Speargrass weevils thriving on Mana Island

Weevils get a lot of bad press. A few species are serious crop pests or despoilers of cereal products, and they give the rest of the family a bad name. In New Zealand, a dozen or so weevil species and populations are recognised as being threatened with extinction, and Wellington speargrass weevils (Lyperobius huttoni) are one of the most threatened. They are the only North Island population of a group of weevils characteristic of the South Island high country.

A Wellington speargrass weevil browses on an Aciphylla squarrosa leaf, Mana Island. Image: Colin Miskelly, Te Papa

A Wellington speargrass weevil browses on an Aciphylla squarrosa leaf, Mana Island. Image: Colin Miskelly, Te Papa

These 2 cm-long weevils have two traits that make them vulnerable to extinction. Firstly they are monophagic, meaning that they eat only one plant species – in this case the ferociously well-armed speargrass (taramea) Aciphylla squarrosa. The larvae live in the soil, feeding on the roots, while the adults nibble the needle-tipped leaves and flower stalks.

Speargrass (Aciphylla squarrosa) in flower, Mana Island, November 2015. Image: Colin Miskelly, Te Papa

Speargrass (Aciphylla squarrosa) in flower, Mana Island, November 2015. Image: Colin Miskelly, Te Papa

Most speargrasses are alpine plants, but around the Wellington south coast and on Mana Island, A. squarrosa occurs on coastal slopes to near sea-level. Mainland populations suffer from browsing by pigs and goats, which is the first problem the weevils face. But the main problem is that they are flightless and slow-moving, making them highly vulnerable to predation by rats and mice. By 2006, the Department of Conservation (DOC) estimated that there were fewer than 150 adult speargrass weevils surviving on the Wellington south coast.

Aerial view of the west coast of Mana Island, January 2015. Speargrass weevils were released at the left-centre of the image, between the track and the cliff-top. Image: Colin Miskelly, Te Papa

Aerial view of the west coast of Mana Island, January 2015. Speargrass weevils were released at the left-centre of the image, between the track and the cliff-top. Image: Colin Miskelly, Te Papa

Fortunately there are no introduced mammals on Mana Island, and during 2006-07 DOC (with financial support from the Friends of Mana Island) moved 40 adult weevils from Hawkins Hill (on the Wellington south coast) to the island. The animals were released in a patch of speargrass on the western coastal slopes of Mana Island, and kept a low profile for the next 9 years. Occasional browsing sign was noted on speargrass stems at the release site, and 1-2 animals per year were seen most years.

Feeding sign left be a Wellington speargrass weevil on an Aciphylla squarrosa leaf, Mana Island, November 2015. Image: Colin Miskelly, Te Papa

Feeding sign left by a Wellington speargrass weevil on an Aciphylla squarrosa leaf, Mana Island, November 2015. Image: Colin Miskelly, Te Papa

During a recent visit to Mana Island to monitor translocated seabirds, my departure was delayed a day, allowing time to search for the weevils. Two animals were found during 90 minutes of searching, but the most encouraging sign of their successful establishment was how far they had spread. Previous sightings were all near the release site, but one of the animals seen on 18 November was 400 metres to the north – an impressive distance over rugged terrain for a flightless insect. Dozens of the speargrasses between the two sites bore the characteristic feeding sign of the weevils.

Feeding sign left by a Wellington speargrass weevil after browsing on an Aciphylla squarrosa flowerstalk, Mana Island, November 2015. Image: Colin Miskelly, Te Papa

Feeding sign left by a Wellington speargrass weevil after browsing on an Aciphylla squarrosa flowerstalk, Mana Island, November 2015. Image: Colin Miskelly, Te Papa

Unlike the related flax weevils also introduced to Mana Island, the speargrass weevils appear to do very little damage to their host plants, and you have to look quite closely to see their feeding sign. In contrast, the flax weevils have obliterated several hectares of solid flax sward, and are in the process of changing the island’s plant communities.

A Wellington speargrass weevil on Mana Island, November 2015. Image: Colin Miskelly, Te Papa

A Wellington speargrass weevil on Mana Island, November 2015. Image: Colin Miskelly, Te Papa

With thanks to Brent Tandy (DOC) for information on the 2006-07 translocations.

Related blogs

A plague of flax weevils – a conservation hyper-success story

One step forward after three steps back – slow progress with restoring populations of New Zealand seabirds

A box of fluffy birds – the sequel. Fairy prion chicks fly from Mana Island

4 Responses

  1. Graham Randle

    Colin, I wish to advise you that I saw a rare bird on the mainland a Red Fronted Parakeet at Holdsworth Lodge close to the old lodge three weeks ago. I do have a photo which is not that good but it does show that it is a parakeet. I was also wondering it DOC has released some their. I also went to the Sounds for a week with my friend who has a ship that services Salmon Farms and I noticed a patch of white on a point opposite Allports Island that turned out to be a shag roost. I did notice it last year when I was there and saw a King Shag with some Spotted Shags on it but this time there were up to 7. I was to far away to see if they were nesting but I thought you should know about the site. It they were nesting they are only about three quarters of an hour from Picton and are nesting on the main land. I also visited the Wairau River where I saw up to 30 Black Fronted Turns hawking up and down over the water close to Highway no1’s bridge just out of Tua Marina. I do have a telephoto of the Shag site and some of the Terns. I hope you will be interested in this. Graham Randle

    Reply
    • Colin Miskelly

      Hi Graham
      Thanks very much for these observations. They are a bit off-topic in relation to speargrass weevils, so I’ll respond by personal email
      Cheers
      Colin

  2. Mark Anderson

    Fascinating report and great news about the successful translocation. It would be really interesting to know how this species has managed to exist in two distant communities which would be difficult to move between for this slow moving creature – did they evolve on the coast and as temperatures gradually warmed moved with their host or is there a plant that it can eat, but isn’t seen on due to its low population numbers? It would be very interesting to know how many of these charismatic animals there are there in case this can be used as a source population – there is some speargrass (not sure of the species) on islands in the Marlborough Sounds.

    Reply
    • Colin Miskelly

      Hi Mark
      I presume you are referring to the original distribution of Lyperobius huttoni, which was mainly in the South Island (Marlborough to South Canterbury, east of the main divide), plus the outlying population on the Wellington south coast. It is conjecture how this flightless insect came to be in the North Island. Its very limited distribution compared to the distribution of speargrasses in the Wellington region and the North Island axial ranges led to the suggestion that some animals had rafted across Cook Strait on clumps of vegetation washed down a South Island river. It would be interesting to compare the genetic profile of Wellington animals with the different South Island populations to see if there is an evident source population, and to estimate the time since divergence.
      Regards
      Colin

Leave a Reply

  • (will not be published)