Parasitoid wasp

Megarhyssa macrurus, a parasitoid, ovipositing into its host through the wood of a tree. The body of a female is c. 2 inches (51 mm) long, with an ovipositor c. 4 inches (100 mm) long.

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Females of the parasitoid wasp Neoneurus vesculus ovipositing in workers of the ant Formica cunicularia.

Parasitoid wasps are a large group of hymenopteran superfamilies, all but the Orussoidea (wood wasps) in the wasp-waisted Apocrita. As parasitoids, they lay their eggs on or in the bodies of other arthropods, often lepidopteran caterpillars, leading to the death of these hosts. Different species parasitise Coleoptera, Diptera, and Hemiptera, and some, the Pompilidae, specialise in spiders. Eggs are laid by different species in host eggs, larvae, pupae, and adults.

Parasitoidy evolved only once in the Hymenoptera, during the Permian, leading to a single clade, but the parasitic lifestyle has secondarily been lost several times including among the ants, bees, and yellowjacket wasps. The Apocrita emerged within that group during the Jurassic.

Host insects have evolved a range of defences against parasitoid wasps, including hiding, wriggling, and camouflage markings.

Many parasitoid wasps are considered beneficial to humans because they naturally control agricultural pests. Some are applied commercially in biological pest control. Historically, parasitoidy in wasps influenced the thinking of Charles Darwin.^[1]

Description[edit]

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.

Evolution and taxonomy[edit]

Evolution[edit]

Based on genetic and fossil analysis, parasitoidy has evolved only once in the Hymenoptera. All parasitoid wasps are descended from this lineage, all except the Orussoidea in the Apocrita. The Aculeata, which includes bees, ants, and parasitoid spider wasps, evolved from within the Apocrita; it contains many families of parasitoids, though not the Ichneumonoidea, Cynipoidea, and Chalcidoidea.^[2][3] The common ancestor in which parasitoidism evolved was probably an ectoparasitoid wood wasp 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 Ichneumonoidea.^[4][5] 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.^[3][5]

Parasitoidy evolved only once in the Hymenoptera, during the Permian, leading to a single clade. The Apocrita emerged within that group during the Jurassic.^[6][7][8][2] The cladogram gives a condensed overview of the positions of some of these groups (boldface), amongst non-parasitic groups like the Vespidae.

Taxonomy[edit]

The parasitoid wasps are paraphyletic since the ants, bees, and non-parasitic wasps such as the Vespidae are not included, and there are many members of mainly parasitoidal families which are not themselves parasitic. Listed are Hymenopteran families where most members have a parasitoid lifestyle.^[9]

Parasitoidy[edit]

Hosts[edit]

Apparently healthy parasitised moth caterpillar

Caterpillar has been killed by the growing wasp larvae after starting to spin a cocoon

Many parasitoid wasps use larval Lepidoptera as hosts, but some groups parasitize different host life stages (egg, larva or nymph, pupa, adult) of nearly all other orders of insects, especially Coleoptera, Diptera, Hemiptera and other Hymenoptera. Some attack arthropods other than insects, such as spiders. Adult female wasps of most species oviposit into their hosts' bodies or eggs. Some also inject a mix of secretory products that paralyze the host or protect the egg from the host's immune system; these include polydnaviruses, ovarian proteins, and venom. If a polydnavirus is included, it infects the cells of the host, causing symptoms that benefit the parasite.^[10]

Host size is important for the development of the parasitoid, as the host is its entire food supply until it emerges as an adult; small hosts often produce smaller parasitoids.^[11] Some species preferentially lay female eggs in larger hosts and male eggs in smaller hosts, as the reproductive capabilities of males are limited less severely by smaller adult body size.^[12]

Some parasitoid wasps mark the host with chemical signals to show that an egg has been laid there. This may both deter rivals from ovipositing, and signal to itself that no further egg is needed in that host, effectively reducing the chances that offspring will have to compete for food and increasing the offspring's survival.^[13][14]

Life cycle[edit]

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 almost so. A meconium, or the accumulated wastes from the larva is cast out as the larva transitions to a prepupa.^[15][16] 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. Females of some species will also drink hemolymph from hosts to gain additional nutrients for egg production.^[17]

Polydnavirus[edit]

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.^[18][19]

Some parasitoidal wasps 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.^[18]

Host defences[edit]

The hosts of parasitoids have developed several levels of defence. 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.^[20][21]

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.^[22]

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.^[23][24]

Certain caterpillars eat plants that are toxic to both themselves and the parasite to cure themselves.^[25] Drosophila melanogaster larvae also self-medicate with ethanol to treat parasitism.^[26] D. melanogaster females lay their eggs in food containing toxic amounts of alcohol if they detect parasitoid wasps nearby. The alcohol protects them from the wasps, at the cost of retarding their own growth.^[27]

Pest control[edit]

Encarsia formosa, an endoparasitic chalcid wasp, bred commercially to control whitefly in greenhouses

Trioxys complanatus, (Aphidiidae) ovipositing into a spotted alfalfa aphid, a commercial pest in Australia.^[a]

Parasitoid wasps are considered beneficial as they naturally control the population of many pest insects. They are widely used commercially (alongside other parasitoids such as tachinid flies) for biological pest control, for which the most important groups are the ichneumonid wasps, which prey mainly on caterpillars of butterflies and moths; braconid wasps, which attack caterpillars and a wide range of other insects including greenfly; chalcid wasps, which parasitise eggs and larvae of greenfly, whitefly, cabbage caterpillars, and scale insects.^[29]

One of the first parasitoid wasps to enter commercial use was Encarsia formosa, an endoparasitic chalcid. It has been used to control whitefly in greenhouses since the 1920s. Use of the insect fell almost to nothing, replaced by chemical pesticides by the 1940s. Since the 1970s, usage has revived, with renewed usage in Europe and Russia.^[30] In some countries, such as New Zealand, it is the primary biological control agent used to control greenhouse whiteflies, particularly on crops such as tomato, a particularly difficult plant for predators to establish on.^[31]

Commercially, there are two types of rearing systems: short-term seasonal daily output with high production of parasitoids per day, and long-term year-round low daily output with a range in production of 4–1000 million female parasitoids per week, to meet demand for suitable parasitoids for different crops.^[32]

In culture[edit]

Charles Darwin[edit]

Parasitoidy in wasps influenced the thinking of Charles Darwin.^[b] In an 1860 letter to the American naturalist Asa Gray, Darwin wrote: "I cannot persuade myself that a beneficent and omnipotent God would have designedly created parasitic wasps with the express intention of their feeding within the living bodies of Caterpillars."^[1] The palaeontologist Donald Prothero notes that religiously-minded people of the Victorian era, including Darwin, were horrified by this instance of evident cruelty in nature, particularly noticeable in the Ichneumonidae.^[34]

Notes[edit]

References[edit]