Читать книгу Parasitology - Alan Gunn - Страница 24

The term parasitoid is restricted to certain parasitic insects whose hosts are almost exclusively other insects – although a few species attack certain crustacea, spiders, millipedes, centipedes, and earthworms. Some parasites cause mortality and may even depend on the death of their host to effect transmission to the next stage of their life cycle, but host death is not inevitable. By contrast, parasitoids slowly consume their host’s tissues so that the host remains alive until the parasitoid has completed its development. At this point, the host dies either through the loss of vital tissues or through the parasitoid physically eating its way out of its host. Parasitoids are all parasitic during their larval stage, and the adult insect is free living and feeds on nectar, pollen, dead organic matter, or is predatory, depending upon the species. Parasitoids can develop as endoparasites within their host or as ectoparasites attached to the outside but with their mouthparts buried deep within the host’s body. The larva has only the one host in or on which it develops and those that are endoparasites tend to exhibit the most host specificity. This lifestyle is therefore distinct from those insects such as warble flies (e.g., Hypoderma bovis) and bot flies (e.g., Gasterophilus intestinalis), which exhibit a more ‘traditional’ parasitic way of life that does not inevitably result in the death of the host. Many species of the order Hymenoptera (bees, ants, wasps) are parasitoids, and it is also a common lifestyle among the Diptera (true flies), but it is absent or very rare among the other orders. By contrast, most of the insect orders are hosts to parasitoids. Hyperparasitism is also common in which a parasitoid parasitizes another species of parasitoid. Parasitoids are effective for the control of agricultural pests, particularly within closed environments such as greenhouses. However, they have had limited success as control agents for parasites, their vectors, or intermediate hosts.

The parasitoid lifecycle typically begins with the adult female locating its host and either injecting one or more eggs or attaching them to the host’s outer surface. Sometimes she also injects a toxin that temporarily or permanently disables her victim. The host is chosen based on its stage of development, which may be anywhere from the egg to the adult stage.