Dramatic increases in weta (Orthoptera) following mammal eradication on Maungatautari – evidence from pitfalls and tracking tunnels

Opportunities now exist to establish pest-free areas on the mainland of New Zealand by eradicating introduced mammals from within predator-proof-fenced areas. This has increased opportunities to investigate how the native insect fauna responds to the eradication of introduced mammals. We examined the response of weta populations to mammal eradication in a before-after-control-impact (BACI) experiment within the southern exclosure on Maungatautari. A novel monitoring technique (footprint tracking tunnels) was used in combination with a conventional technique (lethal pitfall traps) to monitor weta populations. Within 2 years after mammal eradication, there were dramatic increases in weta pitfall captures, weta tracking rates and the incidence of weta footprinting per tracking card (proportion squares tracked out of 100). The mean number of weta per pitfall increased 12-fold (95% credible interval 7–20) after mammal eradication for adult Hemideina thoracica and 52-fold (95% credible interval 30–89) for other weta. Before and immediately after mammal eradication approximately equal proportions of juveniles, subadult and adult weta were caught in the pitfall traps. The age structure of weta caught in pitfall traps changed after mammal eradication, with the percentage of adult weta in the samples increasing markedly from 30% in summer 2004/05 to 66% in summer 2008/09. The sex ratio of weta caught also changed, the percentage of females in pitfall traps increasing from 25% in summer 2004/05 to 55% in summer 2008/09. The two monitoring techniques have different advantages and disadvantages, which are discussed, but key advantages of tracking tunnels are that they are non-lethal and time-efficient. Counting squares tracked per card gave a more sensitive and precise indicator of response to mammal eradication than simply recording presence and absence of tracks, but required much more processing time.