If you are visiting Shrawley Wood between late April and late June look for out a cylindrical orange beetle about 2cms long (Figure 1). Hylecoetus dermestoides is the creature in question and if you manage to spot one then you have been lucky enough to see one of the few animals, besides humans, leaf-cutter ants and a few others, which ‘farms’ another organism for food.
The story of this fascinating beetle begins with a female depositing her eggs in crevices and cracks in the bark of mature trees, usually no more than 2m above ground level. She lays her eggs on a variety of diseased and dead trees, but avoids pines, larches and hornbeams. Birch is a favourite and a large, diseased birch with the correct conditions (warm and no less than 30–40% moisture content) is perfect.
Lots of beetles use dead wood as a food source for their young, but this species has some remarkable differences. On the female’s ovipositor there is a small pocket, the mycangium, which is filled with fungal spores and every egg she lays gets coated with spores from this pocket. After a few days the eggs hatch and small, whitish larvae wriggle out in to the world and loiter around their empty egg-shells for a while to pick up some of the fungal spores left by their mother. After a while they begin to tunnel into the wood using their powerful mandibles, carrying some of the fungal spores with them. Initially, the tunnels are very narrow as the larvae are small, but as they grow, the tunnel must also increase in width to accommodate them. The tunnel may run for over 30 cm, snaking into the wood, but it is not the wood the larva is eating. The tunnel is, in fact, a sheltered fungus farm.
The spores provisioned by the female and carried by the larvae infect the wood until the walls of the tunnels are lined with a white layer of fungus. The fungus Endomyces hylecoeti is a type of yeast that has struck up a symbiotic relationship with the beetle. The fungus gets access to wood in the safety of tunnels and in return the larvae get something to eat. The larva takes excellent care of its fungus garden doing everything it can to keep the conditions just right for the fungi so that it will have enough food to complete its development. The fungus requires oxygen to thrive, so the larva must rid the tunnel of any debris to maintain a good flow of air. The larva shuffles along the confines of its tunnel pushing any wood dust and waste to the outside, where it falls to the base of the tree (Figure 2).
By the winter, the larva will not be fully grown, so it will have to retreat to the deepest reaches of its tunnel and enter a resting state to survive the cold, harsh conditions. Come the spring, the larva reawakens and continues to feed on whatever fungi survived the winter. Soon, it is ready to pupate and it wriggles to the tunnel entrance and enlarges its width to form a chamber for its imminent metamorphosis.
H. dermestoides and its relatives are commonly known as ship timber beetles. They are sometimes regarded as forestry pests because of the damage they do to timber, affecting high quality wood that today is destined for construction, but was once used for ship building. In actual fact, their effects on commercial timber operations are minimal, as they tend to go for diseased or dead trees.
Although this species is widespread the adults are rarely to be seen in significant numbers. Look out for the adults flying in warm, calm conditions. The flight of the female, heavy with eggs, is very laboured. The male is around half the size of his mate, with the same general body shape and the addition of feathery palps on his head (Figure 3). A suitable brood tree will be home to many larvae, which give their presence away by the sawdust that accumulates at tree’s base after it has been ejected from their tunnels.