Feature

Working With Weevils

Conservation of the less-than-cute'n'cuddly is carried out in the remoter parts of the South Island.

Bruce Thomas

The study of several species of native weevils is an important component in a multi-faceted, islands restoration research programme in Breaksea Sound, Fiordland. The knobbled weevil (Hadramphus stilbocarpae), the flax weevil (Anagotus fairburni), and the teteaweka weevil (Anagotus sp. [turbotti group]) are large (20-25 mm), nocturnal, and flightless -- somewhat akin to dinosaurs of the invertebrate world.

Like so many of our ancient, specialised insects, weevils are vulnerable to rat predation and now have very disjunct, relict distributions, mostly confined to rodent-free offshore islands. Ideas conceived during a biological survey of islands in Breaksea Sound in 1974 culminated in successful, milestone campaigns to eradicate Norway rats from nine-hectare Hawea Island in 1986 and 170-ha Breaksea Island in 1988.

Rat eradication was achieved using new generation anti-coagulant poisons distributed in bait stations made from plastic drainage pipe. An important aspect of the ongoing conservation management strategy for these islands has been the translocation and monitoring of several species of native birds, lizards, and invertebrates, including weevils.

In Fiordland, knobbled weevils inhabit the "shoreline carrot" Anisotome lyallii and, despite extensive surveys from Doubtful Sound to Dusky Sound, populations of this species were found on only a few islands in the mouth of Breaksea Sound and along the exposed outer coast of Resolution Island and Fivefingers Peninsula. During the surveys, flax weevils and teteaweka weevils, which are host-specific to flax (Phormium cookianum) and the coastal tree daisy teteaweka (Olearia oporina), were recorded only on the tiny 3ha rat-free Wairaki Island in Breaksea Sound.

The transfer of 40 knobbled weevils and 20 flax weevils into suitable habitat near the Breaksea Island hut in March 1991 has been extremely successful and the new founder populations are now well established and increasing rapidly in both numbers and range. Dispersal by knobbled weevils has been quite exceptional -- they have crossed 70 metres of open boulder beach to colonise a semi-detached, vegetated rock stack and have been found in plants over 200 metres from the point of the original release.

Knobbled weevils on the edge of the main release area are now being marked with numbered bee-tags in an effort to determine longevity and the extent of movements of individuals during this dispersal phase. The current distribution of knobbled weevils in the Breaksea area indicates there has also been natural dispersal of this species between islands in recent years; natural dispersal is most likely to have occurred by rafting, but no weevils were found during checks of several clumps of Anisotome retrieved from the sea after storms.

An intriguing insight into the world of weevils has come from monitoring a population of knobbled weevils as they have progressively demolished their habitat on a small, unnamed islet, "OG3", in the Breaksea group. When we first stumbled on this particular population in 1991, weevil densities seemed extremely high in several of the 136 Anisotome plants that were growing on this rock. However, at that time not all plants were inhabited by weevils and there was a total of almost 9m² of foliage available to them.

Systematic checks showed the Anisotome was being reduced by as much as 50% per year by the weevils, and by April 1996 only six plants, a mere 2.5m² of foliage, remained. With grubs feeding at the roots and, from mark-recapture data, at least 106 adult weevils munching at leaves and stems, the complete annihilation of the remaining plants seems inevitable, with a resultant population crash of knobbled weevils likely sometime in the near future. In contrast, our control population of knobbled weevils on Wairaki Island has remained relatively stable over this period.

Weevil Wanderings

In April 1993, five years after rats were eradicated, several teteaweka weevils were found near the Breaksea hut during routine invertebrate monitoring on Breaksea Island. There has been a steady increase in the numbers of these weevils at this study site since then, so at first it seemed that we were recording a nice example of species recovery as a response to the elimination of rats.

However, the explanation for their presence near the Breaksea hut may not be quite so simple. A full arboreal life-cycle makes teteaweka weevils less vulnerable to rat predation than flax weevils or knobbled weevils, and neither teteaweka weevils nor their sign have been found during our searches of good habitat elsewhere on the island, as would be expected if they were in a general population recovery mode. Of concern is the possibility that some have been inadvertently transported between islands on the clothing of the research team.

In similar circumstances, flax weevils seem to have become established in the weevil monitoring area on Hawea Island (albeit in low numbers) yet they have not been seen elsewhere on the island. Also, on three occasions over the years, knobbled weevils have been found in the Breaksea hut and on board the support vessel after people have returned from doing observations and monitoring at night. It might be that the grasping nature of weevils makes them ideally adapted for this sort of chance dispersal. However, it has real implications for how careful we must be if we are to avoid accidental introductions, particularly of pests, into our special conservation areas.

On the positive side in this particular instance though, with a rapidly declining population of teteaweka weevils on Wairaki Island (a consequence of natural degradation of their tree daisy habitat), it is nice to know a second population is thriving in the newly created safe haven of Breaksea Island -- no matter how they come to be there.

These studies of weevils in Breaksea Sound have broken much new ground and been a key element in the development of a successful, fully integrated programme involving both conservation and science. Undertaken on a shoe-string budget, the Breaksea programme would not have been achieved were it not from grants and/or logistic support from several providers, good inter-organisational collaboration, high quality field assistance from a myriad of dedicated volunteer conservationists, and a rare long-term approach to monitoring.

Set in nature's wonderful, dynamic, outdoor "laboratory" known to us as Breaksea Sound, there is no doubt that this project has proved to be a fine example of the conservation catch-cry "research by management".

An ecologist with Manaaki Whenua Landcare Research in Nelson, Bruce Thomas has been involved in the Breaksea project since its inception.