ichneumon wasp

ichneumon wasp, (family Ichneumonidae), any of a large and widely distributed solitary wasp family of considerable ecological and economic importance. One of the most diverse animal groups in the world and the largest group of parasitic wasps, there are around 25,000 ichneumon wasp species known to science and as many as 75,000 more that are yet to be discovered and named.

The name sometimes refers to any member of the superfamily Ichneumonoidea, which includes the families Stephanidae, Braconidae, and Ichneumonidae. The Stephanidae, a largely tropical group, consists of several hundred species, and the Braconidae is a large, widely distributed group found in both temperate and tropical regions.

Physical description

Given the sheer number of species, ichneumon wasps vary considerably in shape, color, and size. Adults average about 1.2 cm (about 0.5 inch) long, though many are larger. The largest ichneumons of North America, those of the genus Megarhyssa, are more than 5 cm (2 inches) in length, not including the extremely long ovipositor (egg-laying organ) of the females, which may be 5 to 10 cm (2 to 4 inches) in length. Ichneumon wasps are similar in appearance, though typically larger in size, than the related braconid wasps, another group of parasitic wasps. Many species exhibit sexual dimorphism in which the coloration of the males and females differs.

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As insects, ichneumon wasps have a body formed of a head, thorax, and abdomen, and six legs. They have a pair of large compound eyes on the side of the head and three ocelli (simple eyes) on the top of the head. Most species resemble other wasps in having a long, slender, curved abdomen with a fairly narrow waist. They usually differ from the stinging wasps in having longer antennae with more segments, or sections. Often confused for an intimidating stinger, the ovipositor is generally longer than the body; only a few species use the ovipositor for defense. Many species have a dark spot on the fore wing.

Natural history and parasitism

Ichneumon wasps are parasitoids, meaning their larvae feed and develop on or within the bodies of other arthropods. They parasitize many insect groups, especially Lepidoptera (moths and butterflies), Coleoptera (beetles), and other Hymenoptera (ants, bees, and wasps). Some utilize spiders as hosts.

Adult wasps typically feed on nectar and pollen, and as solitary wasps, only come together to mate. The females lay their eggs in or on the larvae or pupae (rarely eggs or adults) of the host. To attack wood-boring hosts deep inside woody plant tissues, some ichneumon wasp species actually use their ovipositors as a drill capable of penetrating solid wood. Once the ichneumon wasp egg hatches, the larva feeds on the fats and body fluids of the doomed host until fully grown, then usually spins a silken cocoon or pupates within the host’s body. The species that parasitize hosts in open habitats usually develop as internal endoparasites, whereas those that attack hosts in concealed places, such as wood burrows, usually feed on the host externally as ectoparasites. In most cases a single larva develops in one host. However, in some cases, many larvae develop in a single host.

Members of the subfamily Ophioninae, including Ophion, Enicospilus, and Thyreodon, primarily parasitize moths and are thus active at night. They form one of the few groups of ichneumon wasps that can sting vertebrate animals. Certain members of the genus Netelia (subfamily Tryphoninae) also use their short ovipositors defensively.

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A few species are hyperparasitic, meaning they feed on the primary parasitoid already developing within the original host. Such species attack a wide range of insects parasitized by other hymenopterans, including heteroperans (true bugs), lepidopterans, and symphytans (sawflies, horntails, and wood wasps).

Importance

Ichneumon wasp larvae kill their hosts and thus fulfill an important role in the regulation of insect populations. The group parasitizes many insect pests, serving to keep their abundance in check in both natural and agricultural settings, and only small number of species parasitize other beneficial insects. Specific ichneumon wasps have been actively encouraged or introduced in some places to serve as biological control. The western giant ichneumonid wasp (Megarhyssa nortoni), for example, has been introduced to South Africa, Australia, and New Zealand as a biological control agent to reduce the populations of the invasive wood wasp Sirex noctilio. Similarly, the yellow ichneumon wasp (Xanthopimpla punctata) is an economically important parasitoid of stem boring caterpillars that feed on cereal plants and other crops.

Beyond their parasitism, ichneumon wasps are also at least incidental pollinators for many plant species. The group is understudied, but there is evidence that some species have a degree of floral specialization and exhibit specific foraging behaviors, suggesting that their importance as pollinators may be under appreciated.