Museum Notebook: Pepetuna a stunning signal of spring

Pinned puriri moths (Aenetus virescens). Photo / Whanganui Regional Museum

When is the first day of spring? It's hard to pin down exactly when the season changes, especially when wintry blasts keep us warmly wrapped up indoors.

Spring is signalled for me by the call of the pīpīwharauroa, or shining cuckoo, which returns to Aotearoa around late September or early October.

Another notable signal of springtime in Aotearoa is the emergence of the pepetuna, or the pūriri moth - a huge bright green moth with a wingspan of 150mm for the female and 100mm for the male. Although rarely seen in city areas, if you are in or near a patch of native forest in the North Island, you may hear the noisy blundering of a pūriri moth attracted to outside lights, especially from October through to December, although they can also hatch later in summer or in early autumn.

Part of a worldwide family known as "ghost-moths", the pepetuna (Aenetus virescens) is one of several endemic ghost-moths found only in New Zealand. Although pepetuna are usually bright green with white or brown mottled markings, males can also be more of a yellow colour or even albino - very ghostly.

A live pepetuna. Photo / Whanganui Regional Museum

Strangely, the adult moths have no mouthparts, so they never eat, and live only for a few days, during which brief time they seek a mate and reproduce before expiring. Prior to this, pepetuna caterpillars live for up to seven years, although their lifespan varies. They start out as tiny larvae on the forest floor, eating bracket fungus. A few months later, they climb up and bore a hole in a native tree - usually houhere (lacebark), putaputaweta or pūriri, which is how they acquired their common name of the pūriri moth.

The caterpillar creates a large, angled tunnel inside the host tree, leaving a noticeable scar at the entrance which they cover with tough silk. The larva feeds on the callous tissue created by the tree around this entranceway. As the caterpillar grows, it gradually enlarges the tunnel.

When they reach about 12cm long, the larvae pupate for 150 to 170 days before emerging as adults. After mating, the female moth scatters up to 2000 eggs on to the forest floor, near a potential host tree, then both the male and the female flutter away to die or to be gobbled up by a hungry ruru, or a night-prowling cat.

I will be looking out for pepetuna over the next few months, especially out at Bushy Park Tarapuruhi. I will count myself lucky if I do, as I haven't had one of these lovely creatures alive in my hands for many years.

There is a collection of Aenetus virescens moths on display in the Whanganui Regional Museum exhibition "The Smallest Creatures - Aitanga-a-Pēpeke". Their beautiful green colour fades very quickly once they die, so these pinned specimens are more of a pale greenish-white than they were when originally collected - more of a ghost-moth, and less of a bright signal of springtime.

• Margie Beautrais is the educator at Whanganui Regional Museum.