It’s because of this potential damage from caterpillars that preventative and curative measures are taken in Spain to stop them becoming out of control.
Trapping by using G-traps baited with the pheromone pityolure is used to monitor population levels in summer.
Other methods employed include the destruction of winter nests by cutting or shooting and burning them, and the aerial application of insecticides during outbreaks.
All in all, the processional caterpillar is not the friendliest creature. But the threat to people and animals is can be lessened by trying not to come into contact with the hairs.
Thankfully, because of the distinct line that the caterpillars form when marching, it is easy to spot them and give them a wide birth.

CATERPILLARS:
THE FACTS

 It’s their hairs that can cause irritation.

 They are easy to spot because they travel in lines, or processions, hence the name.

 Eggs are laid in batches, wrapped around a couple of needles.

 They live in groups and young ones make small provisional nests.

 They feed on pine needles and cause yellow-brownish discoloration.