Parasitoids

What is a parasitoid?

Parasitoids spend a part of their life obtaining nourishment from a ‘host’ organism, which they ultimately kill during their developmental process, usually preventing the host from reproducing. Parasitoids should not be confused with parasites – click here to learn how to tell the difference. All known parasitoids are invertebrates, almost all are insects, and most are wasps. However, parasitoids do include other invertebrates, such as species of flies, beetles, moths, mites and worms.

Parasitoids are not responsible for any human diseases, but are important for controlling agricultural pests, such as aphids and are the inspiration behind 20^th Century Fox’s Alien movies.

An adult parasitoid wasp (Rhyssa persuasoria) © Roger Key

Mysterious wasps

Parasitoid wasps make up the largest group of insects in the UK – including Scotland – and new species are being discovered all thetime.

Parasitoid wasps are poorly recorded, with only a very small number of people studying them, and so we don’t know which species are rare or endangered, nor can we say where different species are found with any certainty. They belong to the order Hymenoptera, which also includessocial wasps, ants, bees and other insects. The largest group of parasitoid wasps are known as ichneumon wasps, although there are other groups as well.

Parasitoid wasps are often ‘host specific’, meaning that a species of parasitoid wasp relies on only one or a small number of closely related host species, with which they have evolved closely. Host specific parasitoids are tied to the fate of their host organisms – if their host species becomes extinct, so will they.

Although parasitoid wasps may have formidible looking egg laying tube (ovipositors), these are not used to deliver a sting as in some other species of wasp, so there’s no need to fear them – if you’re a human...

Parasitoid wasp larvae emerging from a caterpillar host © Ben Hamers

If you happen to be the host invertebrate, that’s a different matter! These wasps employ different methods to ensure their eggs end up on or inside the host species – for example, some cut small pockets in leaves where they lay their eggs, which are later eaten by caterpillars feeding on the plant! In fact, some plants have developed very close and mutually beneficial relationships with parasitoid wasps. In these cases a fragrance is emitted to attract parasitoids when a caterpillar’s saliva mixes with the plant’s juices. The parasitoid wasp then knows where to find a host for its young, which is beneficial for the plant as it controls the numbers of caterpillars.

The parasitoid wasp’s young then develops within the host – feeding on bodily fluids and non-vital organs, before finally killing the host and pupating. Where the host is a caterpillar, it never reaches maturity itself – dying before it can pupate and become a butterfly or moth.

Macabre Russian dolls

Some parasitoids (mainly parasitoid wasps) use other parasitoid wasp larvae as their hosts, and are known as ‘hyperparasitoids’ or ‘secondary parasitoids’. Some secondary parasitoids lay their eggs within their host parasitoid while they are pupating, while others insert them directly into live primary parasitoid larvae within a live host – much like a macabre Russian doll!Of course, the primary parasitoid larvae is killed by the secondary parasitoid before it reaches maturity.

Hardy parasitoid worms

Gordian worms, or horsehair worms, belong to the Nematomorpha phylum, and follow a similar life strategy as parasitoid insects. Unlike parasitoid insects, though, Nematomorphs can be found in the sea, as well as on land, where the adults live in fresh water.

Just like parasitoid insects, Nematomorphs are free-living as adults, while their larvae develop within the bodies of other invertebrates – often Orthopterans (e.g. grasshoppers and crickets) in terrestrial worms and Decapods (e.g. crabs, lobsters and shrimp) in marine worms.

Gordian worm emerging from ground beetle host © Chris Cathrine

Gordian worms are also phenomenally hardy – quite amazingly, gordian worms can survive their host being eaten by a predator! After their host is devoured, the gordian worm larva will wriggle free of the predator, and go in search of a new host.

Are germs parasitoids?

Parasitoids are not known to be responsible for any human diseases, but instead are very important for controlling agricultural pests.

The term ‘parasitoid’ is generally used only for multicellular organisms. Therefore, although some viruses and bacteria do follow similar life strategies, they are not generally considered to be parasitoids.