When in Spain and thinking about wild life many people ask about venomous snakes, scorpions, spiders and will not be aware that they are much more likely to endanger themselves and their pets by getting too close to an innocent looking line of caterpillars that can be found from December to April of each year, crossing your path.
There are several stages within their lifecycle but they are only dangerous to people and pets during the caterpillar phase. In the adult phase they are a simple and unremarkable, short lived moth which emerges in the summer and flies at night. The male moth is attracted to the female moth by pheromones that she emits. They will mate and a single female can then lay up to 300 tiny eggs which she attaches in a mass to a pine needle. Around one month later these eggs hatch into minute caterpillars. These larvae have 5 growth stages that are called “instars”. They grow quickly in body size, moult their skin and that denotes the start of the next instar.
These social caterpillars living in family communities eat pine needles by night and sleep in little temporary silk nests by day. At this point they are nomadic and the nests are not really visible. However at the third instar (moult) the siblings build a permanent white silky nest on the tip of a pine branch. These appear during the winter as white cotton or ‘candyfloss’ like structures and a single pine tree may have many. If there are half a dozen or more in the top of a tree they can easily strip the leaves with their nightly foraging, possibly clearing all greenery off some branches and in many cases damaging the tree badly. By feeding under the cover of darkness they avoid attack by birds and predatory wasps. At dusk the caterpillars leave their communal nest in search of food, there is no single entrance hole, they simply push through the silk layers and once onto a branch they will leave a scent trail to help themselves find their way back before the morning light arrives.
This period of night time eating occurs during the winter months and whilst low temperatures may slow them down, it would need to be below minus 16 degrees Celsius to kill them. The silken homes are carefully positioned to take advantage of the sun’s heat and this warmth is absorbed into the nest thereby aiding the resting caterpillars to digest their previous night’s meal. Also the fact that there can be up to 300 caterpillars in a nest helps to keep the nest warmer by as much as a degree or two than the outside temperature.
During the stage of the 5th and last instar the nest will be looking dirty as it has over wintered and excrement has collected at its base. Generally this stage happens around February and March but depending on spring temperatures can begin in January or continue until April as well. This is the time for them to leave the nest in preparation for the next part of their lifecycle. And it is this point when most people and pets come into contact with the caterpillars, sometimes with very painful consequences. The colony follows a leader, nose to tail, in a long procession. These processions can vary greatly in length, depending on how many have survived to this final caterpillar stage and whether they have been disturbed. 60 or so caterpillars each about 4cm long in a chain can be an impressive sight and if seen along a road may be mistaken for a snake. While searching out a pupation site, they may travel a distance of 30 or so metres to find soft soil to burrow into.
Once underground they change into pupae and during this part of their lives they look nothing like a caterpillar at all, now they are covered in a tubular brown protective casing and they will lay dormant until the summer months. They have stored nutrients in their bodies on which they will survive the pupal stage of development. If the weather conditions are not favourable, they may remain underground until the following year. This is why some years seem to have many more visible nests than others, it may literally be because two years of moths (this years and last years) emerged at the same time.
And so on to the dangers involved, the best advice is to avoid these innocent looking creatures at all costs. The caterpillars are covered in tiny barbed hairs which are their defence mechanism. These hairs are often being shed and so can be airborne around infested pine trees, on the branches where they have travelled and also left in the line of the migrating procession.
When humans and our pets come into contact with these hairs, they can cause reactions ranging from mild inflammation and irritation to severe anaphylactic shock. The worst problems occur if you make contact with the caterpillar directly and ingest the hairs, either by picking it up, stepping on it or moving them in some manner. Once on your skin a rash soon forms which can be incredibly itchy. Medical advice should be sought if you are unfortunate enough to experience this. The rash can be painful, very itchy and lasts for as much as three weeks.
Moving the caterpillars, their nests, or even the branches that they have walked along, may release these hairs into the air where they can be inhaled or come to rest unnoticed on clothing. The nest material that remains on the tree after the caterpillars have left will still contain the “urticating” hairs. (The word “Urtica” is Latin for Nettle, a plant that has barbed hairs which cause a rash). Even burning infected pine branches should be avoided as the hairs can be lifted into the air and fall anywhere or be inhaled.
Veterinary services have many emergency calls at the time when the caterpillars are migrating to the ground as inquisitive dogs can get too close to the intriguing procession and may pick up the hairs onto their paws, these irritate and so they lick them. Once the hairs are on the lips/tongue it will induce itching, swelling and possibly vomiting. Look out for the symptoms of : small white spots in the mouth and on the tongue, excessive drooling and chomping. In some cases partial amputation of the tongue is the only course of action.
These defoliating pests, which can attack all types of pine trees, are found in warmer parts of Southern Europe, North Africa and across to the Near East. Their numbers have increased partly due to large plantations of pine trees in the Mediterranean areas. This facilitates their ability to multiply successfully. Milder winters, as have been occurring recently, are allowing these insects to expand into new areas, both into more northern latitudes and higher elevations.
Their favored food tree is Black pine (Pinus nigra) followed by Canary Island pine (Pinus canariensis), Scots Pine (Pinus sylvestris), Maritime Pine (Pinus pinaster), Aleppo Pine (Pinus halepensis) and Stone Pine (Pinus pinea).
Whilst there has been some work done on pheromone traps/mating disruption systems to try to halt the spread of these pests, at present there seems to be no testing being carried out in Andalucia. Another form of control that is regularly used in Spain is the indiscriminate aerial spraying of pine forests with lethal mixes of insecticides and diesel oil as a fixative to stick the liquid to the pine needles. Of course this kind of treatment kills a diverse range of insects as they ingest the varied affected leaves throughout the forest and also the birds that eat these insects have been found dead.
(Having witnessed this first hand and having been sprayed by an aeroplane with this mixture I have to say that whilst the pine processionary caterpillar is a serious pest this kind of treatment is simply not acceptable).
If you are aware of the presence of processionary caterpillars in your area then contact your local town hall and ask them what their policy is for control and eradication. In large forests this is a complicated problem to solve but in individual and small plantations on urbanisations or in towns there should be an eradication system in place. If not, ask why not.
Under no circumstances should you try to handle the caterpillars, cut down the nests or try to burn them.
Insects and Creepy Crawlies 
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Thankyou for your excellent article.
Fred Bricknell