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The term parasitoid wasp refers to a large evolutionary grade of hymenopteran superfamilies, mainly in the Apocrita. The parasitic or parasitoidal Apocrita are divided into some dozens of families. They are parasitoids of various animals, mainly other arthropods. Many of them are considered beneficial to humans because they control populations of agricultural pests. Others are unwelcome because they are hyperparasitoids, attacking beneficial parasitoids.
Parasitoidal wasps range from some of the smallest species of insects, to wasps about an inch long. Some are parasitoids that complete their metamorphosis in a single small egg of a small insect, and such a wasp is necessarily less than 1 mm long. Most females have a ‘spine-like’ ovipositor at the tip of the abdomen (Drees and Jackman). The egg and larval stage are usually not observed unless dissected from the host in which the adult female parasitized, except in species that practically fill the skin of the host with parasitoid larvae.
Distinguishing features: Ichneumonidae and Braconidae 
Most are brownish or black, not brightly colored (Borror and White). These species are widely distributed. They also have a wide range of hosts that they parasitize. Some species of Braconids pupate in silken cocoons on the outside of their dead host (Miller and Ball). Wing venation: Only one recurrent vein, or no recurrent veins.
Ichneumonids vary greatly in size and their color varies from brightly colored yellow to uniform black. The family is cosmopolitan. Most are very specific in the species of the host they parasitize. Wing venation: Two recurrent veins.
Caterpillars provide major examples of larval Lepidoptera as a class of host, but various species parasitoid wasps in various Hymenopteran families parasitize their own favoured life stages [egg, larva, adult] of particular species in many other orders of insects including Coleoptera, Diptera, Hemiptera and other Hymenoptera. Some attack other Arthropoda, such as spiders. Adult female wasps parasitize insect species, most species ovipositing into their hosts' bodies or eggs. The females of some parasitoid species also insert secretory products (combinations that may include polydnaviruses, ovarian proteins, and venom) that protect the egg from the immune system of the host. Once a host of a parasitoid that expresses polydnavirus particles has been parasitised, the virus that accompanied the egg during oviposition, infects the cells of the host in ways that benefit the parasitoid.
Life Cycle 
Inside the host the egg hatches into a larva or larvae. The larva feed inside the host until ready to pupate, which by then is generally either dead or moribund. Depending on its species, the parasitoid then may eat its way out of the host or remain in the more or less empty skin. In either case it then generally spins a cocoon and pupates. As adults parasitoid wasps feed primarily on nectar from flowers. If they happen to be species that rely on polydnavirus then all adults include the DNA for their associated species of the virus in their genomes.
Polydnaviruses are a unique group of insect viruses that have a mutualistic relationship with some parasitic wasps. Wasps benefit from this mutualism because the virus provides certain protection of the parasitic larva inside the host by weakening the host’s immune system and by altering the cells of the host to be more beneficial to the parasite. The polydnavirus, like all viruses, need a host to replicate and in this case is the oviducts of the adult female. The relationship between these viruses and the wasp is obligatory in the sense that all individuals are ‘infected’ with the viruses; the virus has been incorporated in the wasp’s genome (Miller and Ball). These relationships between virus and parasitoid have been and are currently studied as model systems to study parasitoid-host immune interactions (Beckage).
Some species have direct aggressive mechanisms for parasitizing a host by actively targeting the host immune system and then suppressing it; polydnaviruses are responsible for this approach (Miller and Ball). Others have more passive approaches to parasitism by ovipositing into a part of the host that has little exposure to the host’s immune system or by avoiding activation of the host’s immune system (Miller and Ball); ovarian proteins are more involved in this approach and PDV less so.
There are two especially recognized genera of polydnaviruses: Ichnoviruses (IV) and Bracoviruses (BV). The ichnoviruses occur in ichneumonid wasp species and bracoviruses in braconid wasps. The genome of the virus is composed of multiple segments of double-stranded, super-helical DNA packaged in capsid proteins and a double layer (IV) or single layer (BV) envelope. The large genome of polydnaviruses is what distinguishes PDV from other viruses (Feming and Summers). While both have segmented DNA genomes, little or no sequence homology exists between BV and IV, suggesting that the two genera evolved independently. Specifically, BV and IV radically differ in morphology, methods of nucleocapsid release from cells, and possible packaging of multi-genomic DNAs; it is also unknown whether the two follow the same strategy of replication (Wanjiru).
Pest Control 
Typically, parasitoid wasps are not considered agricultural pests and are considered a beneficial insect as they control the population of host insects. They are also increasingly being released directly into regions specifically for the use of agricultural pest control.
“A number of parasitic wasp species are commercially available from insectaries and are purchased and released in augmentative biological control programs. Other species have been imported from other countries from which pests have been accidentally introduced without their natural enemies and released to reintroduce the natural enemy with its host, a practice called importation, or "classical" biological control and which occasionally results in sustained suppression.” (Drees and Jackman)
Host defenses 
The hosts of parasitoids have several levels of defense against them. Many hosts try to hide from the parasitoids in inaccessible habitats. They may also get rid off their frass (body wastes) and avoid plants that they have chewed on as both can attract parasitoids. The egg shells and cuticles of the prey are thickened to prevent the parasitoid from penetrating them. When they encounter the egg laying female, prey use defenses like dropping off the plant they are on, twisting and thrashing so as to dislodge or kill the female and even regurgitating onto the wasp to entangle it. The wriggling can sometimes help by causing the wasp to "miss" laying the egg on the host and instead place it nearby. Wriggling of pupae can cause the wasp to lose its grip on the smooth hard pupa or get trapped in the silk strands. Some caterpillars even bite the female wasps that approach it. Some insects secrete poisonous compounds that kill or drive away the parasitoid. Ants that are in a symbiotic relationship with caterpillars, aphids or scale insects may protect them from attack by wasps.
Even parasitoid wasps are vulnerable to hyperparasioid wasps. Some parasitoid wasps change the behaviour of the infected host to build a silk web around the pupa of the wasps after they emerge from its body to protect them from hyperparasitoids.
In endoparasitoids, host immune cells can encapsulate the eggs and larvae of parasitoid wasps. In aphids, the presence of a secondary endosymbiont, Buchnera aphidicola that carries a particular latent phage makes the aphid relatively immune to their parasitoid wasps by killing many of the eggs. However, wasps counter this by laying more eggs in aphids that have the endosymbiont so that at least one of them can hatch and parasitize the aphid.
Certain caterpillars eat plants that are toxic to both themselves and the parasite to cure themselves. Drosophila melanogaster larvae also self-medicate with ethanol to treat parasitism. D. melanogaster females lay their eggs in food containing toxic amounts of alcohol if they detect parasitoid wasps nearby. Despite the alcohol retarding the growth of the flies, it protects them from the wasps.
See also 
- Richards, O. W.; Davies, R.G. (1977). Imms' General Textbook of Entomology: Volume 1: Structure, Physiology and Development Volume 2: Classification and Biology. Berlin: Springer. ISBN 0-412-61390-5.
- Lois K. Miller; Laurence Andrew Ball (1998). The insect viruses. Springer. ISBN 978-0-306-45881-1. Retrieved 10 May 2013.
- Strand, M. R.; Pech, L. L. (1995). "Immunological Basis for Compatibility in Parasitoid-Host Relationships". Annual Review of Entomology 40: 31–56. doi:10.1146/annurev.en.40.010195.000335. PMID 7810989.
- Gross, P. (1993). "Insect Behavioral and Morphological Defenses Against Parasitoids". Annual Review of Entomology 38: 251–273. doi:10.1146/annurev.en.38.010193.001343.
- Tanaka, S.; Ohsaki, N. (2006). "Behavioral manipulation of host caterpillars by the primary parasitoid wasp Cotesia glomerata (L.) to construct defensive webs against hyperparasitism". Ecological Research 21 (4): 570. doi:10.1007/s11284-006-0153-2.
- Oliver, K. M.; Russell, J. A.; Moran, N. A.; Hunter, M. S. (2003). "Facultative bacterial symbionts in aphids confer resistance to parasitic wasps". Proceedings of the National Academy of Sciences 100 (4): 1803. doi:10.1073/pnas.0335320100.
- Oliver, K. M.; Noge, K.; Huang, E. M.; Campos, J. M.; Becerra, J. X.; Hunter, M. S. (2012). "Parasitic wasp responses to symbiont-based defense in aphids". BMC Biology 10: 11. doi:10.1186/1741-7007-10-11. PMC 3312838. PMID 22364271.
- Singer, M. S.; Mace, K. C.; Bernays, E. A. (2009). "Self-Medication as Adaptive Plasticity: Increased Ingestion of Plant Toxins by Parasitized Caterpillars". In May, Robin Charles. PLoS ONE 4 (3): e4796. doi:10.1371/journal.pone.0004796. PMC 2652102. PMID 19274098.
- Milan, N. F.; Kacsoh, B. Z.; Schlenke, T. A. (2012). "Alcohol Consumption as Self-Medication against Blood-Borne Parasites in the Fruit Fly". Current Biology 22 (6): 488–493. doi:10.1016/j.cub.2012.01.045. PMC 3311762. PMID 22342747.
- Kacsoh, B. Z.; Lynch, Z. R.; Mortimer, N. T.; Schlenke, T. A. (2013). "Fruit Flies Medicate Offspring After Seeing Parasites". Science 339 (6122): 947. doi:10.1126/science.1229625.
Beckage, N.E. (2008). Insect Immunology. Elsevier’s Science and Technology Rights Department.
Borror, D. J. & White, R. E. (1970). Insects. Peterson Field Guide. Houghton Mifflin Company.
Drees, B.M. & Jackman, J. (1999). Parasitic Wasp. Field Guide to Texas Insects. Gulf Publishing Company.
Dupas, S., Wanjiru, G., Branca, A., Pierre Le Ru, B. & Silvain, J. (2008). Evolution of a Polydnavirus Gene in Relation to Parasitoid–Host Species Immune Resistance. Oxford Journals 99, 491-499.
Jo-Ann G., Feming, W. & Summers, M.D. (1991). Polydnavirus DNA is integrated in the DNA of its parasitoid wasp host. Proceedings of the national academy of sciences 88, 9770-9774.
Miller, L.K & Ball, L.A. (1998). The Insect Viruses. The Viruses.
Marziano, N.K., Hasegawa, D.K., Phelan, P & Turnbull, M.W. (2011) Functional Interactions between Polydnavirus and Host Cellular Innexins. Journal of Virology 85, 10222-10229