Tiny hunting wasps seldom noticed

Recently I was brought for identification tiny hunting wasps less than 3mm long, writes Anthony Harris.

They had been flying in and out of borer beetle holes in the outside walls of an old wooden villa in Maori Hill. The tiny wasps were using the holes as nesting sites and are harmless. The minuscule wasps (four Spilomena species) are widespread in Dunedin but are very seldom noticed on account of their small size.

A few years ago years ago, a Dunedin resident reported many thousands of tiny insects flying in a dense cloud under an open veranda along the entire 18.5m frontage of a magnificent wooden two-storied house set in a beautiful flower garden. Some of the insects were moving big piles of sawdust from 1.5mm-wide holes, and would repeatedly fly away and then return. When I visited the house, I saw the largest concentration I have ever seen of the minute hunting wasp Spilomena nozela. These were 2.6mm to 3mm long, and did not make the holes, but were nesting in abandoned Anobium borer-beetle galleries after first cleaning out the debris left by the beetle larvae.

These wasps fly to flowers on which there are thrips, sting them to paralysis, then fly with them to borer beetle holes, where the thrips are stored inside, as food for the tiny wasps’ larvae.

Thrips are minute insects, frequently 1mm to 2mm long and often seen as little black specs in flowers. Six or more Spilomena females sometimes share the same burrow and make up to 72 cells within. This communal nesting is not found in any other New Zealand genus of hunting wasps.

Four species of Spilomena occur in New Zealand, all of very similar appearance and behaviour. When Spilomena wasps are active, they first clean out the old borer-beetle galleries, so that mounds of wood-dust appear at the entrances. It looks as though fresh, active, borer is present in the wood.

A minute bee is also nesting in abandoned borer beetle holes at present on the outside walls of wooden houses. This is Euryglossina proctotrypoides (superfamily Apoidea, family Colletidae), which is gregarious but solitary. This pigmy bee is 3.3mm to 4.5mm long and is very similar to Spilomena wasps in size, shape and general appearance. The bees’ antennae, however, are situated slightly higher on the face than those of Spilomena. The tiny bee takes nectar from flowers including manuka and kanuka into the burrow and like Spilomena, is harmless.

On account of its small size and superficial similarity, the minute bee E. proctotrypoides is misidentified as a Spilomena species in some New Zealand insect collections.

E. proctotrypoides was accidentally introduced into New Zealand from Australia in the early days of European settlement, possibly by sailing ships, along with Spilomena wasps, in old borer holes in wood.