The prickly prize of ongaonga

It's spectacularly spiky and delivers a painful or even deadly sting. Why are a team of conservationists growing and planting up Orokonui Ecosanctuary near Dunedin with more and more native tree nettle, ongaonga? It's all because of a pretty little pollinator called the kahukura, or red admiral butterfly, and its prickly preferences. Claire Concannon visits Orokonui to learn more about the ongaonga-kahukura relationship, as well as new research investigating whether these native butterflies are the victims of a sneaky ecological 'trap'.

With stems and leaves covered in sharp, needle-like spines, it's a native plant that screams 'don't touch me'.

And rightly so. If you do touch the native tree nettle ongaonga, the consequences can be painful - and even deadly.

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Walking down the Butterfly Track at Orokonui Ecosanctuary just north of Ōtepoti Dunedin, you'll come across plentiful ongaonga, purposefully planted.

Why would we want more of this 'ferocious' plant that can deliver an unpleasant sting?

Part of the ecosanctuary's mission is to provide as many native species as possible with habitats and ecosystems, says botanist and educator Taylor Davies-Colley.

"A lot of species require very specific host species for their caterpillars to live on," he tells Our Changing World.

Native butterflies like the kahukura / red admiral rely on the sharp barbs of ongaonga (aka Urtica ferox), which provide a safe haven for their precious eggs and larvae.

As the caterpillar grows, it folds the spiky leaves around itself as a protective tent during the day.

University of Otago Master's student Greer Sanger has been researching how the kahukura's preferences may be leading the butterfly species into an 'ecological trap' - a situation where an animal inadvertently opts for a less-than-ideal habitat.

Dragonflies attracted by the shiny surfaces of black headstones because they resemble the surface of water are an example of this.

In the case of kahukura, could introduced nettles be swindling butterflies with the offer of inferior habitat?

To test this idea out, Sanger caught some kahukura from around Ōtepoti and placed them in special cages with either ongaonga, a Chatham Islands hybrid nettle, or the introduced dwarf nettle. She then watched to see where the butterflies chose to lay their eggs.

"They definitely had a preference for the native over the introduced ," Sanger says. "None of them chose to lay on the introduced nettle." …

Go to this episode on rnz.co.nz for more details