13. Insect predation and parasitism
We saw in Chapter 11 that many insects are phytophagous, feeding directly on primary producers, the algae and higher plants. These phytophages comprise a substantial food resource, which is fed upon by a range of other organisms. Individuals within this broad carnivorous group may be categorized as follows. A predator kills and consumes a number of prey animals during its life. Predation involves the interactions in space and time between predator foraging and prey availability, although often it is treated in a one-sided manner as if predation is what the predator does. Animals that live at the expense of another animal (a host) that eventually dies as a result are called parasitoids; they may live externally (ectoparasitoids) or internally (endoparasitoids). Those that live at the expense of another animal (also a host) that they do not kill are parasites, which likewise can be internal (endoparasites) or external (ectoparasites). A host attacked by a parasitoid or parasite is parasitized, and parasitization is the condition of being parasitized. Parasitism describes the relationship between parasitoid or parasite and the host. Predators, parasitoids, and parasites, although defined above as if distinct, may not be so clear-cut, as parasitoids may be viewed as specialized predators.
By some estimates, about 25% of insect species are predatory or parasitic in feeding habit in some life-history stage. Representatives from amongst nearly every order of insects are predatory, with adults and immature stages of the Odonata, Mantodea, Manto- phasmatodea and the neuropteroid orders (Neuroptera, Megaloptera, and Raphidioptera), and adults of the Mecoptera being almost exclusively predatory. These orders are considered in Boxes 10.2, and 13.2–13.5, and the vignette for this chapter depicts a female mecopteran, Panorpa communis (Panorpidae), feeding on a dead pupa of a small tortoiseshell butterfly, Aglais urticae. The Hymenoptera (Box 12.2) are speciose, with a preponderance of parasitoid taxa using almost exclusively invertebrate hosts. The uncommon Strepsiptera are unusual in being endoparasites in other insects (Box 13.6). Other parasites that are of medical or veterinary importance, such as lice, adult fleas, and many Diptera, are considered in Chapter 15.
Insects are amenable to field and laboratory studies of predator—prey interactions as they are unresponsive to human attention, easy to manipulate, may have several generations a year, and show a range of predatory and defensive strategies and life histories. Furthermore, studies of predator—prey and parasitoid—host interactions are fundamental to understanding and effecting biological control strategies for pest insects. Attempts to model predator—prey interactions mathematically often emphasize parasitoids, as some simplifications can be made. These include the ability to simplify search strategies, as only the adult female parasitoid seeks hosts, and the number of offspring per unit host remains relatively constant from generation to generation.
In this chapter we show how predators, parasitoids, and parasites forage, i.e. locate and select their prey or hosts. We look at morphological modifications of predators for handling prey, and how some of the prey defenses covered in Chapter 14 are overcome. The means by which parasitoids overcome host defenses and develop within their hosts is examined, and different strategies of host use by parasitoids are explained. The host use and specificity of ectoparasites is discussed from a phylogenetic perspective. Finally, we conclude with a consideration of the relationships between predator/parasitoid/parasite and prey/host abundances and evolutionary histories. In the taxonomic boxes at the end of the chapter, the Mantodea, Mantophasmatodea, neuropteroid orders, Mecoptera, and Strepsiptera are described.