Tuesday 9 May 2017

Redbacks are going down the rabbit hole and rare NZ beetles pay the price

Venomous Australian redback spiders are increasingly “sinking their fangs” into New Zealand’s Central Otago region and their invasion is being aided by the activities of another introduced pest – European rabbits – new University of Otago research has shown.

Otago scientists undertook a study near the town of Cromwell that found members of this 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 other native insects.

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

Ms 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.

Ms 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 of them were still present after this time period.

Ms 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 says.

The Otago study findings were published earlier this year in the international journal Biological Invasions.

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