Curator of Entomology, Natural Sciences, John Early discovered a species of introduced wasp previously unrecognised in New Zealand thanks to last year’s Level 4 Lockdown. He discusses how he made his unusual discovery.

Lockdown 2020 was a great opportunity to investigate some of the insects in my garden and I joined many other kiwis and became a backyard naturalist. My main research interest is in the small (by that I mean 1-3 mm long) parasitic wasps of New Zealand - discovering what species we have in our native forests and other natural ecosystems but not paying too much attention to modified urban and suburban environments. This was the perfect opportunity to get to know the locals.

One of my main methods to survey these little insects is to use yellow pan traps, small bright yellow bowls containing tap water plus a few drops of an unscented detergent. Not a very sophisticated technique but it delivers the goods. Yellow is strong entomological magic - many insects of all kinds are attracted to it, land on the water and drown. Two days later I sieve the contents, wash them in fresh water and transfer them to a jar with 70% alcohol preservative, then examine the catch under the microscope. It's quite exciting to sort the sample - a real lucky dip.

The Lockdown period was mild and sunny, there was a lot of insect activity and I had great success, from traps placed around the garden and lawn, and particularly in my compost heap (a euphemism for the place where I randomly toss the kitchen scraps and garden waste). There were many parasitic wasps I had never seen before, some cosmopolitan species that are known to parasitize pupae of house flies and blowflies which are prolific in the compost, but also some native species well known to me from native bush habitats. Given the wealth of specimens I decided to continue trapping to monitor changes over time and so I ran the traps for two consecutive days in the middle of each month (as long as it wasn't cold, windy and wet), and I'm continuing this even now - over a year since I started.

I got a big surprise last December when I could see a couple of larger (about 10 mm long) black wasps in two of the traps and I found more through to March 2021. They looked a bit like native bees but weren't quite right, certainly not like the two species that I was already familiar with from my garden. Nor were they any of the other handful of black solitary native wasp species. Examination under the microscope quickly showed that they were a species of mason wasp, scientific name Pison species, but quite different from the only three species known from New Zealand. The task now was to identify precisely what species of Pison they were. I already knew that Australia was where the world's greatest diversity of Pison species was found so the probability of it being an accidental introduction of one of these was very high. Fortunately, Dr Wojciech Pulawski of the California Academy of Science in San Francisco had recently (2018) published a huge technical work on them, cataloguing 163 species from Australia. Following his identification key I concluded that it was the species Pison marginatum, and he confirmed that when I later sent him a pair to check.

But what does this species do? and will it become a problem like other introduced insects? All mason wasp species sting and paralyse spiders which they carry in flight to their mud nest cells. They lay one egg per nest cell on one of the still living spiders and when it hatches the mason wasp larva feeds on them over a period of several weeks. It's like the mother wasp has provided a lot of fresh food in the fridge for her baby. When fully fed the mason wasp larva transforms to a pupa, then eventually an adult wasp which chews its way out of the mud nest cell and flies free.

I couldn't find any mud nest cells of this new species, so I'll be keeping a sharp lookout for them this coming summer. I want to know the structure of their nest cells, what spider species they prey on and the best way is to find freshly made nest cells then break them open to retrieve the spiders. Given the wide geographic distribution of Pison marginatum they probably won't be very specific. Prey choice will most likely be determined by whatever spiders are found in the habitats where they prefer to hunt, and which are of a size they are able to handle and carry back to nests.

Where else in New Zealand have they been found? So far, the only other records I can find on iNaturalist are from Auckland's North Shore (January 2021) plus one older record from Hawkes Bay (February 2020). It will undoubtedly spread throughout much of New Zealand. Mason wasps are good hitch hikers because of their adaptability to make nest cells in all kinds of cavities and grooves, both natural and in human made objects which are easily transported. It turns out that this particular species is widespread throughout Australia as well as Indonesia, Malaysia, the Philippines, and across the Western Pacific islands to Hawai'i. Hitching a ride on shipped cargo seems the most likely way it spread so far and wide and most recently here to Aotearoa. Given its wide geographical distribution it's a wonder that it didn't find its way here much sooner.