Redbacks threaten Cromwell beetle

Thursday, 29 October 2015

Bryan S A, Van Heezik Y, Vink C J, Seddon P J, Phillips C B, Barratt B I P. 2015. Invasive redback spiders (Latrodectus hasseltii) threaten an endangered, endemic New Zealand beetle (Prodontria lewisii). Journal of Insect Conservation 19: 1021–1027.

Latrodectus hasselti (redback spider)

Redback spiders are threatening the endangered Cromwell bettle

Australian redback spiders (Latrodectus hasseltii) preying on the endemic, nationally endangered Cromwell chafer beetle (Prodontria lewisii), which is only found an 81 hectare reserve near Cromwell. Surveys estimate that there are up to 371 redback spiders in the reserve and 278 Cromwell chafer beetles have been captured and eaten by this introduced spider. Other prey caught by redback in the reserve include a McCann’s skink (Oligosoma maccanni).

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.

See the paper here.

Image Credit: Bryce McQuillan