Pine processionary moth

Thaumetopoea pityocampa

Processionnaire du pin

Pine processionary moth

The moths of Thaumetopoea pityocampa, are fairly small and a non descript drab greyish brown, generally nocturnal and unlikely to be noticed, normally they only live for one night so they need to go about their business quickly.  

They lay their eggs in summer

generally fairly high up in most species of pine trees and in some cases types of fir are used. All other conifers and other tree species are avoided.

The nests start small

when the young caterpillars make their initial nests and gradually they make the larger ones which are clearly visible, this is where they spend the winter, often in colonies of several hundreds. These webs are amazingly dense, virtually impenetrable and provide not only a safe place for the caterpillars but also an environment that is often several degrees warmer than the surroundings throughout the winter period, all clever stuff! Like the adult moth, the caterpillars are nocturnal, forcing their way out of the nest at night to forage on the pine needles where they can cause a fair amount of damage, and although this will obviously weaken the tree it is unlikely to actually cause its death. They then return to the nest before daylight arrives.

The descent to the ground

takes place in winter or spring, anytime from December, (or even November), until May depending on weather conditions the caterpillars leave the trees and go down to the ground, this is when we will see them forming their long nose to tail processions as they make their way to find a place in the soil to pupate. The period of pupation can last a couple of months or several years. They actually touch each other to make a long chain, hence the common name of Pine processionary moth, this is important to remember as there are other caterpillars which follow each other in lines but do not touch (there is also another Processionary caterpillar, the Oak Processionary, but these are rarely of any concern and spend their lives in the trees).

The caterpillars

 have orange brown backs with bluish grey bands and bluish grey protrusions in pairs at each segment, these protrusions have little bunches of hairs “growing” from them and it is these “hairs” that can be a danger. These irritant hairs contain a highly allergenic protein which in humans can cause reactions ranging from mild itching to anaphylactic shock, all contact with sensitive skin regions, your mouth, nose or eyes should be avoided. Show caution with your animals, dogs in particular may sniff them and this should be avoided at all cost, Necrosis of the tongue has been observed in some domestic animals and severe swelling may occur causing breathing difficulties. If you suspect this has happened take your animal to a Veterinary practice without delay.

Ways to avoid contact

This may all sound a bit “shock and horror”, but it really isn’t that bad as they are easily seen in their long chains, it is usually only necessary to be on the look out from December until May and they will only be in proximity to pine or fir trees, so no need to get too worked up about it. However it is possible during mild autumns and winters that development will be rapid and descent from the trees could take place as early as November. If, on the other hand, you have them in your garden you should perhaps consider having the trees removed. Ideally this would be in late summer when any caterpillars are too small to be a danger and any birds should have finished nesting.

Expansion towards the North

They are widespread in the southern half of France and have been progressing northwards with increasing use of pine trees, both commercially and aesthetically along with the climate changes that are taking place, having now extended their range to north of the Loire. Interestingly the caterpillars die at a temperature below -16°C, and their nocturnal feeding is conditioned by the succession of temperatures higher than 9°C in the nest during the day and air temperatures higher than 0°C in the night, so with the milder conditions that we are now experiencing overall they are having a “boom time”.

Long tailed tits taking web for nesting material

Natural predators are:

Natural predators are: Crested tits that will eat the eggs and very young caterpillars. Cuckoos that will eat the caterpillars. Hoopoes that will eat the pupa from the ground and most bats that will eat the moths but in reality they will have very little impact anymore than birds have on any other moth or butterfly caterpillars.

Other hairy caterpillars aren't a danger

There are of course a large number of “hairy” caterpillars and web forming caterpillars. All hairy caterpillars can cause irritation to some degree but not an actual danger and I would encourage you to leave them alone, they are often the caterpillars of some of our more colourful butterflies. 

See also Pine processionary moth and Napoleon 3rd of France HERE