Ichneumonoidea

This is one of the largest groups of parasitic insects and by far the most well known hymenopteran parasites (over 100,000 estimated species worldwide) of primarily parasitoid insects that attack other arthropods, especially the immature stages of other insects (Gauld and Bolton, 1988; Wahl and Sharkey, 1993). A few taxa are secondarily phytophagous, and several groups that attack egg masses or food provisions of other arthropods are difficult to classify as either "parasitic", predaceous or parasitoid. It probably ranks first in effectiveness of reducing or holding in balance numerous phytophagous pests. The species of Ichneumonoidea vary tremendously in size, from approximately 1 mm in length to 5-6 cm (even larger if the ovipositor of some larger species is included).

Dominant families are Ichneumonidae and Braconidae (Clausen 1940), as well as one fossil family Eoichneumonidae, and a fossil genus, Tanychora, of uncertain relationship to the other families (Sharkey and Wahl, 1992). Past classifications have sometimes included other groups, such as Stephanidae and Megalyridae, now transferred elsewhere, or they have recognized subfamilies of Braconidae (e. g. Aphidiinae and Apozyginae) and Ichneumonidae (Agriotypinae and Paxylommatinae) as distinct families. In this section the families Agriotypidae, Aphidiidae, Apozygidae, Braconidae, Ichneumonidae and Paxylommatidae will be treated separately. An immense and incredably varied group which can be easily recognised by having more than 16 antennal segments and the prominent stigma in the forewing. The front edge of the forewing in thickened due to the virtual fusion of the first long vein with the front margin and the consequent obliteration of the long narrow cell found in most other hymenopterans.

Ichneumonoidea have been present since at least the early Cretaceous, being represented by Tanychora from Transbaikalia (Townes, 1973), as well as several genera of Eoichneumonidae from Australia, Siberia and Mongolia (Jell and Duncan, 1986; Rasnitsyn and Sharkey, 1988). The placements of the early Cretaceous Praeichneumon townesi as an ichneumonoid and of Eobracon inopinatus as a braconid (Rasnitsyn, 1983) have been questioned recently (Sharkey and Wahl, 1992; Whitfield, 2002); nevertheless it is clear that the superfamily was still represented at least by extinct groups from the very early Cretaceous.

Wahl & Sharkey (1993) noted that in this superfamily veins C and R of the forewing are adjacent or fused, so that cell C is absent or apically nearly so. The antennae are not elbowed, and mostly have more than 11 flagellar segments. The ovipositor is often extended and long. Trochantelli are present. Metasomal sternum 1 is divided in half and the apical portion is weakly sclerotized. Metasomal tergum 1 is often with a lateral pit (glymma) on the anterior half. The mandible usually has 2 teeth. Wahl & Sharkey (1993) included only two families in this superfamily: Braconidae and Ichneumonidae. They justified this by the following account:
"Several other families have been placed in the superfamily. The family Stephanidae has often been included (Townes 1969, Carlson 1979), but it possesses none of the autapomorphies that define Ichneumonoidea (Sharkey & Wahl 1992). The ichneumonid subfamily Paxlommatinae has been various treated as a subfamily of braconids (van Achterberg 1976) or as a separate family (Mason 1981). The paper by Mason showed that it could not be regarded as belonging in the Braconidae; Rasnitsyn (1980)and Gauld (1984a) treat the group as a subfamily of ichneumonids. Some workers, such as Mason (1971), regard the ichneumonid subfamily Agriotypinae as having family status, placing emphasis upon certain specialized attributes. Ichneumonid specialists see no compelling reason to regard them as anything other than a derived group of ichneumonids. Aphidiine braconids are often treated as a family. A major autapomorphy of Braconidae, tergum 2 fused with 3, is present in the aphidiines, although the fusion is weakened and some bending occurs. The braconid subfamily Apozyginae was originally described as a separate family (Mason 1978) but is best considered as belonging to the cyclostome braconids."

In this article some of the subfamilies in Ichneumonidae are treated as distinct families, using an earlier system, as much of the biological data is found under this classification. Wahl & Sharkey (1993) noted that ichneumonoids parasitize mainly the larvae and pupae of holometabolous insects, excluding the Megaloptera and Siphonaptera. Whereas ichneumonids are almost completely restricted to the immature stages of the Holometabola (a few groups use egg nests of Pseudoscorpionida, egg cocoons of Araneae or adult Araneae), many braconids parasitize nymphal Hemimetabola (Homoptera-- Aphididae, Heteroptera, Isoptera, and Psocoptera). No braconids are known to parasitize Araneae or their eggs. A few braconids also parasitize adult Coleoptera and Hymenoptera. Unlike microhymenoptera, ichneumonoids rarely parasitize individual eggs, although many braconids and a few ichneumonids are egg-larval parasitoids, laying an egg in the host egg but consuming the host in its larval stage. Symphyta parasitism is quite common in Ichneumonidae, having arisen on several separate occasions. In braconids, only Ichneutinae and a few scattered species of other groups are sawfly parasitoids.

Ectoparasitism (living on the surface of the host and feeding through an integumentary wound) is the primitive condition for ichneumonoids (and Apocrita). External parasitoids generally parasitize hosts in concealed locations, such as stem tunnels, pupal cells, leaf rolls, or cocoons. Many species inject venom before the eggs are laid. The resulting paralysis may be temporary or permanent, or fatal. The egg is sometimes deposited next to the host, especially when paralysis is permanent. If only temporary paralysis is induced, the egg is often deposited on the host but where the host cannot reach it.

Endoparasitism evolved independently on several occasions within the ichneumonoids, the exact number of times within each family being unclear. Although certain advantages are gained by developing inside the host, the ichneumonoid is subject to attack by the host's immune system. A variety of strategies are used to overcome this, including the injection of viruses at the time of oviposition. These serve to control the immune reactions of the host (Edson et al. 1981).

Besides ecto- and endoparasitic modes of development, ichneumonoid biology may be viewed differently. Askew & Shaw (1986) distinguished between idiobionts, which do not allow the host to develop after oviposition, and koinobionts, which allow host development after oviposition and do not kill until a later stage. Mature larvae, prepupae, or pupae are the hosts of idiobionts, which are often ectoparasitoids. A venom that paralyzes or kills the host is usually injected at oviposition. Gauld (1987) noted that the host is an "immobile piece of meat." Idiobiont endoparasitic taxa are known, some of which are quite speciose, such as Pimplini and most Ichneumoninae in Ichneumonidae, and most Euphorinae in Braconidae. Koinobionts are usually endoparasitoids, parasitizing the eggs or early larval stages of the host. Parasitoids development is delayed or protracted, allowing the host to reach the later larval instars or pupal stage before it is consumed. Gauld (1987) elaborated on this subject especially with respect to patterns of diversity in tropical ichneumonid fauna.

Gregarious development is more common in braconids than ichneumonids. In contrast, hyperparasitism is only infrequently found in braconids, while many ichneumonids are hyperparasitoids of other ichneumonoids or Tachinidae (Diptera).

Three to 5 larval instars occur. The mature larva is shaped like a grub and apodous, resembling the larvae of Aculeata. Several heavily sclerotized rods and bands occur around the mouthparts and are valuable for taxonomy. The cast skin of the mature larva is retained in the parasitoid's cocoon, or in the host remains if no parasitoid cocoon is formed, along with the larval meconium and the cast pupal skin. The larval skins when mounted on slides may enable a study of head structures (Wahl 1984, 1989). The cocoon and its contents is usually preserved with reared specimens and retained in gelatin capsules with the reared adult (Wahl & Sharkey 1993).

Image by Brian Valentine