Photo essay: In praise of parasites
I am not an entomologist, but a few weeks ago I was photographing insects in my garden and spotted a lot of green vegetable or stink bugs (Nezara viridula) on the spider plants. These bugs have fairly broad tastes and will feed on a wide variety of plants. They particularly like developing flowers and fruit and world-wide they are an economically destructive pest on a number of crops.
So it was interesting to see a small wasp apparently stalking one of the vegetable bugs.
The green vegetable bug
By the way, when entomologists refer to bugs, or ‘true bugs’, they usually mean a group of insects which have piercing and sucking mouth parts.
Despite their pest status green vegetable bugs are an attractive insect. They go through five moults or growth stages, called instars, and several of these instars were present at the same time on the spider plants.
Instars of green vegetable bugs
Identifiying the wasp
The small wasp I saw was identified by my entomologist colleague, John Early as Aridelus rufotestaceus. And it was indeed stalking the bugs. It is parasitic and lays its eggs inside a host insect. The eggs hatch into larvae which then go about consuming the bug’s body from within.
It is an introduced species which was first discovered in New Zealand some years ago by another entomologist, Stephen Thorpe. We don’t know how it got here or even how long it has been here, but this is possibly the first time that its parasitizing behaviour has been observed and documented in this country.
In the next series of photographs you can see Aridelus stalking its victim and then striking to lay its eggs.
Aridelus stalks its victim
This all happens in a matter of seconds.
Studies overseas have shown that more than one egg is laid, but only one develops into a larvae.
They have also shown that Aridelus was able to parasitize all but the very smallest of instars.
A second small wasp also spent a considerable time stalking two vegetable bugs, but never quite managed to gets its approach right.
A less successful approach
As it happens, I was lucky to make these observations, because in the following week night time temperatures continued to drop and the bugs all but disappeared. However, I will be looking out for them, and their parasites, again next year.
Post by: Wilma Blom
Wilma is the Curator Marine Invertebrates at Auckland Museum. She also looks after palaeontology and geology.