In 1996, Xylotoles costatus was assessed as being Extinct. However, animals have since been found on South East Island in the Chatham Islands group. There is only a single population in an area of 219 ha. It is believed that there are fewer than 200 mature individuals in the total population. The population is probably stable, but it is unknown if it is subject to extreme fluctuations that increase the threat of extinction. Historically (over last 100 years) there has been a > 60% decline in the habitat area (and probably the population numbers). This was human induced. Recruitment of host plants is possibly also an issue.

Xylotoles costatus has been collected from Pitt Island and South East (Rangatira) Island in the Chatham Islands group (Early et al. 1991, Emberson et al. 1996, Emberson 1998b). All recent collections have been on South East Island. It has not been seen on Pitt Island since 1907 (R. Emberson, pers. comm. 1999). The species is believed to be extirpated from Pitt Island.

The species was not found on Pitt Island during a survey in 1990 (Early et al. 1991) or during several subsequent searches on the island (R. Emberson, pers. comm.). It has not been seen on Pitt Island since 1907. South East Island has been searched four times since 1992. A specimen has been reared out of a C. chathamica branch (R. Emberson, pers. comm. 1999). Current population size and abundance is not known, but there are believed to be <200 mature individuals.

A flightless, blackish longhorn beetle with a variable green-bronze sheen and ridges on their wing cases. The body is 15–20 cm long (Emberson and Marris 1993, Emberson 1998a). They are very similar to, though generally smaller than, Xylotoles traversi which is found throughout the Chatham Islands (J. Marris, pers. comm. 2000). The antennae are long and able to fold back against the body.

Most species of Xylotoles are thought to be non-host specific, feeding as larvae on dead twigs (Emberson et al. 1996). They are usually found on Chatham Islands Coprosma (Coprosma chathamica) at night, either on tree trunks or dead branches (Emberson and Marris 1993c, Emberson 1998b, Emberson, pers. comm. 1999). A specimen has also been found by beating a dead branch of ngaio (Myoporum laetum) caught up in a tangle of Muehlenbeckia (Emberson and Marris 1993, Emberson et al. 1996).

The species is vulnerable to mouse predation on Pitt Island (Emberson and Marris 1993). This, along with a reduction in habitat, may have resulted in their possible extirpation from that island.

X. costatus is listed as ‘Nationally Critical’ (Hitchmough 2002), the highest threat ranking available in the New Zealand Department of Conservation threat classification system (Molloy et al. 2002). It is listed as a highest priority threatened species for conservation action (category A) in the New Zealand list of threatened invertebrates (McGuinness 2001).

Further surveys are required on Pitt Island to determine whether the species is still present there, but current evidence points to it being extirpated from that island.

A variety of trees on South East Island should be marked then searched for the beetle to determine whether the apparent relationship between Xylotoles and C. chathamica is true. Most searches have focused on C. chathmanica because this is where they have been known to be found. However, this may not necessarily be the main host.

Branch traps should be set across South east Island and checked in 6 weeks time for the presence of the beetle. This will provide some idea of distribution and abundance. Wood borers are hard to sample, and the low numbers sampled make it difficult to obtain accurate population estimates (R. Emberson, pers. comm., 1999).

Further taxonomic work is required to enable X. costatus to be clearly separated from X. traversi (J. Marris, pers. comm., 2000).

Rodent quarantine procedures are required on South East Island.