Emma and I have been putting up with each other for three years now. We first met working with kiwi, that is, New Zealand’s iconic flightless bird, down in a place called Franz Josef. A year later we witnessed the Aurora Borealis (northern lights) in Iceland, and a year on from then we had a sunrise encounter with yellow-eyed penguins, one of the world’s rarest penguin species. This year, our planned encounter was to get hands on with the Mahoenui giant wētā and, inevitably, the expanse of gorse which they call home.
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New Zealand’s Endemic Wētā
The Department of Conservation states that there are 70 different species of wētā, all of which are endemic to New Zealand. Wētā are classically separated into 5 broad groups; Giant wētā, Cave wētā, Ground wētā, Tree wētā and Tusked wētā. The number of species continues to change with new research and genomic data, as well as with the discoveries of brand new species.
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The giant wētā is arguably the heaviest insect in the world and is the biggest member of the grasshopper order, Orthoptera. They are a great example of Island Gigantism.
Threats Facing Wētā
Wētā, especially the larger of the wētā, seem to have had a pretty rough ride in recent times. Although the wētā had native predators in the form of birds (especially the weka and kiwi), reptiles, and bats before the arrival of humans to New Zealand, introduced species such as cats, hedgehogs, rats, stoats, ferrets and weasels have caused a considerable rise in the rate of predation. They are also vulnerable to habitat destruction. Habitat modification caused by introduced browsers also has a marked effect. Many of the giant species now only survive on offshore predator free islands and many are endangered.
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Despite the supposed dire outlook for many wētā species, there is actually quite a high potential for recovery given the chance. Some of the reasons for this good chance of recovery are as follows: Invertebrates have a high rate of productivity; many wētā adapt well to modified habitat; invertebrates also require smaller areas to survive than vertebrates, and can survive in tiny fragments of original habitat; wētā also thrive in captive breeding programmes, providing the opportunity for research.
The Mahoenui Giant Wētā
We were lucky enough to meet the Mahoenui giant wētā on this expedition. The story behind the endangered Mahoenui giant wētā is an interesting one. The Mahoenui giant wētā was long considered extinct on the mainland until it was rediscovered in 1962 holding on in a pocket of tawa forest at Mahoenui (Waikato, North Island). A later study in 1987 revealed greater numbers of wētā on farmland utilising the invasive weed, gorse. By August 1990, about 240 hectares of gorse habitat was purchased by the Department of Conservation as a reserve for these wētā.
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Mahoenui Giant Wētā Habitat: An Unlikely Combination
The unlikely combination of cattle, goats and gorse bushes had set up a unique habitat for the wētā. Cattle establish trails by pushing and breaking through the gorse bushes. This, in turn, allows a good supply of sunlight to penentrate the lower foliage promoting fresh green growth. These tunnels created by cattle then allow access for wild goats to browse on gorse and other plants in the area. Browsing by the goats forces the gorse bushes to grow in dense clumps. Thanks to all these introduced species, the prickly fortresses have become a protective sanctuary shielding the giant wētā from predators.
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This scenario is both bizarre and counter-intuitive. Most conservation work in New Zealand involves managing weeds and pests with the aim of reducing the impact on native organisms and ecosystems. Whereas here, non-natives are being utilised to manage other non-natives, and are effectively protecting endangered native organisms. The question remains as to whether the giant wētā would have managed to do as well in native forest without the protective gorse and its hoofed pruners.
An interesting challenge to DOC over the coming years is to establish whether the natural succession of tree ferns growing above the gorse, shading the bushes of light, and eventually the reestablishment of native trees, will need to be halted. Again, this being almost completely against the normal practice of management.
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Mahoenui Giant Wētā Biology
Mahoenui giant wētā can be found in two different colour morphs. 69% of Mahoenui wētā are a dark brown mahogany colour, 31% are an interesting yellow.
Females are larger than males, with adults measuring up to 75mm (weighing 19g) and 50mm (weighing 12g) respectively.
The total life cycle of Mahoenui giant wētā is up to 2 years and is short compared to some tree wētā (12 years in captivity). Their eggs are laid in the soil in autumn and incubation takes ~10 months with the nymphs emerging in March/April each year. As they grow, the Mahoenui giant wētā moult shedding their skin ten times over a lifetime. Each stage between moults is called an instar. Only in the first 6 instars, any appendages that are lost or broken can be regrown during each moult.
Once the females lay eggs, all the adults will die before winter. Predation from hedgehogs, possums, rats, cats and stoats is highest for females when they come down to the ground to lay 200-400 eggs. Females lay their eggs with a long ovipositor that protrude from the rear of her abdomen. The young nymphs are cannibalistic eating other insects including caterpillars, spiders, cicadas, crickets, and even their brothers and sisters. Leaves, bark and dead leaf litter are also on the menu.
Mahoenui giant wētā are generally solitary insects, which is probably down to their nasty habit of eating each other when young. For defense from other wētā entering their territory, they produce warning clicks by extending their abdomen in and out. They also strike their hind legs against their abdomen to create a rasping sound. They will attempt to push or scare off predators by quickly raising their spiny hind legs.
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A Fantastic Experience
Our experience with the Mahoenui giant wētā was an encounter courtesy of a few groups that we pestered with emails. Waitomo Education Services were my first port of contact who then passed me on to DOC, who then passed me on to a BSc graduate, Hannah, who had just finished a project tracking Mahoenui giant wētā using tiny, tiny transmitters super-glued to a metal plate mounted on their backs.
Despite having finished her project, Hannah very kindly took Emma and I out on a private tour and giant wētā hunt. We were spiked endlessly by gorse that we had to rummage through in search of them, but over the four hours of trawling, we managed to find many tree wētā, two early instar giant wētā, and two pretty big male giant wētā. Sadly, we didn’t come across any females, as these tend to be considerably larger as they are egg laden at this time of year. Saying that, the two we came across were beautiful, prehistoric-looking creatures of a deep mahogany colour. They were also very well behaved and photogenic as you can see.
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Despite not finding the biggest wētā in the world, we had a wonderful time and I guess finding everything at once would spoil the fun. Next time, the monstrous females we have heard so much about are on the cards.
References and Further Reading
Department of Conservation – Weta Facts http://www.doc.govt.nz/nature/native-animals/invertebrates/weta/
(Retrieved 31 January 2017)
Mahoenui Giant Weta (Deinacrida mahoenui) http://www.waitomocaves.com/downloads/Mahoenui%20Giant%20Weta%20(Deinacrida%20mahoenui).pdf
(Retrieved 31 January 2017)
Wikipedia – Weta https://en.wikipedia.org/wiki/Weta
(Retrieved 31 January 2017)
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