David Chapman investigates the mysterious oak eggar moths he encountered in Exmouth

PUBLISHED: 19:04 20 January 2010 | UPDATED: 16:08 20 February 2013

A newly hatched cocoon of the grass eggar moth

A newly hatched cocoon of the grass eggar moth

A strange find during a summertime Exmoor ramble leads to an interesting discovery in the world of moths. Words and photos by David Chapman.

David Chapman investigates the mysterious oak eggar moths he encountered in Exmouth

A few years ago in August I found something unusual whilst I was wandering along the coast path. Scattered amongst the short grass around the path close to the heather-clad moors of Exmoor were lots of small papery cocoons in the shape of acorns. These brown papery cases had a slightly furry texture to their surface, all of them were empty having been split open, but what were they?

I asked a few people, did a bit of research and found the answer. These were cocoons of a species of moth known as the oak eggar moth (Lasiocampa quercus). The name 'eggar' belongs to a group of moths of which there are about 1,000 representatives worldwide, with ten species in Britain and eight in Somerset. All are thickset and most are medium-sized or large moths. They are generally coloured in various shades of brown and have a single white spot on the forewing.

I soon found out that it was the egg-shaped cocoons, typical of this family, which gave rise to the name 'eggar'. Strangely enough it wasn't the eggar part of this moth's name that intrigued me so much as the reference to 'oak'. I checked my field guides to see if the larva of this species feeds on oak, fully realising that this was very unlikely since I had found the cocoons adjacent to moorland. I found one reference which suggested that the larvae may sometimes feed on oak but this is not the reason for the name. The name came about because the cocoon resembles an acorn. I really should have thought of that since I had already likened it to an acorn myself!

The oak eggar is a beautiful moth with a distinctive pale band across its wings. The female is much larger than the male with a lighter buff or yellow colouring, but both have the same wing pattern. The female's antennae are quite small but those of the male are large and feathery. He flies by day using his large antennae to pick up the scent of unmated females. Meanwhile she flies by night whilst laying her eggs. Egg laying for an oak eggar moth is a fairly random affair - she simply drops them onto the ground whilst flying around.

The eggs hatch in late summer and the caterpillars (more often called larvae when referring to moths rather than butterflies) begin feeding. Their main food plants on heath and moor are heather and bilberry though oak eggar moths are also found in lightly wooded areas where they feed on the leaves of bramble, blackthorn, hawthorn and other woody plants. The larvae are still quite small when they go into hibernation but their growth continues at an ever-increasing rate the following spring.

In spring and early summer we may see these hairy larvae crawling around amongst heather. The larvae of the oak eggar moth can grow up to 6.5cm long. They are dark brown with a line of white spots along their flanks and sometimes a row of red markings is visible lower down. The hairs grow in tufts and act as a defence against predation since they can cause skin irritation, but they are still eaten by some specialists such as the cuckoo.
Last year I noticed a similar type of caterpillar on the dunes near Burnham during June. These were not quite as large as the oak eggar larvae but were much more attractively coloured. Their flanks were dark with white hairs but along their backs was a mass of mustard-yellow-coloured hairs. These were larvae of the grass eggar moth and one characteristic feature of their larva is an area of orange on the segment just behind the eyes.

The grass eggar moth is much rarer than the oak eggar moth in Britain, being found at just a few isolated locations in the North West and South West of England, including Somerset, of course! The specimens shown here were actually photographed last year on the Isles of Scilly, another hotspot for this species.

Grass eggar moths have a similar life cycle to the oak eggars but they tend to eat grass, which is more plentiful in their chosen habitat. The adult grass eggar closely resembles the oak eggar but is often just a little paler and smaller. At some point, usually during July, the larvae of both species begin pupation and form an acorn-shaped cocoon around themselves. Pupation takes place above the ground, sometimes in leaf litter or amongst the leaves or branches of heather, but around the coast this often happens on bare ground. In August the adult moths emerge, leaving their cocoons for interested naturalists to find and the cycle starts over again.

It isn't uncommon in the countryside to come across something about which we have no knowledge. This is one of the things that makes wildlife watching so exciting. As is so often the case with new wildlife sightings, seeing one almost seems to guarantee seeing lots more, and I now can't go for a walk on a coastal heath in summer without seeing the cocoons of oak eggar moths, and I am left wondering why I had never noticed them before!
If you are interested in identifying moths I recommend the Field Guide to the Moths of Great Britain and Ireland by Paul Waring and Martin Townsend, illustrated by Richard Lewington. This book is published by British Wildlife Publishing, ISBN 0-9531399-2-1.

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