Rabbits aid redbacks' Central Otago invasion

The Australian redback spider, characterised by the red strip across its swollen black abdomen, is found in several areas in Central Otago and New Plymouth. Photo / File

Venomous Australian redback spiders are fanning out further in Central Otago - and scientists say they're being aided by another troublesome introduced species.

Otago University scientists undertook a study near Cromwell that found members of the invasive spider species are finding happy homes in old rabbit holes, in which they spin webs that catch scores of critically endangered Cromwell chafer beetles.

And it isn't the only endangered species under threat.

A species of weta is now considered "nationally critical" - the most serious category for threatened wildlife - because of redback spiders invading its territory, while another was recently reclassified as "nationally vulnerable".

The Hemiandrus "furoviarius", found in Tekapo riverbeds, was reclassified to nationally critical because of observed decline, while the Hemiandrus "Cromwell" moved from "naturally uncommon" to "nationally vulnerable" as a result of reassessment of the likely impacts of redbacks now abundant in their habitat.

As part of a study for her master's degree, Jackie Spencer investigated the spiders' habitat in the 81ha Cromwell Chafer Beetle Nature Reserve (CCBNR).

Spencer and the Department of Zoology's Professor Phil Seddon and Associate Professor Yolanda Van Heezik and AgResearch's Dr Barbara Barratt found that old rabbit holes were providing a very effective shelter from Central Otago's hot dry summers and very cold winters for at least 455 redback spiders.

Redbacks first became established in the region in the 1980s, and likely arrived with goods such as steel products from Australia.

In recent years they moved into the reserve.

Spencer recorded and analysed prey caught in redback webs on the reserve over a four-month period and then studied what effect filling in rabbit holes had on spider populations.

The reserve is surrounded by a rabbit-proof fence and none of the warrens are currently inhabited by rabbits.

The research team found that 99 per cent of redback spiders in the CCBNR had built their webs in old warrens.

The reclusive Cromwell chafer beetle, which is listed internationally as a critically endangered species, was the second-most common prey found in these redback webs.

After removing fifteen female spiders from their burrows, and filling these and surrounding holes in and releasing the spiders, four months later the researchers found none living in the vicinity of the filled-in holes.

Of the control group of 15 females who were not removed from their holes, 13 were still present after this time period.

Spencer says the clear implication of the study is that conservation efforts to save the chafer beetle must focus on eliminating rabbit holes.

"These flightless nocturnal beetles are easy prey for redbacks - if we want to see this rare and enigmatic species survive we must get rid of these spider lairs."

The Department of Conservation has experimented with filling in all the burrows in the central area of the reserve, and the results so far have been promising, she said.

The redback, characterised by the red strip across its swollen black abdomen, is found in several areas in Central Otago and New Plymouth, and its spread to other regions has remained a constant biosecurity worry.

Its incestuous nature means it can breed more freely than other species, and its thick web has helped it get past Customs checks.

But it's more notorious for the toxic venom that a single bite can deliver to a person unlucky enough to encounter one.

One in three bites comes with a severe reaction - but even "mild" ones are often extremely painful.