A necessary weevil – an introduction to speargrass weevils and other invertebrates associated with speargrasses

A necessary weevil – an introduction to speargrass weevils and other invertebrates associated with speargrasses

Speargrasses, with their sharp leaves and flower spikes, may look like a plant you want to avoid at all costs but a number of critters call them home, including the charismatic speargrass weevils. Science researcher Lara Shepherd introduces these weevils, plus some other critters that utilise speargrass plants.

When I used to think of weevils my first thought was of those pesky weevils that invade the flour-based products in kitchen cupboards. But I recently found out that Aotearoa New Zealand has around 1500 native species of weevil and these play an important role in our ecosystem.

During our recent speargrass fieldwork an unexpected distraction was finding speargrass weevils (Lyperobius) on our target plants. Speargrass weevils are particularly charismatic because they are some of the largest beetles in Aotearoa New Zealand, reaching close to 3 cm in length!

A black and yellow weevil on the tips of a person's fingers.
The speargrass weevil, Lyperobius spedenii at Mt Dick, Kingston. Photo by Lara Shepherd

Speargrasses and their close relatives Anisotome are important host plants for speargrass weevils. The weevils larvae live amongst and feed on the speargrass roots and the adults feed on the leaves, flowers stalks and developing seeds. The spiky leaves of some speargrass species are no match for the weevils, who manage to avoid being impaled on the sharp leaf tips.

Lyperobius cupiendus feeding on a speargrass leaf. Photo by Taylor Davies-Colley. CC-BY
Lyperobius cupiendus feeding on the surface of a speargrass leaf. Photo by Taylor Davies-Colley, CC-BY via iNaturalist

There are currently 16 species of Lyperobius speargrass weevil recognised, although there may be more undescribed species. All species are only found in Aotearoa New Zealand. Most species are restricted to the South Island, with only populations of Lyperobius huttonii reaching the North Island and Lyperobius nesidiotes restricted to the Snares Islands.

A spiky plant on a snowy hillside. The plant has clumpy flowers, one of the flowers has a weevil on it.
Lyperobius barbarae on the floral inflorescence of an Aciphylla dobsonii. Lyperobius barbarae was named in 1999 after the entomologist Dr Barbara Barratt. Photo by Lara Shepherd

Some speargrass weevils are very rare, a result of habitat clearance and predation by rodents and one species (Lyperobius nesidiotes) may even be extinct. Damage to speargrass plants such as uprooting by pigs and browsing by hares is also likely to have a negative impact on speargrass weevils, as well as the speargrasses.

During our fieldtrip we found some speargrass weevil species to be quite common in places, mainly in the alpine zone where they might be more likely to escape being eaten by rats.

A top-down view of a spiky plant with large weevils on it.
Aciphylla simplex from the Pisa Range crawling with Lyperobius hudsoni weevils. Photo by Lara Shepherd
Close up of a black and white weevil on a spiky plant.
A Lyperobius hudsoni on Aciphylla simplex. The milky sap of the speargrass is visible and it is possible that the weevil is feeding on this sap. Photo by Lara Shepherd

Two species of knobbled weevils (Hadramphus) are also restricted to speargrass plants. The coxella weevil (H. spinipennis) lives on the Rēkohu/Chatham Islands. The Canterbury knobbled weevil (H. tuberculatus) was once widespread across the Canterbury Plains but sightings ceased after 1922, so it was considered extinct. However, it was rediscovered in 2004 at Burkes Pass Scenic Reserve by a university student studying speargrasses. The significance of this finding has been compared to the rediscovery of the ‘extinct’ takahe in the Murchison Mountains in 1948!

A close up image of a brown weevil.
The Canterbury knobbled weevil (Hadramphus tuberculatus) was rediscovered in 2004. Photo by Warren Chinn, CC-BY-NC via iNaturalist

As well as being the specific hosts for a number of weevils, speargrasses are also important plants for many other insects with the flowers providing a significant source of food. In return the insects move pollen between plants. Speargrasses have separate male and female plants. It is thought that the flowers of female plants produce nectar to entice insects, and the flowers of male plants produce both nectar and pollen.

We often saw speargrass flower spikes teeming with life and research has shown that these the larger the flower spike then the more insects it attracts. A few insects we found on speargrasses are shown below.

Thanks to those on iNaturalist who identified our insect observations, especially Samuel Brown, Stephen Thorpe, and Steve Kerr.

Further reading

Craw RC. 1999. Molytini (Insecta: Coleoptera: Curculionidae: Molytinae). Fauna of New Zealand 39.

Young LM. 2006. Masting and insect pollination in the dioecious alpine herb Aciphylla. MSc thesis. University of Canterbury.

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