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