Processionary Pine Caterpillars (Thaumetopoea Pityocampa)

End of the summer when the weather is cooler through to spring is generally the time when we tend to see these caterpillars (larva of the silk moth) Whilst they are on the move they can cause problems for us and our dogs. Having said that they can be seen moving in their processions at any time during the year here in Portugal and the Veterinary Surgeons do see reactions to these caterpillars all year round, therefore it is wise to be aware of what they look like and the potential problems they can cause.

The Processionary Caterpillars are found in certain Pine Trees.

  • Black pine (Pinus Nigra) – their favourite
  • Canary Island Pine (Pinus Canariensis)
  • Scots Pine (Pinus Sylvestris)
  • Maritime Pine (Pinus Pinaster)
  • Aleppo Pine (Pinus Halepensis)
  • Stone Pine (Pinus Pinea)

The caterpillars can be found in city parks where there are infested Pine trees as well as in the countryside Pine forests.

The Life Cycle of the Processionary Pine Caterpillar.

There are several stages to the life cycle, however they are only dangerous during the caterpillar stage, not during the adult moth stage.

The adult a short lived moth, emerges from the soil during the summer (around August) and flies at night. After mating they seek out Pine trees to lay their eggs.

A single female moth can lay up to 300 tiny eggs which she attaches to the pine needles in a moss type mass.

Approxiamtely one month later these eggs hatch into minute caterpillars which go through 5 growth stages where they grow quickly and moult their skin ready for the next stage.

Between December and April the caterpillars come down from the nests looking for food.

They eat the pine needles at night by pushing their way through the nest at sunset avoiding their predators e.g. birds and wasps. There is a scent trail left as they move around so they can find their way back to the nest before morning light.

Up to the 3rd stage of growth they sleep in small temporary silk nests during the day. By the time of the 3rd moult they have built a permanent white silky nest on the tip of a pine branch which looks like white cotton candyfloss structures.

A single tree can have many nests which are positioned to take advantage of the suns heat, providing warmth to help them with digestion of the previous nights meal.

By the 5th stage of growth the nest looks darker, about February/March (sometimes January depending on the temperature), and the caterpillars will now leave the nest in preparation for the next part of their lifecycle.

They follow a leader in a procession which can vary in length of up 60+ caterpillars with each one being about 4cm long.

They can travel 30+ metres to find suitable soft soil to burrow into where they change into pupae, covered in a tubular brown protective casing. It is here they will lay dormant during the warmer summer months until they are ready to emerge as a moth and start their life cycle again.

If the weather is not favourable for them to emerge they will remain underground till the next year.

They are active during colder temperatures but it needs temperatures of -16° C to kill them.

It is usually during this 5th stage of growth when people and pets are likely to come into contact with them.

Why are these caterpillars dangerous?

They are covered in tiny barbed hairs which they use as their defense mechanism. During the 5th stage of growth they are able to eject these hairs when they feel threatened or stressed.

These hairs have harpoons that penetrate and irritate the skin with an urticating protein known as thaumetopoein which can cause a very rapid pronounced allergic reaction in humans and animals.

Our dogs and cats can pick up these hairs in their paws, which will then irritate causing them to lick their paws.

If they actually eat the caterpillars it is almost likely to be fatal.

Signs that your pet has come into contact with these caterpillars:

  • Within 5 minutes the lip, tongue and nose can swell to a huge size and eventually the whole head and their airway may swell.
  • Once the hairs reach the lip/tongue itching, swelling and possible vomiting occur.
  • If the tongue becomes large and swollen it may need amputating due to the swollen area of the tongue dying.
  • You may also see small white spots in their mouth or on their tongue.
  • Excessive drooling and chomping is seen due to the horrible taste of the protein produced by the caterpillars.
  • Their eyes can become irritated, with the risk of blindness from the hairs the caterpillars eject getting into the eyes.
  • Hyperthermia – an increase in body temperature.
  • Tachypnoea – an increase in rate of breathing.
  • Respiratory distress – where your pet will be struggling to breathe.
  • Cyanosis – tongue and mucus membranes turning a blue/grey colour.
  • Very bad smell – from the destruction the hairs/protein cause to your pets tongue.
  • Death can occur if not treated very early, it only requires 3-4 caterpillars to have fatal consequences for a medium sized dog.

Why are dogs interested in the caterpillars?

They produce a bittersweet smell and taste which attracts dogs and cats to them.

What should I do if I suspect my dog has come into contact with these caterpillars?

It is very important to take your dog immediately to the Veterinary Surgery for treatment.

Be prepared always carry the local Veterinary Surgery’s telephone number on you and ring in advance so they know you are on your way and can be ready for you.

Know the Veterinary Surgery opening times and whether you need to telephone a different number if they are closed.

Whilst on your way to the Veterinary Surgery you can give your dog an antihistamine called Lepicortindo Prednisolona (Prednisolone).

Speak to your Veterinary Surgeon beforehand about having an antihistamine tablet ready to hand and the dose you should administer to your dog if required.

However the most important medication your dog requires has to be administered by a Veterinary Surgeon and involves a cortisone injection, antibiotics and fluid therapy, hence why it is very important to get to the Veterinary Surgery as soon as possible.

For more information on the dangers of these caterpillars and how to deal with any nests etc. please go to:


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