The caterpillar, once very hungry, now looks under the weather, no longer a plump, cabbage-consuming obsessive. Its skin crawls; it involuntarily twitches. Beads of yellow erupt over its velveteen flanks. They swell before your eyes, then follows a moment of awful realisation: these are grubs, pushing out through rupturing skin. The caterpillar has become host to the parasitic wasp Cotesia glomerata.
Allotmenteers all over the UK may notice holes appearing in the leaves of their prize cabbages, by which time the culprits, caterpillars of the small and large white butterflies, are fairly well developed. Wouldn’t it be nice to have an early-warning system that told us these cryptic species were starting to deconstruct your crop? Maybe if cabbages could light up or scream?
Well, in a way, they do. When brassicas are attacked by a herbivorous insect, they let out a call for help, an odour cloud called a kairomone. If you are a female Cotesia, this stuff gets your antennae twitching. The discreet insect homes in on gauzy wings and stabs her rapier-like ovipositor into the caterpillar. She deposits 15–50 eggs inside its body. This is a beautifully sinister symbiotic relationship between cabbage and wasp.
But the real magical malevolence is microscopic and molecular. To stop the caterpillar host’s immune system fighting back, each egg is coated with a Bracovirus that has become integrated into the very cells of the wasp.
The infection hijacks and chemically disables the caterpillar’s defences, altering its immune system and controlling its growth and behaviour to suit its parasitic crew. Over the next two weeks the larvae slowly consume their host from the inside-out, leaving only the essential organs, to keep the surrogate womb ticking over.
When they are fully grown, they paralyse the host and start to rasp at the inside of the caterpillar’s skin with tooth-like projections around their mouths, before bursting out in a grizzly mass extrusion. But that still isn’t the end of the caterpillar – the larvae have one task left for it.
The virus corrupts the caterpillar’s behaviour, so rather than limping off to die, it spins a silken pad over the top of the fuzzy mass of Cotesia cocoons. Here it stays put. The zombie security guard protects its killers for another 10 days.
However, the saga can be much more complicated. It’s a ‘bug-eat-bug’ world and the developing wasps have their own predators. In some summers over half the Cotesia broods don’t get this far, themselves falling victim to an equally dastardly cousin: the furtive, hyperparasitic wasp Lysibia nana.
One of the side-effects of the Cotesia infection is the alteration of the salivary concoctions of the caterpillar. The cabbage reacts to this in a different way, changing the ‘smell’ of the gaseous cocktail it releases that attracted the wasp in the first place.
This altered recipe is picked up by Lysibia, which sets out to take over the already complicated food-chain. It is attracted to the parasitised caterpillars and lays its own eggs in the developing Cotesia lava inside. And it doesn’t necessarily end here – there are other species that will predate Lysibia, and even others upon them.
Main image: Cotesia glomerata© Lennart Tange on Flickr, CC BY 3.0, via Wikimedia Commons
