Alexandra’s trainee ranger, Abby Toseland, explains the importance of catching introduced redback spiders to help protect our Cromwell chaffer beetle.
What is the Cromwell chafer beetle?
The Cromwell chafer beetle (Prodontria lewisi) – is a large flightless beetle that lives underground, only emerging at night in spring and summer to feed on plants and to breed.
Why is it special?
This beetle is like the Maui’s dolphin of invertebrates. It’s nationally critical and can only be found in one place in the world – the Cromwell Chafer Beetle Nature Reserve (between Cromwell and Bannockburn, in Central Otago). The 81 hectare reserve was established in 1979 and is the only reserve in the world created solely for the protection of an invertebrate.
How many chafer beetles are left?
It’s hard to give a definitive answer, population estimates are not precisely known and estimates vary. Chafer beetles are tricky to find as they burrow deep and only come out at night. Work is happening to gain an understanding of the population though. Core sampling with an excavator mounted rotator corer has been carried out. The samples taken were searched by hand, on site and any invertebrates found were recorded then returned to the hole from which the core had been taken.
Why are redback spiders a threat?
Rabbit holes in the reserve have proven to be ideal spaces for redback spiders to build their webs. A study carried out by Ag Research, Department of Conservation and the University of Otago showed 99% of spiders built their webs in rabbit holes. Under inspection of those webs it has been found that the Cromwell chafer beetle is common prey for the introduced red back spiders which is greatly decreasing their already dwindling numbers.
How are they being protected?
Conservation efforts to protect this endangered species include stomping in the thousands of rabbit holes formed in the reserve every year. Research has found filling in the holes, as well as carrying out regular rabbit control efforts, is an effective way of eliminating the presence of the red back spiders in the reserve and therefore reducing the rate of the chafer beetle being preyed upon.
Over the course of the last four years, redback spider abundance has greatly decreased from 460 spiders found in 2014 to 123 spiders found in 2016/17.
This winter I took part in rabbit hole stomping along with other Alexandra based DOC staff and we will be returning this summer to see if there is another drop in numbers to coincide with this.
For more information on the Cromwell chafer beetle, listen to our DOC Threatened Species Ambassador, Nicola Toki, on her Radio New Zealand Critter of the Week series.