By Dr Mike Dickison
In a drawer in the Whanganui Regional Museum's collection storeroom are several curious objects. They resemble large caterpillars, with all their legs and wrinkles depicted, made out of very light wood, quite solid to the touch. From the head of each protrudes a long spike, like a shoot, but also woody. Known as vegetable caterpillars, these curios were once highly prized by Māori and Pākehā alike, and represent the last remains of a moth consumed by a fungus.
The vegetable caterpillar, or āwheto, begins its life as the larva of one of the species of native ghost moths or porina moths. Although related to the porina moth that plague farmers' pastures, these species only live in native bush, and their large caterpillars inhabit silk-lined burrows in the forest floor, emerging at night to eat fallen leaves.
From time to time a few unlucky caterpillars become infected by spores of the fungus Ophiocordyceps robertsii, either by eating them or inhaling them, and are thereafter doomed. The fungus gradually fills and consumes the caterpillar's entire body, leaving only the skin. By this time the dead caterpillar is underground in its burrow, so the fungus sends up a thin stem, or stroma, up to 30 centimetres long, which eventually pokes above the leaf litter.
The end of the stroma is coated in spores, which rain down over the forest floor to infect more caterpillars, and thus the cycle continues. In some spots in the bush you can see half a dozen fruiting spikes protruding from the ground, each a caterpillar's grave marker.