Leaping out of the leaves the beast impales its victim, injecting it with paralysing venom along with several eggs. One of these eggs, nurtured and protected by the host eventually hatches and sets about destroying its brother and sister eggs. The hatchling then starts devouring the flesh of the host from the inside. It takes care to eat only the fat stores and non vital parts, such as the sexual organs, of the host. Eventually the creature bursts out of the back of its host, but not before paralysing its victim by chewing through the nerves controlling its legs. However, the host’s misery is not yet over.
Having left the essential organs, the host becomes a zombie dancing grotesquely above a cocoon, spun between the paralyzed legs. The host’s struggles and defences scare away those wishing to destroy the developing monster. When the adult finally hatches from its cocoon, the victim is left to starve to death.
Although sounding like the plot of a sci-fi horror film, this happens daily here on planet earth. Thankfully (for us) the monster described above is a parasitoid wasp, and its victim a ladybird. There are a large number of parasitoid wasps around the world, and these insects all implant the victims with anything from an individual to thousands of eggs.
Theses wasps are usually referred to as parasitic. This is incorrect, as although parasitic creatures live on a host, they keep the host alive, with either few noticeable effects, or in a weakened state. Parasitoids, on the other hand, always kill the host once they have used them to their advantage. There are some true parasitic wasps, include the gall wasps, which are often parasites of plants.
The parasitoid wasps can be split into two groups. The idiobionts and the koinobionts. The idiobionts are, relatively speaking, the kinder of the two in that the venom paralyses the host completely, requiring the wasp eggs to hatch quickly and devour the tissues of the host before or soon after it dies.
Koinobionts, such as the one described in the introduction, only paralyse their victim for a short time, in order to inject the host with eggs. After this initial attack, the host will generally recover, and continue to develop and feed, whilst the wasps young grow inside. The host is only fatally injured when the wasp larvae is ready to leave, in some cases death is instantaneous, while in others the host is further used as defence as it slowly dies.
The koinobionts are typically internal parasitoids, however the Polysphinctine wasps are unusual in that they lay their single eggs on the outside of a spider, in a position on the abdomen which the spider cannot reach to scrape the egg off. The larvae hatches out and stays fixed to the same position, remaining very small and vampirically drinking small amounts of the spiders blood, whilst the spider continues to grow. When the spider reaches a certain size, the larvae suck the spider dry and grow 5 – 6 times their original size overnight. Just before drinking them dry, several species also manipulate the spider into building a webbed shelter which the wasp larvae is able to use as protection whilst it develops into an adult.
The largest species of these wasps in the UK is the Sabre Wasp (Rhyssa persuasoria). This 8cm long wasp has a fearsome looking ‘sting’ but this is actually an ovipositor and is harmless to humans. The ovipositor is used to inject grubs burrowing deep inside wood.
Despite some of the instances mentioned above, many of these wasps prey on insects which are pests in human civilisation. Organisms preyed upon by wasps include cockroaches, aphids and caterpillars. The wasps act as an important biological control in agriculture, especially on organic farms, or in place of many banned pesticides.
Lee Ismail, Curator of Natural Sciences