Parasitoid wasp

Parasitoid wasps are a vast group within the Hymenoptera order, and almost all of them belong to the wasp-waisted subgroup called Apocrita—the only exception being the wood wasps (Orussoidea). True to their name, these wasps are parasitoids: they deposit their eggs onto or inside other arthropods, and the developing young eventually kill that host. Different species target different insect orders, with caterpillars (Lepidoptera) being the most common choice, though some go after beetles, flies, or true bugs. The spider wasps (Pompilidae) are specialists that hunt only spiders. Parasitoid wasps also vary in which life stage of the host they attack—eggs, larvae, pupae, or adults. Their parasitic strategies generally fall into two main types. Some are endoparasitic, meaning they develop inside the host, and are koinobionts, allowing the host to keep feeding, growing, and molting. Others are ectoparasitic, developing on the outside, and are idiobionts, paralyzing the host right away. A number of endoparasitic wasps in the superfamily Ichneumonoidea have a mutualistic partnership with polydnaviruses, which help suppress the host's immune defenses.

Parasitoidism evolved just once among Hymenoptera, back in the Permian period, giving rise to a single lineage called Euhymenoptera. However, this parasitic lifestyle has been lost several times since then—for instance, among ants, bees, and vespid wasps. As a result, the order Hymenoptera today includes many parasitoid families mixed in with non-parasitoid groups. Some parasitoid wasp groups are enormous: estimates suggest the Chalcidoidea may contain as many as 500,000 species, the Ichneumonidae around 100,000, and the Braconidae up to 50,000. Host insects have developed various defenses against them, such as hiding, wriggling, or camouflage markings. Many parasitoid wasps are considered beneficial to humans because they naturally keep agricultural pests in check. Some have been used commercially in biological pest control since the 1920s, starting with *Encarsia formosa* to manage whiteflies in greenhouses. Historically, the concept of parasitoidism in wasps even influenced Charles Darwin’s thinking.

Parasitoid wasps range from some of the tiniest insects to species about an inch long. Most females have a long, sharp ovipositor at the tip of the abdomen; it sometimes lacks venom glands and, outside of the aculeate groups, is almost never modified into a stinger. Parasitoids can be classified in several ways. They may live inside the host as endoparasitoids or feed on it from the outside as ectoparasitoids—both strategies occur among wasps. They can also be grouped by their effect on the host: idiobionts stop the host’s development after immobilizing it, while koinobionts let the host keep developing as they feed. Again, both types are found. Most ectoparasitoid wasps are idiobionts, since a moving or molting host could damage or dislodge them. Most endoparasitoid wasps are koinobionts, which gives them the advantage of a host that continues to grow larger and stay safer from predators.

Many parasitoid wasps use lepidopteran larvae as hosts, but some groups target different life stages (eggs, larvae or nymphs, pupae, or adults) of nearly all other insect orders, especially beetles, flies, true bugs, and other Hymenoptera. A few attack non-insect arthropods: for instance, the Pompilidae specialize in catching spiders—quick, dangerous prey often as large as the wasp itself—but the spider wasp is faster, swiftly stinging its prey to immobilize it. In most species, adult females lay their eggs directly into the host’s body or eggs. More rarely, parasitoid wasps may use plant seeds as hosts, such as *Torymus druparum*. Some inject a mix of secretions that paralyze the host or protect the egg from the host’s immune system; these can include polydnaviruses, ovarian proteins, and venom. If a polydnavirus is present, it infects the nuclei of host blood cells and other cells, causing symptoms that benefit the parasite. Host size matters because the host is the parasitoid’s entire food supply until it emerges as an adult; smaller hosts often produce smaller parasitoids. Some species preferentially lay female eggs in larger hosts and male eggs in smaller ones, since a male’s reproductive success is less limited by a smaller body size. Some parasitoid wasps also mark the host with chemical signals to show an egg has been laid there. This can deter rivals from laying more eggs and signal to the same wasp that no further egg is needed, reducing competition for food and boosting the offspring’s survival chances.

Once an egg is laid on or inside the host, it hatches into a larva—or sometimes two or more larvae through polyembryony. Endoparasitoid eggs can absorb fluids from the host and grow several times larger before hatching. The first-stage larvae are often highly mobile and may have strong mandibles or other structures to compete with other parasitoid larvae. Later stages are generally more grub-like. Parasitoid larvae have incomplete digestive systems with no rear opening, which prevents the host from being contaminated by waste. The larva feeds on the host’s tissues until it is ready to pupate; by then, the host is usually dead or nearly so. As the larva transitions to a prepupa, it casts out a meconium—the accumulated waste. Depending on the species, the parasitoid may then chew its way out of the host or stay inside the more or less empty skin. In either case, it typically spins a cocoon and pupates. As adults, parasitoid wasps mainly feed on nectar from flowers. Females of some species drink hemolymph from hosts to get extra nutrients for egg production.