Host Selection by Insect Parasitoids

Insects that are parasitic only during their immature stages are termed protelean parasites (11). The protelean parasites that attack invertebrates nearly always de­ stroy their hosts. These parasites are often described as parasitoids, a term coined by Reuter (167) to differentiate them from the typical parasites. Parasitoids include a vast number of species of the so-called parasitic Hymenoptera, the Strepsiptera, and a few of the Diptera, primarily in the family Tachinidae. Although there are a few exceptions, insect parasitoids appear as typical parasites during their early developmental stages and later destroy the host to live as free adults. Placing emphasis on the latter aspects of their feeding behavior, Flanders (62) has recently referred to such insects as carniveroids. The evolutionary strategy of the parasitoid­ host relationship is different from that of either the predator or the parasite-host relationship in that the host's future development is of importance only to the parasitoid (226). The adult female parasitoid upon emergence is often in an alien habitat and removed from a host population. She must locate a suitable host in order to propa­ gate. Salt (179) concluded that the parasitoid first seeks a suitable environment. Laing (109) divided the host selection process into environmental and host factors and believed that the parasitoid is guided to a host habitat by chemical and physical parameters. Once a female has located a host habitat, she then searches systemat­ ically. Combining the information of Salt (179) and Flanders (60), Doutt (41) divided the process that results in successful parasitism into four steps: (a) host habitat location, (b) host location, (c) host acceptance, and (d) host suitability. More recently, a fifth step, host regulation, has been added (226) in order to adequately describe the factors necessary for successful parasitism. The first three of these steps can be combined as aspects of the host selection process. The host selection process may consist of only two or three steps in one relationship or of many steps in others. Because of this, there is often some overlap in describing and