The larvae of the rare goat moth feed inside tree trunks for up to five years. They create runs of fermented sap on the outside of the tree which attract a wide range of other insects.

One of the more interesting wildlife features that we’ve discovered on Dundreggan is the presence of ‘goat moth’ trees there. These are so-called because the larvae of the goat moth (Cossus cossus) burrow into the trunks of broadleaved trees (birch trees on Dundreggan), where they live for up to 5 years. They go through various instars, or stages of growth, before leaving the tree to pupate and then fly as adult moths. The species’ common name comes from the goat-like smell of the larvae.

It was Jane Bowman who first discovered these trees on the estate, and she now knows of four of them, with another that is abandoned. Jane had observed the adult moths on Dundreggan before, but until this summer hadn’t seen the larvae, as they can only usually be spotted when they emerge from their trees – an event that takes place perhaps on just one day in 5 years! There was considerable excitement therefore when larvae emerged from several trees on different parts of the estate within a period of a few days in August this year, including a day when the whole Trees for Life staff team was there for a site visit!

The larvae live in the trees for so long because it’s a slow process to digest the tough cellulose of the tree’s wood, and they reach a large size – fully grown caterpillars are up 10 cm long and a spectacular bright red colour. While they feed, they produce frass (as the excrement of caterpillars is known) and runs of fermented tree sap that drips out of their holes. This latter appears to be irresistible to a wide range of other insects, which come to feed on it. Red admiral butterflies are particularly attracted to goat moth trees, and the sap seems to make them intoxicated or ‘drunk’, as their behaviour is different when they are feeding, and they can be approached much closer than usual! In fact it was the presence of insects, the caterpillars’ frass and the discolouration of the host tree’s trunk that first tipped Jane off about the goat moth being on Dundreggan.

When they are fully grown, the larvae leave their host tree in summer and look for a place on the ground to pupate through the winter.

The adult moths emerge the following summer, and are on the wing in June and July. They are amongst the heaviest of all moths in the UK and have a wingspan of up to 9.5 cm. The adults don’t feed at all, and are also notable for adopting a distinctive angled posture when they are at rest on a tree. The goat moth is listed as a priority species in the UK Biodiversity Action Plan (BAP), so its presence on Dundreggan adds to the conservation importance of the estate.

Sources and further reading