14 Jul 2022

The battling beetle

From Our Changing World, 5:00 am on 14 July 2022

This is a(nother) tale of a native New Zealand underdog, and the passionate people who study them. No flashy feathers, no YouTube livestream, no pretending to be something its not for the limelight. And yes, it too is battling predators and habitat loss.

Luna standing in the bush surrounded by ferns. She is wearing a rain jacked and backpack.

Luna on Rakiura Photo: Luna Thomas

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Meet the Helm’s stag beetle, Geodorcus helmsi, so named because of their enlarged mouthparts, or mandibles, that look like stag antlers. In general, they are pretty chill, moving slowly through life. The spend their days in the soil and at night they find a favourite tree to climb up to sip some sap.

A stag beetle on a tree trunk.

A stag beetle drinking sap from a tree. Photo: Luna Thomas

It’s one of ten native stag beetles in the Geodorcus genus. It is also the most widespread – it has been found along the west and south coasts of the South Island – from Karamea in the north to Tapanui in the south and also on Rakiura Stewart Island.

Which is one of the field work sites for PhD student Luna Thomas’ research. She wants to get a better understanding of the behaviour and size and gender distribution of these beetles. To do that, she has to go beetle hunting, at night.

On Rakiura Luna has a natural experimental set up – she can compare findings between the predator-free Ulva island and around the town of Oban. Unfortunately, in the latter, she is coming across the grizzly remains of rat predation – large caches of beetle heads.

Hopefully the recent announcement of Manaaki Whenua and Predator Free Rakiura’s joint plan to remove predators from the island means a brighter future for the beetles there.

Lit by red light two beetles are set inside a cake tin. A camera on a tripod is set up above it to record.

Observing beetle battles. Photo: Luna Thomas

However, other species in this Geodorcus group are not so lucky. Some have only been found in very restricted habitat, the most extreme example of which is Geodorcus ithaginis or the Mokonhinau stag beetle. It had been restricted to one tiny offshore rocky stack, but couldn’t be found in a 2019 survey.

As part of her research, Luna also has a DOC permit to bring some of the Helm’s stag beetles back to the University of Otago, where she is trialling conditions to keep them in captivity. Though different species, she is hopeful that the information gleaned from Geodorcus helmsi may also help some of its critically endangered cousins.

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