Native hunting wasp less common
Otago Museum
THE gadflyhunter, Podagritus carbonicolor, is a magnificent 18mmlong native hunting wasp that captures big flies such as gadflies and blowflies in dry sandbanks in riverbeds. It is a member of the family Crabronidae, sometimes called ‘‘policeman’’ wasps. Much larger than related wasps overseas, it is New Zealand’s largest species.
The female gadflyhunter sometimes patrols gaps and paths in sand dunes, where it hawks for large flies — especially big native species — that use the passages as flyways. These convenient warm passageways are protected from strong winds and are often utilised by flying insects. The wasps also take prey in the lee of sandbanks and on flowers, as well as on and around foliage.
The wasp pounces on a fly and stings it to paralysis. It then wraps its middle legs around the fly, holding the fly close to its underside, with the fly upside down and facing forward. The wasp and its large prey make a bulky and obvious sight as the wasp flies slowly to its nest — a big round hole dug into dry sand.
The wasp drops its prey inside the entrance, with the tip of the fly’s abdomen just visible. Inside the nest, the wasp remains invisible beyond the fly, while it goes further down into the nest. Fifteen seconds later the fly disappears completely as it is taken deep into the burrow by the wasp. The wasp deposits the fly upside down, facing the entrance, in a pearshaped nestcell dug in moist sand, often at a depth of many centimetres. Several flies are placed in each nest cell.
The wasp is harmless and does not sting humans.
I have observed the gadflyhunter wasp throughout New Zealand and believe that its numbers are decreasing.
For many years, there was a big population at the mouth of the Eglinton River, near Lake Te Anau, but recent floods have reduced its numbers, even there, almost to zero.
In 1972, I observed this species at the Oreti River, near Lumsden, but have not seen it there since. Regular large floods in the Oreti River convert the sand, shingle and rocks in its course into a fluid mixture. Sandbanks reappear after the flood in completely different places. The large numbers of specialised native insects that live in sandbanks in river courses presumably reestablish nests in areas that have survived the flood.
Because more farmland commonly encroaches right up to the banks of rivers now, there are increasingly fewer places from which surviving sandloving insects can repopulate suitable rivers after floods.