Tuesday, February 02, 2016
Pine processionary caterpillars
Caterpillars are not usually perceived as a threat but processionary ( the English word is actually processional but everyone uses the Spanish word adapted to sound English) caterpillars can cause problems to humans who tangle with them. Dogs, which tend to sniff and paw things they come across can get into big trouble. Cats don't usually have problems with the caterpillars because they are supremely indifferent to any life form that doesn't feed them.
The pine processionary moth usually flies around May to July and only lives for a day or so. On that day the moths have to get busy. They have to mate and the successful females then lay around three hundred tiny eggs usually in the foliage of pine trees though some firs and cedars are also targets. Just so I don't have to keep repeating tree types we'll pretend they only live in pines. The eggs take about a month to hatch.
Once hatched, the tiny caterpillars start to eat the pine leaves. Over time they pass through five growth stages which are technically referred to as instars. The caterpillars strip the leaves or needles from the trees but usually the foliage grows back. At each instar the beasts moult a skin and increase in size. At the third moult these gregarious little caterpillars build a nest which looks a lot like spun white candyfloss in the branches of the pine trees. At the fifth and last instar, which is usually sometime between February and April, the caterpillars come out of the tree and search for a place to pupate. This year, with the much warmer winter, they are already on the move and there are more of them because fewer have been killed by cold and frost.
When they leave the tree they do so in a long line. A neighbour, who came to warn me that the caterpillars were on the move told me that she'd seen or heard of a line that was five metres long. There are lots of Internet photos of caterpillars which have been easily persuaded to form circles which just go round and round and round. In this processionary stage each caterpillar follows a scent produced in the stomach of the caterpillar in front. None of the websites I read explained what drives the caterpillar at the head of the line! To digress slightly I heard a news story recently which explained that the way the caterpillars maintain a constant speed can be taught to drivers as a way of stopping the formation of traffic jams.
Eventually, when the caterpillars find a good place to dig a burrow underground where they can pupate, the line will disperse. While the beasts are on the move they can be a danger to humans and other animals. It's the hairs on the caterpillars that cause the problems. If the caterpillar feels under threat they can eject both a tiny cloud of toxins and little toxin loaded hairs which are harpoon like and stick to flesh really well.
In humans this toxin usually leads to unpleasant and very painful rashes and eye irritation which can last for several weeks. Dogs sniff the caterpillars, maybe they bat them around with their paw. The frightened caterpillar releases hairs which stick to the dogs nose or tongue. The dog licks at its paw. In turn the dog's tongue will become irritated, sometimes so much so, that they have to be amputated to prevent sepsis and necrosis - infection and gangrene. Some dogs have such a severe reaction to the poisons that they die through kidney failure.
I know that most people around here go hunting for the nests on or near their land, cut them down and burn them. This advice was repeated on several websites but there were lots of warnings that this could go terribly wrong as tossing a nest onto a roaring fire can be an effective way to spread the little hairs around in the smoke and rising air. A couple of websites suggested drenching the nests with water first as even the loose hairs are loaded with the toxin.
There do seem to be lots of methods for dealing with the caterpillars at almost every stage but I suspect that many of them are not particularly effective. One I read about several times was to break the nests particularly in cold spells so that the caterpillars die of cold without the protection of the warming cocoon. Several of those websites suggested that for the out of reach nests a good blast of light shotgun pellets would do the job!
There are pheromone traps to attract the males - the lads go looking for a bit of hanky panky but end up trapped. The girls have to do without. Result no eggs. Then there are several "traditional" insecticides which have to be applied in the autumn. The treatment that seems to be most in line with current thinking and, apparently works well, involves using bacteria. There are a couple of effective processes. One interrupts the life cycle of the caterpillar whilst the other poisons their food. Oh, I nearly forgot, there's another method that seems eminently sensible. That's to put a physical barrier in the way, the main basic design seems to be like a cone or envelope which the caterpillars walk into but then can't escape from (though I'm not sure why). Another one was simply to put a walled moat around the base of the affected trees using flexible plastic in which the caterpillars drown.
Blue tits and other tits apparently love to eat the caterpillars so a longer term solution my be to attract birds to breed nearby.
The information about who to turn to for help was very contradictory. It seems that some town halls will send people to remove the nests. There were also lots of adverts for commercial firms very happy to remove the nests and caterpillars for you - at a price. I saw it suggested several times that Seprona, the environmental arm of the Guardia Civil, has a statutory responsibility to destroy the nests but there was nothing on the Sepona website that I could find to confirm that. If you want to give it a try the Seprona number is 062.
Otherwise the best advice may be to stay away from pine trees for a while!