Invasive redback spiders (Latrodectus hasseltii) threaten an endangered, endemic New Zealand beetle (Prodontria lewisii)

Abstract

Australian redback spiders (Latrodectus hasseltii Thorell, 1870) are invasive, opportunistic predators that threaten New Zealand fauna. Initially recorded in Central Otago in 1981, they were observed in 2012 preying on the endemic, nationally endangered Cromwell chafer beetle (Prodontria lewisii Broun, 1904) in the species’ last occupied habitat, the 81 ha Cromwell Chafer Beetle Nature Reserve (CCBNR). We surveyed the redback spider population over the entire reserve in October (spring) and December (summer) 2013, recording web occupants, web condition, and prey caught. During the 5 weeks between surveys, prey caught in half of the recorded spider webs were monitored each week. We estimated a redback population of between 208 and 371 spiders, an average of between 2.57 and 4.58 spiders per hectare within the CCBNR, with maximum web densities ranging between 8.7 and 10.7 webs per hectare. In the final survey in December, 278 P. lewisii cadavers were found in redback spider webs. Redback spiders occupied a large portion of the habitat to which the Cromwell chafer beetles are restricted. Ten of the 26 prey species recorded in webs were native, including McCann’s skink (Oligosoma maccanni); the first record of skink predation by redback spiders in New Zealand.