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In September, the Otago Daily Times reported that the Department of Conservation had set up a breeding programme for the beetle, but only a small proportion of larvae were pupating.

It was suspected the problem might have been connected with their diet.

Wild beetles feed on such things as tussock root, but the laboratory beetles were given carrots because they are more easily available.

The larvae ate carrot, but AgResearch scientist and project head Dr Barbara Barratt said there was a "hang-up" at the time when the beetles finished feeding as larvae and were supposed to pupate.

An experiment is being set up to determine if the problem was caused by diet or by other factors such as the length of "chilling" beetles underwent.

"We know that pupation is the time that synchronises the whole population. They all pupate at the same time and we suspect a chilling period is required before they will do that."

Dr Barratt said Cromwell's Wooing Tree Vineyard, which produces a pinot noir called "beetle juice", had given "a little bit of money" to help pay for the next step of fieldwork.

Field staff would be back in Cromwell on January 8 to collect eggs and larvae from beetle "cages" installed at the 81ha reserve.

"That's the only place they occur on the whole planet so it's a very important reserve. It's a nature reserve which has the highest ranking of all reserves."