Steve August, of Dunedin, asks :-

These caterpillars grew in an open gas pipe in a gas-fired hot tub in Central Otago, blocking off the flow of gas. What are they?

Robert Hoare, an entomologist with Landcare Research, responded.

The caterpillars in the gas pipe are those of the Light Brown Apple Moth (Epiphyas postvittana), an Australian species of leaf-roller moth that has been present in New Zealand since 1891, and is now very common.

Their bright green colour is for camouflage on leaves. They tie leaves together or fold them with silk and hide within. More than 250 host-plants are known in New Zealand and this moth can be a pest in orchards, hence the English name.

So what are they doing congregating in a pipe? The answer is that they have been gathered as prey by a small species of wasp, the European tube wasp (Ancistrocerus gazella), which specialises on leaf-roller moths. This small (7-15 mm), yellow-banded black wasp first turned up in New Zealand in the 1980’s, presumably as an accidental import: like the moth, it is now very common and widespread. It was first seen in Central Otago in 1989.

The life history was studied in detail by Anthony Harris of the Otago Museum. Each female wasp seeks out a tubular hole 2.5 to 7.5 mm in diameter in which to construct its nest. e.g. holes in mortar, nail-holes, cracks between stones on the outside of houses, beetle-holes in dead wood, occasionally in grass stems, or in this case a tubular gas pipe.

The nest consists of 1 to 8 cells separated by partitions which the female makes from moulded mud. Inside each cell she places up to 20 caterpillars which she finds by searching nearby vegetation. The caterpillars are lightly paralysed by her sting, but still capable of some movement. Once the cell is stocked, she suspends an egg from the roof and seals the cell with mud. The wasp’s larva will hatch in 3 days and feed on the caterpillars for 12 days, before spinning a cocoon, turning into a pupa and eventually hatching as an adult wasp.

So here we have a novel interaction between a European wasp and an Australian moth in the unexpected setting of a New Zealand hot tub.

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