Parasitoid is an insect and especially a wasp that completes its larval development within the body of another insect which it eventually kills and that is free-living as an adult an insect and especially a wasp that completes its larval development within the body of another insect which it eventually kills and that is free-living as an adult. 1
A parasitoid is an organism that lives on or in a host organism and ultimately kills the host. 2
Parasitoids are an important biological tool used widely in agriculture for the suppression of various pest species. 3
Karma is a real pest for parasitoids, tiny parasitic wasps that lay their eggs on caterpillars. 4
Parasitoid was firstly studied by a naturalist named Maria Sibylla Merian in 1647-1717. 5 Swedo-Finnish writer Odo Morannal Reuter gave the term of “parasitoid” in 1913. 6 A reviewer of Swedo, William Morton Wheeler adopted this term in English. 7 Reuter used it to describe the process where the parasite develops in or on the body of a single host individual, sooner or later killing that host, while the adult is free-living. Since that time, the concept has been generalized and widely applied. 8
Parasitoids can be classified as either endo- or ectoparasitoids with idiobiont or koinobiont developmental strategies. Endoparasitoids live within their host’s body, while ectoparasitoids feed on the host from outside. Idiobiont parasitoids prevent further development of the host after initially immobilizing it, whereas koinobiont parasitoids allow the host to continue its development while feeding upon it. Most ectoparasitoids are idiobiont, as the host could damage or dislodge the external parasitoid if allowed to move and moult. Most endoparasitoids are koinobionts, giving them the advantage of a host that continues to grow larger and avoid predators. 9
The simplest relationship is in case of primary parasitoids which involves two organisms, the host and the parasitoid. Hyperparasitoids are parasitoids of parasitoids; secondary parasitoids have a primary parasitoid as their host, so there are three organisms involved. Hyperparasitoids are either facultative (can be a primary parasitoid or a hyperparasitoid depending on the situation) or obligate (always develop as a hyperparasitoid). Levels of parasitoids beyond secondary also occur, especially among facultative parasitoids. In oak gall systems, there can be up to five levels of parasitism. 10 Cases in which two or more species of parasitoids simultaneously attack the same host without parasitizing each other are called multi- or multiple parasitism. In many cases, multiple parasitism still leads to the death of one or more of the parasitoids involved. If multiple parasitoids of the same species coexist in a single host, it is called superparasitism. Gregarious species lay multiple eggs or polyembryonic eggs which lead to multiple larvae in a single host. The end result of gregarious superparasitism can be a single surviving parasitoid individual or multiple surviving individuals, depending on the species. If superparasitism occurs accidentally in normally solitary species the larvae often fight among themselves until only one is left. 11, 12
Parasitoids have evolved effective and efficient methods of successful parasitism, many of which involve utilization of multimodal cues 13. Many dipteran parasitoids in the family Phoridae use social insects as hosts due to the reliability of their intraspecific chemical communication signals that make for effective host selection cues 14–15. Phorid fly adults parasitize ants by hovering over insect hosts and then diving down to insert an egg beneath the insect’s exoskeleton 16,17-19. Phorid flies have direct parasitic effects on ants (i.e., cause ant mortality) and also significantly change ant foraging behavior by limiting host resource acquisition behavior, modifying ant competitive hierarchies, and dampening ant effects on herbivores 20-25. There are phorids that attack ants from at least 22 genera across 5 subfamilies. Likewise, more than 20 genera of phorids attack ant hosts 26. With such taxonomic diversification in ant-phorid relationships, the types of cues used by phorids to locate, select, and successfully parasitize ant hosts are also quite diverse.
Successful parasitism requires a series of interactions between a parasitoid and its host. The process can be categorized into five general and sometimes overlapping steps:
(a) host habitat location, (b) host location, (c) host acceptance, (d) host discrimination, and (e) host regulation 13.
For phorid parasitoids, host location involves the use of both habitat and host cues. Host habitat location is the use of environmental cues by the parasitoid to select areas to search for potential hosts. These cues may be directly related to the preferred environment of the host itself (e.g., volatiles from plants commonly used by hosts) or related to the parasitoid’s general habitat preferences (light, temperature, and humidity conditions within a given area) 13. The host location process also requires that a parasitoid use long-range cues to be directed to its’ host. However, unlike host habitat location cues, these cues come directly from the host itself. Ants communicate interspecifically by using complex pheromones. These pheromones often act as host location cues for parasitoids as they can be both reliable (with volatile pheromones highly conserved within a species or genus) and detectable (ants, being eusocial, live in relatively high densities, and can produce large volumes of volatile pheromones) for the parasitoid 27. Once a phorid parasitoid has located a potential host through long-range cues, the parasitoid requires host acceptance cues to trigger the parasitoid’s oviposition behavior. Short-range cues such as movement, host size, and contact chemical cues have all been implicated in triggering phorid fly oviposition 28, 29-39.
In addition to the cues that are required for overall host selection, host discrimination cues, used by parasitoids to detect and reject potential hosts that have been previously parasitized, can be present. While these cues are not necessary for parasitism, they can increase the likelihood of offspring success 13. Parasitoids can also increase the success rate of their offspring through host regulation, whereby parasitoids manipulate their hosts to promote the development of the next generation of parasitoids. Host regulation can involve altering the physiology of the host to facilitate growth and development of egg, larvae, or pupae of the parasitoid or altering host behavior to optimize nutrient intake or location within the external environment 40.