Parasitoid wasps inject their eggs into other arthropods, usually caterpillars, using a stinger-like probe. The eggs hatch and eat the caterpillar from the inside, mostly feasting on bodily fluids and avoiding the vital organs, to keep it alive for as long as possible. Some species then burst out of the caterpillar, killing it, and continue on with their lifecycle.
But other species go further. As some species of Glyptapanteles sp larvae exit a caterpillar they ‘plug’ the holes behind them, using an exoskeleton they have moulted, keeping the caterpillar alive. Then, the caterpillar helps them pupate. It stops feeding and stands guard over the clutch of wasp larvae, defending them with violent head swings to knock off any predators6. Not behaviour that any peace-loving caterpillar usually exhibits. Eventually, the caterpillar starves to death.
It is not clear how the wasp larvae make the caterpillar do this.
A Thyrinteina leucocerae caterpillar with parasitoid wasp pupae. Image credit: José Lino-Neto, CC BY 2.5, via Wikimedia Commons
Around 25 wasp species in Britain specialise in parasitising spiders. The adults paralyse the spider temporarily and lay an egg where the spider can’t reach. The spider recovers, continues to catch insects, and the wasp larva slowly grows by feeding on spider’s haemolymph (like blood).
Other wasps are ‘hyperparasitoids’ – feeding on other parasitoid wasps. Some species can stick their egg-laying probe in a caterpillar and ‘taste’ whether there is another parasitoid wasp larva inside. They then lay their own egg directly into that wasp larva.
One species of wasp, Amblyteles armatorius, is susceptible to infection by parasitic fungus from the Ophiocordyceps genus. Ophiocordyceps unilateralis, a.k.a the zombie-ant fungus, also manipulates behaviour. It makes its insect host climb up a blade of grass or to the underside of a leaf, grip on, and never let go. A perfect place for the fungal spores to grow out of the insect and then be nicely distributed by the wind.
Several parasitoid wasp species have now had their DNA sequenced for the first time by Sanger Institute scientists, as part of the Darwin Tree of Life project7. It may be that the genomic information can be used to find out how the wasps manipulate their hosts – one of nature’s more macabre mysteries.
Sequencing the genomes may also lead to discoveries of new chemical compounds useful to humans. Gavin Broad, from the Natural History Museum, who is also working on the Darwin Tree of Life project said, “Currently, some of the most interesting drugs have come out of things like cone shells, spiders, snakes… But venomous wasps must be a huge untapped resource,”
“They’re doing really complicated things to their hosts, such as arresting development temporarily or permanently, and introducing antibiotics or antifungal agents.”