Master’s student Jackie Spencer’s slideshow about her study of the relationship between redback spiders and rabbits makes the aims of her project clear: to see if redbacks are living in rabbit holes, and find out if they are eating Cromwell chafer beetles — and not get bitten. Pam Jones finds out about a co-habitation that is threatening one of Central Otago’s endangered species.
Jackie Spencer’s predecessor used "a big long stick", but Ms Spencer prefers kitchen tongs.
"I just sort of poke them down there and it seems to work OK. I’ve never had any trouble, but if I get bitten, Dunstan Hospital just said to phone the Cromwell Medical Centre and someone would come and get me. They’ve got antivenin at the hospital."
It is a matter-of-fact emergency plan that, hopefully, will never eventuate, but Ms Spencer’s fieldwork is not for the fainthearted.
It all hinges on finding specimens of an arachnid whose toxic venom is known not only to cause severe pain to humans, but which is now also causing problems for one of Central Otago’s endangered species — the Cromwell chafer beetle.
In March, findings were announced from a joint study that found redbacks were living in old rabbit holes in the Cromwell Chafer Beetle Nature Reserve. Investigation of the redback webs in the rabbit holes found the Cromwell chafer beetle was the spiders’ second-most common prey.
The relationship between the redbacks and rabbits was labelled an "unholy alliance" by AgResearch principal scientist Dr Barbara Barratt, who said 99% of the redbacks in the reserve had built their webs in the rabbit holes.
"Of course the rabbits and spiders aren’t actually plotting together to bring about the demise of the chafer beetle, but these findings do give a fascinating insight into the almost accidental relationships that can develop between species in the natural world, and how that can impact on other species."
Dr Barratt has co-supervised the research of University of Otago master’s student Ms Spencer, which formed the core of the joint study done by the university, AgResearch and the Department of Conservation.
Ms Spencer has spent hundreds of hours in the field and was the lead author for research being accepted by the Journal of Biological Analysis.
She said her main focus was that "the chafer beetles’ plight gets aired", and said the results of her work and follow-up Doc action were promising.
Perhaps unsurprisingly, one of the findings of her research has been that filling in the rabbit holes in the reserve affected spider distribution — when rabbit holes were filled, redbacks did not re-establish in a defined 5m area in all test plots.
She said the "clustering" of both holes and spiders, that is, where spiders lived in the rabbit holes, was significant, and it was crucial to monitor beetle numbers and how many were being eaten. The redbacks’ impact meant there was a danger of the beetles becoming extinct, and it was "time to take action now", she said.
Doc senior ranger, biodiversity, John Keene, of Alexandra, said Ms Spencer’s research was promising.
"While we are very much still in the gathering information stage on the spider/beetle interaction, the results from Jackie’s research are very encouraging to date.
"Key actions Doc are undertaking at the reserve in the next 12 months are rabbit control, including destruction of burrows, and monitoring of beetles and spiders to better inform future management, along with routine fence maintenance and weed control."
It was not known how many chafer beetles and redbacks were at the reserve, Mr Keene said.
But it was fair to assume that if redbacks were living in rabbit holes in the chafer reserve, they would be living in rabbit holes and other habitats elsewhere, Mr Keene said.
It had been known for decades redbacks lived in Central Otago and there would be "thousands and thousands" of them spread throughout the region.
However, the redbacks’ "quiet nature" meant they posed no great risk in the outdoors, Mr Keene said. Like many wild creatures, they would only bite if trapped or threatened.
"They’re pretty shy, retiring creatures ... You’ve got more chance of being bitten by a redback that’s living in your woodpile than you have out in the hills."
There were different theories about how redbacks first arrived in Central Otago from Australia, but the idea they had come in shipments of power poles was a likely one, Mr Keene said.
He emphasised it was still "early days" for the research into redbacks, rabbits and chafers.
Redbacks ate only adult chafers, but it was not known at what stage and whether they had laid their eggs by the time they were eaten.In the meantime, Doc’s other work at the chafer reserve continued.
This winter, routine rabbit control work will be done, and maintenance such as fencing and weed control work is ongoing. The chafer reserve was low maintenance, and Doc had a budget of about $10,000 annually for work there, including staff time, Mr Keene said.
Another survey of redbacks was likely to be done early next year, and once those results were analysed, hopefully, a "rescue plan" could be developed.
"By that time, hopefully, we’ll be well placed to work out a solution, and know more about what to do to protect the chafers from redbacks."
● The co-writers and co-supervisors of Ms Spencer’s master’s study are Dr Phil Seddon, Dr Yolanda van Heezik and Dr Barbara Barratt.
Redbacks timeline
• Early 1980s: Redback spider observed in Central Otago.
• 2010: Redback spiders confirmed to be in the Cromwell Chafer Beetle Nature Reserve.
• 2013: University of Otago student Stacey Bryan confirms there are at least 200-300 redback spiders in the reserve and that they are preying on the chafer beetles.
• 2014-15: Research by University of Otago master’s student Jackie Spencer confirms 99% of redback spiders are building their webs in old rabbit holes in the reserve.
• Spring 2015: Department of Conservation project fills in rabbit holes in central part of reserve.
• March 2016: Another spider survey done by Jackie Spencer — 140 redback spiders located. It is thought Doc work is helping.
• March 2017: Another survey done by Jackie Spencer, 112 redbacks located.
— Source: Jackie Spencer
Cromwell chafer beetle
• The flightless and endangered Cromwell chafer beetle (Prodontria lewisii) is endemic to New Zealand and has a restricted and localised distribution in and near Cromwell. It is not the same as the more common Alexandra chafer beetle.
• Adult Cromwell chafer beetles live in shallow sands and spend the day buried in the soil, emerging at night during spring and summer to eat plants and lichens.
• The Cromwell Chafer Beetle Nature Reserve was formed in 1979 and expanded to 81ha in 1982. It is managed by the Department of Conservation.
Redback spider bites
• If you get bitten by a redback it will hurt — causing severe pain and symptoms that can continue from hours to days — but you are unlikely to suffer any worse fate. In Australia, where about 2000 people are bitten by redbacks a year, an effective antivenin was developed in the 1950s. Varying online reports estimate there had been no deaths in Australia from a confirmed spider bite since either 1955 or 1979. A man died after getting an infection after being bitten by a redback in Australia last year, but it was not confirmed if the bite caused his death.
• In Central Otago, antivenin is held at Dunstan Hospital, in Clyde.
