Have you ever seen what looks like a blue-green grain of rice flying in the sun, like this?
If so, and if you live in Australia, it’s probably an Emerald or jewel wasp from the Primeuchroeus genus. Like many iridescent creatures, the apparent colour changes depending on the angle of the light falling on it, and ranges from pale blue-green through a much darker turquoise to a deep emerald green.
Here’s a closer view of this wasp, which really is about the size of a grain of Basmati rice –:
Primeuchroeus wasps are some of the smaller members of a world-wide family called the Chrysedidae, or Cuckoo wasps, many of which have iridescent colouring. Like its Chrysedid relatives, the Emerald wasp breeds parasitically. It’s thought that its host species are mud-daubing wasps.
The Emerald wasp lays its egg in the nest of the host wasp, and when this egg hatches the larva feeds on either the host’s larvae or on the food store (such as a paralysed caterpillar) laid up in the nest by the host for its own larvae – with fatal consequences for the host larva. If a host wasp mother catches an Emerald wasp invading its nest, it will attack it with sting and jaws. The Emerald wasp responds by curling up defensively and tucking its legs in – it has very thick plates on its back which are impervious to most wasps, and a flattened underside. You can get some idea from this picture of a partly curled wasp which was temporarily caught in a web.
We have recently had two of these Emerald wasps around the front of our house. I’ve seen it suggested that they could be looking for other wasps’ nests to lay their eggs in, as mud-daubing wasps often nest on or near houses and sheds. However I’ve not seen any mud-daubers near the front of our house, and their behaviour didn’t really match that. It wasn’t clear what they were looking for, but they weren’t unduly fazed by my proximity, even though they have weak or non-existent stings. They would disappear for a while, possibly to feed, then return and examine the railings and brickwork by landing and “sniffing out” various spots using those two downward-pointing antennae. Perhaps they were as unsure about their goal as I was – this one was certainly scratching its head.
There doesn’t seem to be a lot known about the habits of Australian Primeuchroeus wasps. Perhaps that’s not surprising. One would have to be keen, and possess keen eyesight, to be able to follow them around on their day’s routine.
However some of their much larger overseas relatives in the Chrysedid family have been closely studied, and consequently become notorious as “zombie wasps”. One – Ampulex compressa, from S Asia and Africa – is able to paralyse a cockroach’s brain but not its legs, and then steer the cockroach zombie-like into a spot where it will not be disturbed. The wasp then lays an egg in the cockroach. After hatching, the wasp larva eats the cockroach from the inside until it is ready to emerge.
Other zombie wasps are able to exercise some control over spiders and caterpillars. Our Primeuchroeus wasps are not known to have the same abilities. They just look decorative on the front porch, like early Christmas baubles - at least if you have a magnifying glass!