Parasitoid wasp

By far the greater number of wasps (over 500,000 species in the superfamily Chalcidoidea alone) are a special type of parasite.^[1] They are parasitoids that lay their eggs in or on the bodies of other insect species (usually the larvae such as caterpillars).^[2]

Megarhyssa macrurus (Ichneumonidae), a parasitoid, ovipositing into its host through the wood of a tree. The body of a female is about 2 inches (50 mm) long, with an ovipositor about 4 inches (100 mm) long.

Braconid parasitoid wasp Aleiodes indiscretus laying eggs into a caterpillar

Braconid parasitoid wasp Apanteles cocoons on a caterpillar

Different species specialize in hosts from different insect orders, most often Lepidoptera, though some select beetles, flies, or bugs; the spider wasps (Pompilidae) attack exclusively spiders.

Almost every species of insect pests has at least one wasp species that preys upon it or parasitizes it. That makes wasps critically important in the natural control of pest numbers (biocontrol). Parasitic wasps are increasingly used in agricultural pest contro, as they do little or no damage themselves to crops. Farmers buy those parasitic wasps for insect control in their fields.

With most species, the adult parasitic wasps themselves do not get any nutrients from their prey. Much like bees, butterflies, and moths, those which do feed as adults usually get all of their nutrition from nectar.

Parasitic wasps are extremely varied in their habits. Many lay their eggs in inert stages of their host (egg or pupa). If the prey is a caterpillar, they paralyze it by injecting it with venom through their ovipositor. Then, they insert one or more eggs into the host or deposit them upon the host externally. The host remains alive until the parasitoid larvae are mature and dies later when the parasitoids pupate, or when they emerge as adults. Basically, the hosts are eaten alive.

Ichneumons

The Ichneumonoidea is a superfamily of wasp parasitoids. It has two main families:

• Ichneumonidae: this is by far the largest family. It is a very specific parasite, each species preying on only one or two prey species.
• Braconidae: this family ahs more all-round parasites, which have a wide range of hosts that they can parasitise.

Polydnavirus

Polydnaviruses (PDV) are a unique group of insect viruses that have a mutualistic relationship with parasitic wasps.

The virus protects the parasitic larva inside the host by weakening the host’s immune system. It also alters the cells of the host in ways that help the parasite. The polydnavirus, like all viruses, need a host to replicate itself in the oviducts of the female wasp.

The relationship between those viruses and the wasp is obligatory; all individuals are infected with the viruses, which have been added to the wasp’s genome.^[3][4]

The two main genera of polydnaviruses are the Ichnoviruses (IV) and the Bracoviruses (BV). Ichnovirus occurs in ichneumonid wasp species and Bracovirus in braconid wasps.

Host defense

The victims of parasitoids have defenses that they can use. Many try to hide from the wasps. The egg shells and cuticles of the prey are thickened to prevent the wasp from penetrating them. When the wasp arrives, prey may drop off the plant they are on, or twist and thrash to dislodge the female. Some regurgitate onto the wasp to tangle it up. The wriggling can sometimes help by causing the wasp to miss laying the egg on the host and instead place it nearby. The 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 bite wasps that approach it. Some secrete poisonous compounds that kill or drive away the parasitoid. Ants that are in a symbiotic relationship with caterpillars, aphids or scale insects defend them from attack by wasps.^[5]

Even parasitoid wasps are vulnerable to hyperparasitoid wasps.^[6] 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.^[7]

In endoparasitoids, host immune cells can encapsulate the eggs and larvae of parasitoid wasps. In aphids, a secondary endosymbiont, Buchnera aphidicola, carries a latent phage, which makes the aphid relatively immune to their parasitoid wasps by killing many of the eggs. However, wasps counter that by laying more eggs in aphids that have the endosymbiont so that at least one of them can hatch and parasitize the aphid.^[5]

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. Although the alcohol slows the flies' growth, it protects them from the wasps.^[8]

Evolution

Parasitoidism evolved only once in the order Hymenoptera, during the Permian, which led to a single clade, Apocrita. Almost all parasitoid wasps are descended from that clade.^[9]

The parasitic lifestyle has been lost several times, including among the ants, bees, and yellowjacket wasps. As a result, Hymenoptera has many families of parasitoids intermixed with non-parasitoid groups. Hymenoptera, Apocrita, and Aculeata are all clades, and each has both parasitic and non-parasitic species. The parasitoid wasps do not form a clade on their own.^[10][11]

The parasitoid wasps include some very large groups. Some estimates suggest the Chalcidoidea has 500,000 species, the Ichneumonidae 100,000 species, and the Braconidae up to 50,000 species. Host insects have evolved a range of defenses against parasitoid wasps, including hiding, wriggling, and camouflage markings.

A large increase of species in Hymenoptera occurred soon after the evolution of parasitoidy in the order. Parasitoidy may have been a main cause.^[11][12] The evolution of a "wasp waist," a constriction in the abdomen, helped Apocrita because it increased manoeuverability of the ovipositor (the organ used to lay eggs).^[13]