Parasitoidism describes a specific type of life-cycle that has evolved in some insects. The majority of these insects are wasps, but not like the usual pesky black-and-yellow kind that you find in the garden during summer. Oh no. These little fellows are much meaner than that. So mean in fact that the makers of the film Alien, when trying to think up a really nasty monster, chose parasitoids as their inspiration.

Description

Parasitoids are an incredibly diverse and successful type of insect. Most parasitoids are members of the orders Hymenoptera* and Diptera*. There are 50,000 and 15,000 described species of parasitoid wasps and flies respectively, along with around 3,000 in other orders. Thus, parasitoids make up about 8.5% of all insect species. Unlike the common wasps and flies that everyone is aware of, parasitoids are tiny things, usually just a few millimetres long. They can be visually distinguished by their abdomen. It has a long probe-like structure called an ovipositor, adapted by evolution to lay eggs deep inside the host that it has chosen.

Their Life-Cycle

Parasitoids have a unique and rather unpleasant life-cycle and are defined by the feeding nature of their larvae. An adult female bearing eggs bites and paralyses a host insect, usually a caterpillar or some other nice and juicy invertebrate. She doesn't kill the poor grub; it's better to keep it alive for the sake of freshness. She then proceeds to insert her ovipositor into the host and lays many eggs inside. The eggs then develop inside the hapless, powerless host until they are ready to pupate into larvae. At this point it gets messy.

These wasps are actually worse than H.R. Giger's Alien in many ways. At least his creation didn't slowly eat its way from the inside out whilst the host was still alive, which is precisely what the larvae of parasitoids do. Their host and home which sheltered them during development doubles up as breakfast once they pupate. The host is the larvae's sole source of nutrients until the adult emerges and bursts out of the now dead host. From this point onwards the parasitoids are free-living and no longer rely on a parasitic relationship.

Parasitoid wasps vary in their behaviour. Some lay their larvae on the outside, which makes the host look like it is dotted with tiny white buds along its body. This strategy is not as effective as laying the eggs inside the host; they are less protected. Some parasitoids actually kill their prey before laying their eggs. Again, this is not such a good strategy as paralysis; the host decomposes slowly and deprives the developing larvae of valuable nutrients. As one might expect, these two variants of parasitoidism are less common than the form described in the previous paragraph. They seem to be "less evolved" and certainly less successful.

It could be argued that parasitoids are somewhere in between predators and parasites in terms of their behaviour. A predator is an organism that survives by preying upon others. Whereas a parasite is an organism that grows, feeds and is sheltered by a different creature, whilst contributing nothing to the host. Like predators, parasitoids always kill the hosts that they lay their eggs into, and like parasites they completely depend upon a host for the continuation of their life cycle.

Biological Control

Parasitoids are not just an entomological curiosity however, they can be very useful. They just happen to be natural enemies of numerous insect pests that attack crops like oilseed rape and other plants. Greenfly are a good example of a pest whose numbers are controlled traditionally with the use of pesticides. There is an ongoing effort in science to introduce alien parasitoid species* into these ecosystems to control the abundance of pests. This is highly desirable as one day carefully introduced parasitoids could replace or reduce the use of harmful pesticides that leach into our water supply and contaminate our food chain.

Wolbachia

Wolbachia are bacteria which inhabit the reproductive organs of parasitoids. They are clever little things as they have completely taken advantage of the parasitoids' horrible way of propagating themselves.

Wolbachia are passed unknowingly down the generations because they piggyback inside the eggs that are laid inside the host. Wolbachia prefer girls; they modify the sex ratio of parasitoids to increase the number of females in the population for their own benefit: The more females there are, the more Wolbachia there are being spread about. Wolbachia use different methods to reach this common goal:

• Host fitness - The distortions that Wolbachia induce in their hosts mode of reproduction favours the reproduction of infected hosts at the expense of uninfected females.
  
• Cytoplasmic Incompatibility (CI) - CI in this case can be defined as "the failiure of a mating to produce female offspring". CI only occurs where Wolbachia-infected males mate with uninfected females. This results in the production of non-viable offspring either by hybrid sterility* or hybrid breakdown*. However, the reciprocal cross (Wolbachia-infected female with uninfected male) is compatible, producing Wolbachia-infected progeny. Wolbachia introduce "cytoplasmic factors"* into the reproductive cells that they inhabit that causes the matings to fail. Therefore infected insects are greatly favoured over uninfected.
  
• Parthenogenesis-induction - Wolbachia can induce "female only" asexual reproduction*. Put simply, they force females to clone themselves!
  
• Male-killing - Wolbachia actively kill infected male embryos during development.
  

The Wolbachia-induced reproductive modifications all appear to isolate populations of invertebrates from each other. "Reproductive isolation" happens when gene flow is restricted between populations, and it is known to be the major factor in new species formation.

Conclusion

Parasitoid insects are a diverse group of species that has such a successful reproductive strategy that they span the insect world. Their unique life style which is a mix of parasite during adolescence and predator during adulthood has drawn the attention of not only science, who want to use these insects as a replacement for pesticides. Even Hollywood has taken note and created an on-screen monster to remember using these fascinating creatures. Finally, a class of specially-evolved bacteria called Wolbachia have taken advantage of parasitoidism to spread themselves about, behaving very much like selfish genetic elements with the help of some cunning tricks.