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II.8 Host–Parasitoid Interactions Cheryl J. Briggs OUTLINE 1. Parasitoid terminology and taxonomy 2. Parasitoids and behavioral ecology 3. Parasitoids in theory: The quest for persistence and stability 4. Heterogeneity in risk of parasitism 5. Large-scale spatial dynamics 6. Stage structure in systems with overlapping generations 7. Parasitoids in biological control: Case study of the successful control of California red scale Parasitoid–host interactions have been popular topics of study in the areas of population biology and behavioral ecology because they represent potentially simple, tightly coupled interactions in which the oviposition behavior of the adult female parasitoid searching for hosts translates directly into fecundity and therefore fitness. The Nicholson-Bailey model, which predicts that the interaction between a single host and a single parasitoid species, in its simplest form, will result in the extinction of one or both species, spawned several decades of research into uncovering the mechanisms that allow real host–parasitoid interactions to persist. Work in this area has focused on the potential stabilizing effects of heterogeneity across the host population in the risk of parasitism , large-scale spatial processes, and stage-structured interactions. GLOSSARY classical biological control. The purposeful release of natural enemies of a pest (often from the pest’s area of origin) with the hope that the enemy will both suppress the density of a pest species and also persist to suppress future outbreaks of the pest. oviposition/ovipositor. The act of laying an egg on or in a host/the specialized structure that many adult female parasitoids have that allows them to lay an egg on or in a host. parasitoid. Parasitoids are insects in which the adult female lays one or more eggs on, in, or near the body of another insect (the host), and the resulting parasitoid offspring use the host for food as they develop , killing the host in the process. population regulation. In the history of ecology, this has been a surprisingly difficult term to define; the tendency of a population to persist within bounds. pseudointerference. A form of temporal density dependence in which the parasitoid efficiency decreases at high parasitoid densities because an increasing fraction of parasitoid attacks are wasted on alreadyparasitized hosts. stability. A population equilibrium is stable if the population returns to the equilibrium following a perturbation. 1. PARASITOID TERMINOLOGY AND TAXONOMY Parasitoids are insects the adult female of which lays one or more eggs on, in, or near the body of another insect (the host), and the resulting parasitoid offspring use the host for food as they develop, killing the host in the process. Some authors have used the term parasitoid more generally to describe parasitic species that spend the majority of their life in close association with a single host individual, ultimately resulting in the death of that host; however, the term has been used mainly in reference to insects with this type of life history. Parasitoids are distinguished from parasites in that parasitoids kill their host in the process of completing their life cycle. They differ from predators because they require only a single host to complete their development. The majority of insect parasitoids are Hymenoptera (wasps) or Diptera (flies), but the parasitoid life history is also present in the Coleoptera (beetles) and occasionally in representatives of other orders of insects. Most parasitoids attack the juvenile stages of their host, and the parasitoid literature is filled with specialized terminology to describe their mode of attack. Parasitoids are often characterized by the stage of host that is attacked, e.g., in egg parasitoids, the adult female parasitoid lays her egg in the egg stage of the host, and in larval parasitoids, it is the larval host stage that is initially attacked. Some parasitoids (termed idiobionts ) immediately kill or permanently paralyze their host at the time of attack, whereas others (koinobionts) permit their host to continue to feed, grow, and develop for some time before it is killed, allowing the developing parasitoid offspring to gain more resources from the host beyond those present at the time of oviposition. In solitary parasitoids, a single egg is laid on a host, whereas in gregarious parasitoids, a few to several hundred eggs can be laid on the same host. In some species, the female parasitoid gains all of the protein needed to produce all of her eggs from the host on which she developed (proovigenic parasitoids), whereas in others, the female continues to develop eggs during her adult life (synovigenic parasitoids), and the adult...


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