The term parasitoid wasp refers to a large evolutionary grade of hymenopteran superfamilies, mainly in the Apocrita. The parasitic or parasitoid 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. A few are unwelcome because they attack other benefical insects.
Parasitoid wasps range from some of the smallest species of insects, to wasps about an inch long. Most females have a 'spine-like' ovipositor at the tip of the abdomen, sometimes lacking venom glands and almost never modified into a stinger. The egg and larval stage are usually not observed unless dissected from the host, except in species that practically fill the skin of the host with parasitoid larvae.
Many parasitoid wasps use larval Lepidoptera as hosts, but various groups of parasitoid wasps different host life stages (egg, larva, nymph, pupa, or adult) of species in nearly all other orders of insects, especially Coleoptera, Diptera, Hemiptera and other Hymenoptera. Some attack non-insect Arthropoda, such as spiders. Adult female wasps of most species oviposit into their hosts' bodies or eggs. The females of some species also inject secretory products that paralyze the host or protect the egg from the immune system of the host. These are produced in combinations that may include polydnaviruses, ovarian proteins, and venom. If a polydna virus is included, it will infect the cells of the host and cause symptoms that benefit the egg or larva.
Host quality can have a major impact on the development of the parasitoid. The nutrition a developing parasitoid can access is limited to what is contained in the host. Small hosts often produce smaller parasitoids. Some wasp females preferentially lay female eggs in larger hosts and male eggs in smaller hosts, as the reproductive capabilities of males will be limited less by smaller adult body size.
On or inside the host the parasitoid egg hatches into a larva or larvae. Endoparasitoid eggs can absorb fluids from the host body and grow several times in size from when they were first laid before hatching. The first instar larvae is often highly mobile and may have strong mandibles or other structures to compete with other parasitiod larvae. The following instars are generally more grub-like. Parasitoid larvae have incomplete digestive systems with no rear opening. This prevents the hosts from being contaminated by their wastes. The larva feeds on the host's tissues until ready to pupate; by then the host is generally either dead or moribund. A meconium, or the accumulated wastes from the larva is cast out as the larva transitions to a prepupa. 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.
Based on genetic and fossil analysis, parasitoidism has evolved only once in the Hymenoptera. All parasitoid wasps are descended from this lineage, most of which occur in the superfamily Apocrita. The superfamily Aculeata, which includes bees, ants, and parasitoid spider wasps, also descended from this lineage. The common ancestor in which parasitoidism evolved was probably an ectoparasitoid woodwasp that fed on wood-boring beetle larvae and lived approximately 247 million years ago. Species similar in lifestyle and morphology to this ancestor still exist in the family Ichneumonidae. A significant radiation of species in the Hymenoptera occurred shortly after the evolution of parasitoidy in the order. Several theories suggest that the adaptive radiation of the Hymenoptera was a result of the evolution of parasitoidy.
Polydnaviruses are a unique group of insect viruses that have a mutualistic relationship with some parasitic wasps. The polydnavirus, like all viruses, needs a host to replicate and in this case its favoured place is the oviducts of an adult female parasitoid wasp. The wasps benefit from this relationship because the virus provides certain protection for the parasitic larvae inside the host, both by weakening the host's immune system and by altering the cells of the host to be more beneficial to the parasite. 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. These relationships between virus and parasitoid have been and are currently studied as model systems to study parasitoid-host immune interactions.
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. 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; 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. 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.
The hosts of parasitoids have developed several levels of defense. Many hosts try to hide from the parasitoids in inaccessible habitats. They may also get rid of their frass (body wastes) and avoid plants that they have chewed on as both can attract parasitoids. The egg shells and cuticles of the potential hosts are thickened to prevent the parasitoid from penetrating them. Hosts may use behavioral evasion when they encounter an egg laying female parasitoid, 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 hyperparasitoid wasps. Some parasitoid wasps change the behaviour of the infected host, causing them to build a silk web around the pupae of the wasps after they emerge from its body to protect them from hyperparasitoids.
Endoparasitoids must deal with host immune cells which can encapsulate the eggs and larvae of parasitoid wasps. In aphids, the presence of a secondary bacterium 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 may 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.
Parasitoid wasps are considered beneficial as they control the population of many pest insects. They are increasingly being released directly into regions specifically for the use of biological 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."
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