Genera Ichneumonorum Nearcticae

The purpose of Genera Ichneumonorum Nearcticae is to present concise current information about the taxonomy, biology, distribution and species-richness of North American Ichneumonidae. In the subsidiary sections (listed on the sidebar) our aim is to provide an introduction to each subfamily present in the Nearctic region, a subfamilial diagnosis, synopsis of biology and classification, and reference to species and available resources for their identification. Each section usually completed by illustrated generic keys, or with keys to species for those subfamilies with one genus and a small number of species. The classification used here is the most generally accepted current overview.

Family Ichneumonidae

GENERAL INTRODUCTION. The Ichneumonidae is the largest family of all the Hymenoptera, and worldwide it probably includes more than 100,000 species (Gauld, 2002), of which 24,281 are currently described and classified into 39 subfamilies. This remarkable diversity makes the Ichneumonidae one of the largest families of organisms on Earth, one that far exceeds, in terms of species-richness, all the vertebrate animals combined. Ichneumonids are common and conspicuous in all New World terrestrial biomes, from Arctic tundra, through equatorial rainforests, to sub-Antarctic islands. They are found in suburban gardens and pristine forests, in deserts and on waterside vegetation. Although ichneumonids are abundant in almost every habitat, some authors believed their species-richness was highest in the north temperate regions (Owen & Owen, 1974; Janzen, 1981; Gauld et al., 1992); the emerging picture, however, is that ichneumonids are, at the very least, not less diverse in the tropics (Sääksjarvi et al., 2004; Veijalainen et al., 2012).

Numerous ichneumonid species may be observed in most localities, such as brightly patterned ichneumonines that are conspicuous on account of their vibrating white-banded antennae, cryptines with strikingly patterned wings flying along sun-dappled forest paths, metallic blue Thyreodon with bright yellow antennae that mimic aggressive pompilid wasps, and the slender brownish-red ophionines that cluster around bright lights at night. Many ichneumonids can be seen on flowers while feeding on nectar or pollen, but most often they are only glimpsed as they fly erratically through the vegetation. A few are physically quite large, with long ovipositors that may exceed 60 mm in length. These ichneumonids excite attention wherever they occur, especially when they are found congregating around tree trunks in old-growth forests. Most, however, are rather small, dark-colored and inconspicuous.

All ichneumonids are parasitoids: they develop by feeding as a larva on a single immature host insect which is eventually killed by the ichneumonid. The adult ichneumonid is free-living. Males may be found in aggregations around sites where females are emerging, but females spend much of their time searching for prospective hosts on or into which they deposit an egg. Many species have a pronounced sexual color-pattern difference, with males conspicuously colored and females rather drab and unicolorous (Gauld & Fitton, 1987).

Ichneumonids attack a wide range of hosts, most frequently the larvae and pupae of the larger holometabolous insect orders (Lepidoptera, Diptera, Coleoptera or other Hymenoptera), although a very few also attack the immatures of other holometabolous insects such as Trichoptera (Agriotypinae), Mecoptera (a few Campopleginae), Raphidioptera (a few Campopleginae), and Neuroptera (Brachycyrtinae, some Cryptinae). No species are known to attack hemimetabolous insects (i.e. those with an egg/nymph/adult life cycle), although some feed on arthropod eggs in sacs, and a very few attack spiders. A common biology is for the female to lay an egg inside a host caterpillar; the ichneumonid larva feeds selectively within its host, avoiding damage to the host caterpillar which continues to feed and grow like a normal larva. When the caterpillar is almost fully-grown, the ichneumonid consumes its insides entirely and breaks free from the caterpillar skin, subsequently spinning a cocoon under or next to the host larval remains. There is tremendous biological diversity within the Ichneumonidae: some drill through two or more centimeters of hardwood to find wood-boring grubs, some lay eggs inside an insect embryo in an egg, others drill into hard exposed pupae, and some temporarily paralyze and parasitize spiders (Eberhard, 2000). Some, such as Apsilops (Cryptinae), search for hosts on plants growing in standing water. Agriotypus species (Agriotypinae) enter freshwater to search for the submerged pupae of caddis-flies (Elliott, 1982) and a few species of Cremastinae, such as Tanychela, enter freshwater to locate their nymphuline hosts (Resh & Jamieson, 1988).


The family Ichneumonidae is a clearly definable monophyletic group on both morphological (Sharkey & Wahl, 1992) and molecular (Sharkey et al., 2012) grounds. It includes the Hybrizontinae (= Paxylommatinae of authors) and the Agriotypinae, two highly derived taxa that have often been treated as distinct families (Mason, 1971; 1981), or in the case of the Hybrizontinae, as a subfamily of Braconidae (van Achterberg, 1976). Gauld (1984: pp. 11-12) presents a cogent argument for including the Hybrizontinae within the Ichneumonidae. Currently the majority of authors follow the interpretation of Sharkey & Wahl (1992).

The internal classification of the Ichneumonidae has been subject to a great deal of change since the 19th century when only five subfamilies were recognized: Ichneumoninae, Cryptinae, Tryphoninae, Pimplinae and Ophioninae. As more natural groupings were recognized, the number of subfamilies steadily increased to 25 in Henry Townes' monographs on the genera (1969; 1970a&b, 1971). Subsequently, with the application of rigorous phylogenetic argumentation, particularly with regards to the Labeninae, Pimplinae and the Microleptinae (sensu Townes), the number increased to 37 in a recent overview (Gauld, 2002). Five new subfamilies have been proposed in the last 21 years:

-Townesioninae: this subfamily, comprising Townesion and Sachtlebenia, was described from the eastern Palaearctic region by Kasparyan (1993) and placed near the Tryphoninae. Subsequently Gauld & Wahl (2000) demonstrated that this was an autapomorphic genus-group of Glyptini, and included it within this tribe in the Banchinae.
-Pedunculinae and Claseinae: Porter (1998) erected two new subfamilies for three Neantarctic genera, the Pedunculinae for Pedunculus, and the Claseinae for Clasis and Ecphysis (formerly treated as a tribe, Claseini, in the Cryptinae). Gauld & Ward (2000) redefined the Pedunculinae to include endemic Australian genera previously placed in the Brachycyrtinae (Gauld, 1984), but saw no reason to remove the Claseini from the Cryptinae.
-Nesomesochorinae: Townes (1970b) placed the Old World genus Chriodes and the Neotropic genus Nonnus together as the Nonnini, a tribe he placed with some reservations in the Campopleginae. Fitton & Gauld (1976) established the correct name of the tribe as Nesomesochorini. Miah & Bhuyia (2001) elevated this tribe to subfamilial status, and we agree that recognition of Nesomesochorinae is warranted since the group shares no unique derived features with campoplegines. The subsequent break-up of the Nesomesochorinae into two subfamilies by Quicke et al. (2005) is unwarranted as the three component genera, Chriodes, Klutiana, and Nonnus, share many morphological synapomorphies.
-Sisyrostolinae: Seyrig (1932) erected the tribe Sisyrostolini in the Pimplinae for Erythrodolius, Icariomimus, and Melandolius. Townes (1971) combined the Sisyrostolini and Townes' Brachyscleromatinae (consisting of Brachyscleroma (Townes et al., 1961)) with the Phrudinae. Quicke et al. (2009) found that the Sisyrostolini and Brachyscleroma were not closely related to the Phrudinae s.s. and removed them to an expanded Brachyscleromatine. Bennett et al. (2013) pointed out that Sisyrostolini Seyrig, 1932 has priority over Brachyscleromatinae Townes, 1961 and that Sisyrostolinae is the correct name of the subfamly. The subfamily consists of the following genera: Brachyscleroma, Erythrodolius, Icariomimus, Laxiareola, Lygurus, and Melandolius (Bennett et al., 2013).

Currently we recognize 39 subfamilies. This is perhaps the most generally recognized classification used at present, although some doubts remain about the status of a few taxa such as Skiapus and Clasis. More detailed comments on the subfamilies are at World Genera and Family Group Names.

The history of ichneumonid classification is complicated further by nomenclatural confusion resulting from Henry Townes, the most influential ichneumonologist of the last century, using an idiosyncratic system of nomenclature, which was followed by his collaborators and adopted by many collections. It was only in the last twenty years that a system of nomenclature based on the International Code of Zoological Nomenclature (vide Fitton & Gauld, 1976; 1978; Wahl & Mason, 1995) has gained wide acceptance. For clarification, the Townesian names of various taxa are given here, and the family-group names of all extant Ichneumonidae are listed here.

We have avoided arranging the subfamilies in any informal groupings, although three natural groupings - the Pimpliformes, Ichneumoniformes and Ophioniformes - seem to be reasonably well supported by morphological synapomorphies, despite continuing debate on their exact limits (Gauld, 1985; Wahl, 1991, 1993; Wahl & Gauld, 1998; Gauld & Wahl, 2000; Quicke et al., 2000, 2009).


New keys to the subfamilies present in North America are included in a separate section (in preparation) and details of each subfamily are given under the appropriate heading on the sidebar. Throughout both of these accounts standardized morphological terminology has been used. Details of this morphological terminology are given in the ichneumonid morphology section.

Ichneumonid Morphology
List of the subfamilies and tribes of the Ichneumonidae used on the GIN site

Catalogues of the described species of Ichneumonidae are: World (Yu & Horstmann, 1997); Eastern Palearctic (Townes, Momoi & Townes, 1965); Nearctic (Carlson, 1979); Neotropic (Townes & Townes, 1966); Indo-Australian (Gupta, 1987); Ethiopian (Townes & Townes, 1973). Taxapad (Yu et al., 2012) is an electronic database of World Ichneumonoidea. Details of the size, distribution, characterization, composition, and biology of the various subfamilies are given below in the various subfamily treatments.

A Detailed Overview of Ichneumonid Biology

a. Life history: eggs.
Ichneumonid females may have as few as ten (in some Ichneumoninae and Cryptinae) to several thousand (in Eucerotinae) potential eggs (Iwata, 1958, 1960). There is a close correlation between the size of the egg and the stage of the host attacked; koinobionts that emerge from the host's pupa but oviposit in early instar larvae tend to have large numbers of small microlecithal eggs, whilst idiobionts attacking pupae usually have few large eggs (Price, 1973. 1974). The eggs of most ichneumonids are ovoid, pale and without apparent chorionic sculpture (Iwata, 1958). However, many ichneumonid species that have very long ovipositors have an extremely elongate egg that is compressible to facilitate its passage down the lumen of the ovipositor. Tryphonines (which are ectoparasitic koinobionts) have eggs furnished with a variety of different types of hooks or anchors that, during the act of oviposition, are embedded in the host cuticle and firmly attach the egg to the outside of the host (Mason, 1967: Kasparayan, 1981). The eggs of Lycorininae are very similar (Coronado-Rivera et al., 2004) and may be attached to the hind gut accessed by oviposition through the host's anus (Shaw, 2004). The eggs of some Anomaloninae have a mushroom-like protuberance which is used to anchor the egg internally within the host's haemocoel (Gauld, 1976), and eggs of one genus of Stilbopinae, Panteles, have a caudal hook that performs an analagous function (Quicke, 2005).

b. Life history: larval and pupal stages.
Probably the plesiomorphic number of larval instars for an ichneumonid is five, but the exact number is often difficult to ascertain (Rojas-Rousse & Benoit, 1977) because the final three instars often closely resemble each other. However, the number of larval instars may be reduced, for example to four in some Cremastinae (Giraldo-Vanegas & Garda, 1992) and to three in some Anomaloninae (Tothill, 1922). The first larval instar of many species is often quite unlike later instars. In many ectoparasitoids it is spindle-shaped with a well-developed head capsule, conspicuous antennae, strongly sclerotized. sharply pointed mandibles and the cuticle may be quite strongly sclerotized and furnished with robust setae (Pschorn-Walcher & Zinnert, 1971; Kasparayan, 1981). Species belonging to the more specialized endoparasitoid groups (e.g. most of the Ophioniformes group of subfamilies) often have a caudate first instar larva which may have a weakly sclerotized head capsule, vestigial antennae and no spiracles. The mandibles may be well-developed (Tothill, 1922; Pschorn-Walcher, 1967) or vestigial (Gauld, 1976; Quicke, 2005). The tail-like caudal appendage may be long or short. but in general its relative length decreases as the larva grows (Bradley & Burgess, 1934; Fisher, 1959; Frilli, 1965). In all known cases the final instar larva is hymenopteriform (Clausen, 1940), with the head capsule small and partially retracted into the thorax. Because the final instar larval skin is generally retained in the cocoon or host pupa, it is relatively easily collected and considerable comparative study has been made of the cephalic sclerites (e.g. Beirne, 1941; Finlayson. 1975; Short, 1978; Wahl, 1990; 1993).

Most koinobionts kill their host as a final instar larva within the host's pupation chamber and are thus afforded some degree of protection during their own pupation. Even so, many ichneumonids spin an ovoid, dense, silken cocoon (Slovák, 1984). Although considerable variation in the amino acid composition of silk has been found between various parasitoid taxa (Quicke & Shaw, 2004) all, except Adelognathus, were dominated by short side-chain amino acids. Which amino acid was dominant varied between taxa, but most silks could be interpreted as fibroins. Some ichneumonids that spend a very short period in their cocoon spin a quite frail cocoon. Multivoltine species may have cocoon dimorphism, with diapausing larvae spinning a thicker cocoon than their non-diapausing siblings (Danthanarayana et al., 1977; Fitton et al., 1982). The cryptine vespid parasitoid Sphecophaga has three forms of cocoons: a white form that gives rise to a brachypterous female; a thin yellow cocoon giving rise to winged females in the same summer; a thick yellow cocoon that gives rise to winged males and females the following summer (Donovan, 1991). The cocoons of some ichneumonids that undergo protracted diapause (e.g. Enicospilus lobophagus) are very dense and have a nacreous inner wall that is probably resistant to desiccation and penetration by micro-organisms. Ichneumonids that pupate within their host's pupa (such as Anomaloninae, Ichneumoninae, Metopiinae and Pimplini) generally do not make a cocoon or spin only a rudimentary cocoon, as do many idiobionts (such as Rhyssinae) that attack hosts deeply concealed in woody tissue. Many species seem to diapause as a prepupa, although a few north temperate species diapause as pharate adults within the cocoon (Morley, 1915).

Some campoplegines (e.g. Cryptophion spp.) are unusual in that they kill their host caterpillar on the food-plant before the caterpillar has become fully grown (Gauld & Janzen, 1994). In such an exposed situation, the ichneumonid cocoon is extremely vulnerable to scavengers or hyperparasitoids and various campoplegines have adopted a variety of strategies for concealing their cocoons from predators. Many species have black and white mottled cocoons that resemble bird-droppings. Those of Cryptophion are concealed under the host larval remains, whilst some northern temperate species of Hyposoter construct a false cocoon (Finlayson, 1966). Other species ensure that their cocoons are removed from the leaf surface. Those of Charops, for example, are suspended from the host food-plant by a long thread, and species of Bathyplectes and some Phobocampe have 'jumping cocoons' that break free of the substrate so that cocoon and pupa fall amongst the leaf litter (Gauld & Bolton, 1988).

c. Adult biology.
Adult ichneumonids, like most other Hymenoptera, feed on pollen and nectar from plants. Such food is necessary to sustain energy requirements, and may increase longevity and fecundity. A study in Britain found that Apiaceae (254 out of 262 visits) constituted the overwhelming majority of ichneumonid flower visits; the remaining visits were to Asteraceae, and none were observed on flowers of 13 other plant families (Jervis et al., 1993). Flower-feeding has been observed in almost all ichneumonid subfamilies. A second feeding strategy, host-feeding, seems to be much more restricted. Host-feeding is where an adult parasitoid feeds from haemolymph exuding from a wound (usually made by the ovipositor) in potential host species. This form of feeding is practised mostly by synovigenic parasitoids that continue to produce macrolethical mature eggs throughout their adult lives. In Britain, it is most commonly observed amongst the Pimplinae (Jervis & Kidd, 1986). Some North American species are obligate host-feeders (Bracken, 1965) and do not oviposit unless they have previously host-fed. Host-feeding is necessary to achieve maximum fecundity in many parasitoids. Related to this is a habit called oösorption, which is the absorption of mature eggs by synovigenic parasitoids deprived of hosts. Some pimplines even resorb part of the flight musclature (Sandlan, 1979) in such circumstances.

The great majority of ichneumonids are bisexual, and like other Hymenoptera arrhenotokous with fertilized diploid eggs producing females and unfertilized haploid eggs giving rise to males. However, a few cosmopolitan synanthropic ichneumonids (e.g., Diplazon laetatorius, Venturia canescens) are thelytokous throughout much of their range. Sphecophaga appears to have a deuterotokous form which exploits locally abundant resources, as well as normal bisexual forms that overwinter (Donovan, 1991). Sexual dimorphism is generally not as pronounced amongst the Ichneumonidae as it is in many other groups of apocritan Hymenoptera, although in a few genera it can be very striking. For example, some species of Cryptinae (e.g. many Gelis spp.) have apterous, ant-like females and normal alate males (Salt, 1952). Sexual dimorphism is most apparent in idiobiont groups, the males of which are relatively short-lived flower-feeders who spend most of their lives flying low through, or resting on the vegetation. The females, on the other hand, are long-lived and spend most of their lives on the ground searching for hosts in leaf litter or in concealment under bark etc. Thus some ichneumonine males are often aculeate mimics and aposematically colored, whilst the females are cryptic black or brownish insects. Many idiobionts (e.g., many Cryptinae, Pimplinae, and Labeninae) show considerable sexual dimorphism in size, a result of facultative arrhenotoky (Flanders, 1956; Aubert, 1959). Females lay a disproportionately large number of female (fertilized) eggs on large hosts and male (unfertilized) eggs on small hosts (Arthur & Wylie, 1959; Kishi, 1970). In general, koinobionts show less pronounced sexual dimorphism; males and females frequently resemble each other closely in shape, color pattern and even size (Gauld & Fitton, 1987) and both males and females can often be seen flying amongst or resting on the vegetation. The nocturnally active koinobionts are the least sexually dimorphic of all the ichneumonids, females and males generally differing only in the density of pectination of the tarsal claws (Gauld, 1988a).

The males emerge from the pupa before the females in many ichneumonid species. In the Rhyssinae (and probably also other wood-borer parasitoids), males may congregate around tree trunks from which conspecific females are in the process of emerging, with some of these males establishing territories which they defend against other males (Eggleton, 1990). Other species show scramble competition for mates and in these cases males are sometimes exceptionally elongate – a specialization for mating with the female prior to her full emergence (Nuttall. 1973). In temperate regions, males of some Orthocentrinae. Tersilochinae and Diplazontinae form conspicuous mating(?) swarms. Gauld (1991) observed that males of Joppidium (Cryptinae) form groups flying above and around newly emerged females. Such behavior suggests that the female may be releasing a pheromone, as has also been suggested for some Campopleginae (Vinson, 1972; Gordh & Hendrickson, 1976). Further evidence for sexual pheromones in Ichneumonidae is the observation that males of Exetastes cinctipes (Banchinae) are attracted to 8-dodecenyl and 11-tetradecenyl acetates (Hrdy & Sedivy, 1979).

Mating is rather brief (usually from one to ten minutes in duration) and has rarely been observed. The females of many species are believed to mate only once (Gordh & Hendrickson, 1976; Slovák, 1986) but some mate repeatedly (Mathur, 1967). The male often approaches the female from the rear and repeatedly strokes her with his antennae (Slobodchikoff, 1973; Barrows, 1976), but some species approach face to face with the male vibrating his wings (Veen, 1982). During copulation the male may be situated behind, above, or rarely to the side of the female. and he usually continues to antennate her throughout copulation (Barrows, 1976; Gordh & Hendrickson, 1976; Danthanarayana et al., 1977).

Little is known about the phenology of ichneumonids. In regions that have a pronounced winter or dry season, the majority of koinobiont species are active in spring or shortly after the start of the rains, exploiting the first flush of phytophagous larvae that appear when plants put on new growth. Most of these ichneumonids will have diapaused as prepupae in their host's pupal retreat. Many show a high degree of synchrony with their hosts and emerge as adults for a very short period. Sometimes, as in the case of Enicospilus lebophagus, only part of a year's population emerges the following year; a small proportion may remain in diapause for another twelve months (D.H. Janzen. pers. comm.).

Idiobionts, which mostly attack prepupae or pupae, tend to appear slightly later in the season. In the temperate region most do not survive the winter, but in the seasonal tropics many continue to fly about well into the following dry season. Thus in the seasonal tropics, unlike the northern temperate regions, idiobiont parasitoids may be active throughout the entire inclement season. Potential hosts that are in diapause are thus susceptible to attack throughout their diapause (Gauld, 1987). A few species of genera such as Camera (Cryptinae) and Clistopyga (Pimplinae) have only been collected in the dry season, suggesting they may diapause in the wetter periods. In the northern temperate regions, some species, particularly of the Ichneumoninae, diapause as adults, concealing themselves during the winter beneath the bark of trees and in grass tussocks (Rasnitsyn, 1964).

Ichneumonids have a variety of defensive strategies against predators. Some are furnished with cuticular processes. which may offer physical protection. For example. some species of Certonotus (Labeninae) have spines on the pronotum, propodeum and metanotum, and Acrotaphus species (Pimplinae) have a pronotal shield and occipital flange protecting the cervical membrane against attacks by asilids. Many medium-sized to large species of tropical mesostenines are black and white striped, an adaptation that renders them very difficult to focus on (at least for the human eye) as they fly through patches of light and shade in the forest understory. Species of the pimpline genera Theronia and Xanthopimpla have large tarsal claws with an internal poison-sac, these claws functioning as a poison fang (Townes, 1940). Most of these species are also aposematically colored and, like those capable of stinging, many are Müllerian mimics of aggressive aculeates (Gauld, 1991). Many other species are also brightly colored and are apparently Batesian mimics of aculeates (Evans, 1966, 1968). Some (e.g. Dolichomitus spp.) are yellow and black banded, or brown and yellow, and apparently mimic polistine vespids. Others, such as Rhynchophion, are metallic blue with violet and orange wings, and in flight closely resemble species of Pepsis (Pompilidae); some mesostenines twitch their wings while running and also can easily be mistaken for pompilids. A few pimplines are orange with black-banded wings and in flight they resemble noxious braconids (Quicke, 1990; Quicke et al., 1997). A few ichneumonids. most notably some species of Pimpla, Apechthis, and Exochus, emit a pungent odour when handled (Townes 1939) and this may serve to deter predators.

d. Evolutionary constraints and ground plan behavioral traits.
Female ichneumonids, like other Hymenoptera, have a "lepismatoid" ovipositor (Grimaldi & Engel, 2005): a tube-like organ that is principally used for egg-laying (this organ is not present in most members of the other hyperdiverse holometabolous insect orders). The ovipositors of ichneumonids vary in length from the massive ones of some Neotropical Dolichomitus, which exceed 30 centimeters in length and are almost ten times as long as the wasp, to the short ones of Ophion and Ichneumon which barely project beyond the metasomal apex. The ovipositor is homologous with the "sting" of the aculeate wasps (such as pompilids, velvet ants, etc.), which is used to immobilize prey and (in the case of social wasps such as ants, honey-bees and yellow-jackets) colony defence. In ichneumonids, the ovipositor is used to introduce the egg onto or into the host, with species like Dolichomitus gaining access to hosts deep in tunnels in wood, and species like Ophion directly encountering and jabbing a caterpillar feeding on a leaf. Species with short ovipositors may stab one with their ovipositor, causing a pricking sensation, but unlike the sting of an aculeate wasp, the sensation generally does not last long. This is because aculeates inject an irritating/painful venom when stinging, whilst ichneumonids do not. Like aculeates, ichneumonids do have venom-producing glands internally at the base of the ovipositor but the venom of the ichneumonid has functions other than annoying vertebrate nest-raiders. Probably the most basic function of venom is to cause paralysis of the host, a trait found throughout the Hymenoptera. Usually paralysis is permanent, but in some ichneumonids, such as the spider-parasitoid Polysphincta, it may only be temporary, allowing the ichneumonid time to precisely position an egg on an immotile host. It the case of endoparasitoids, the venom is injected with the egg and may in some way modify the host, either by disrupting the host's immunodefensive system, inducing supernumerary larval instars, or causing premature pupation. The evolution of an ovipositor and a huge array of host-manipulating venoms have been very important in the diversification of the Hymenoptera.

It is widely suggested that the more primitive apocritan (wasp-waisted) Hymenoptera develop from an egg that has been placed on an immobilized, concealed host by the adult parent. It is this habit of the female – providing the food for the offspring by choosing and immobilizing the host – that distinguishes the higher Hymenoptera from other carnivorous insects that simply lay their eggs near a food source but leave the task of seeking out and attacking the food to the larva. The hymenopteran habit of stinging, permanently immobilizing and rather rapidly consuming the larval food source is termed idiobiosis. Insects that do this are idiobionts. Most idiobionts feed externally on their hosts and they are referred to as ectoparasitoid idiobionts. The few that feed internally within the host are said to be endoparasitoids. Many species of ichneumonids, including the subfamilies Pimplinae, Rhyssinae, Xoridinae, Labeninae and Cryptinae, are ectoparasitic idiobionts. These ichneumonids typically attack concealed hosts, such as the larvae or pupae of endopterygote insects that are in tunnels and mines within plant tissue, or in galls. Most commonly, species of Lepidoptera, Symphyta, Diptera or Coleoptera are exploited as hosts. Many of the morphological adaptations shown by these idiobiont ichneumonids are specializations that enable them to gain ovipositional access to these concealed hosts (Gauld, 1988a). Since different ichneumonid species attack different hosts in subtly different locations, it is often relatively easy to discriminate the various ichneumonid species by differences in the length, shape and structure of the ovipositor

The adult female idiobiont ichneumonid permanently paralyses the host with venom secreted by a gland associated with the ovipositor and injected during the ovipositional sequence. This ensures the host does not damage the delicate parasitoid egg. When the egg (which is usually rather large) rapidly hatches, the food source, alive but immotile, is free from decay. The parasitoid larva simply has to consume a defenseless bag of nutritive fluid and tissue. This type of behavior is relatively undemanding physiologically – i.e. the ichneumonid larva does not have to contend with any reaction against it by the host's immunodefensive system. Physiological constraints consequently have little to do with limiting host ranges. The species of host that the ichneumonid might encounter in a certain situation is therefore probably unimportant, so it is not surprising to find that some idiobionts have host ranges that embrace species belonging to several different orders. For example, Scambus sagax is known to attack species of Lepidoptera, Symphyta and Coleoptera associated with resinous galls, shoots or cones of conifers, whilst Endromopoda detrita has been reared from hymenopterous, lepidopterous and dipterous stem-borers of Poaceae (Fitton et al., 1988). In behavioral terms, host habitat location (a response to a chemical stimulus arising from the immediate environment the host occupies), seems to predominate over host location (a response to a chemical stimulus produced by the host itself).

e. Derived ectoparasitic traits.
Specialized lineages that exploit other categories of hosts, such as the nests of aculeate Hymenoptera or cocoons in concealed situations, have arisen in parallel amongst the Pimplinae, Labeninae and Cryptinae. It has been hypothesized that such associations arose by the ancestors of these parasitoids encountering these hosts in plant tissue (e.g. aculeates nesting in the tunnels of bark beetles or various pupae concealed in crevices in wood) and then becoming progressively more receptive to a cue emanating from the host – such as the saliva used by aculeates in nest-capping (Matsumoto, 2005), or the presence of chemicals in silk (van Baarlen et al., 1996; Thibout, 2005). This has been shown to be plausible by repeated experiments, in which a researcher has removed a potential host from its construct and the parasitoid shows no interest whatsoever (Smithers, 1956). It is therefore assumed that host-location became a more important behavioral step than host habitat-location. Over evolutionary time, species in these lineages became able to exploit similar hosts occurring in a variety of new and different habitats – such as bees nesting in tunnels in the ground, or sphecids in mud nests under rock overhangs (Gauld, 1988a). Groups of idiobionts have thus radiated to exploit nest-building aculeates and cocoons in a wide variety of situations. Cocoons are a huge potential resource, and the enormous ichneumonid tribe Phygadeuontini has radiated to predominantly utilize cocooned immature insects. A few phygadeuontines and some pimplines utilise cocoon-like structures, such as the egg sacs of spiders (Austin, 1985), and in the latter case this seems to have been the origin of an ichneumonid group (the Polysphincta genus-group of the Pimplinae) which attack spiders (Gauld & Dubois, 2006). Not surprisingly, idiobionts that attack these types of host show a higher degree of host specificity than, for example, the Scambus and Endromopoda species mentioned above. Often they are restricted to attacking constructions made by insects of only one or two families, although many are actually capable of developing on whatever is in the host construction, even if it is the prepupa of another parasitoid. Ichneumonids which attack both a host and its primary parasitoids are facultative hyperparasitoids, in contrast to true hyperparasitoids: parasitoids that specifically seek out another parasitoid.

f. Evolution of endoparasitism and koinobiosis.
Although a large proportion of all hymenopterous idiobionts are ectoparasitic, endoparasitic idiobiosis does occur but it is relatively uncommon. This type of host association is more frequent in the Ichneumonidae than in any other family. Some very common species of Pimplinae (such as the ubiquitous Pimpla), a few species of Cryptinae, and many species of Ichneumoninae are all endoparasitic idiobionts attacking the relatively exposed hard pupae of certain Lepidoptera. The evolution of this life-way, probably from cocoon-attacking ectoparasitic ancestors, seems to have been facilitated by the presence of the thick cuticle of these weakly cocooned host pupa. It is assumed that this cuticle offers the ichneumonid larva the necessary protection for its development, protection no longer accorded it by the much reduced cocoon. At its simplest, endoparasitic idiobiosis is merely feeding on an incapacitated host from within, rather than from the outside. However, such an intimate physical association with a host confronts the endoparasitoid with problems that ectoparasitoids avoid. Most notable of these is the host immunodefensive system. This must be overcome, otherwise the parasitoid egg or larva will be encapsulated (Salt, 1968; 1975). Endoparasitic idiobionts achieve this in a variety of ways, including destruction of the host's brain by the newly hatched parasitoid larva (Führer & Kilincer, 1972), by injection of venoms by the adult, or by placement of the egg in a site where it does not encounter host haemocytes (Führer, 1975; Carton, 1978; Osman, 1978).

Many insect larvae, such as the caterpillars of most larger moths, feed in an exposed situation, but they move to a substantially more concealed pupation site upon completion of feeding. If a feeding larvae was attacked by an idiobiont, it would be immobilized in a position where both it and its parasitoid would be highly vulnerable to scavengers, predators and other parasitoids. Such exposed hosts are generally not attacked by idiobionts until they are in concealment. There is, however, a clear selective advantage for any parasitoid that can oviposit onto the easily discoverable larval host stage but postpone permanent immobilization until after the host is in the pupation retreat. The retreat's protection will be afforded to the developing parasitoid as well. The strategy of allowing a host larva to develop for a time after oviposition is called koinobiosis (Askew & Shaw, 1986), It is widespread throughout the Ichneumonidae. Both ectoparasitoid and endoparasitoid forms of koinobiosis are known in the Ichneumonidae, although the former is a fairly uncommon life way.

Ectoparasitoid koinobiosis seems to have evolved separately in three subfamilies: the Adelognathinae, the Tryphoninae and the Polysphincta genus-group of the Pimplinae. At its simplest, as it occurs in the Adelognathinae, there is an apparent evolutionary transition from idiobiosis to incipient koinobiosis. One Adelognathus species develops as an idiobiont ectoparasitoid of a sawfly host concealed in plant tissue (Kopelke, 1987), another develops as an idiobiont ectoparasitoid of an exposed host (Rahoo & Luff, 1987), but several species are koinobiont ectoparasitoids of weakly concealed hosts (Fitton et al., 1982). In all cases, the adelognathine egg is simply glued on to the sawfly host and the emergent ichneumonid larva consumes the host quite rapidly. When fully grown, the adelognathine larva leaves the host and constructs a cocoon, either on the ground or in the host larval retreat (if present). This is a particularly unspecialized form of koinobiosis, since the parasitoid fails to exploit the abilities of the host to construct a more secure pupation retreat. Such a habit may have arisen because the parasitoid larva has been unable to avoid being discarded at the host larva's penultimate molt. Sawflies are particularly effective at being able to divest themselves of larval ectoparasitoids; unlike many other insects with caterpillar-like larvae, sawflies usually have a non-feeding final larval instar. When the sawfly larva is fully fed, it sheds its cuticle (and ectoparasites) before retreating to a pupation site.

A more specialized form of koinobiont ectoparasitism arose in the ancestors of the Tryphoninae. Tryphonines have a highly modified egg which is attached to the host by means of a stalk and anchor mechanism (Mason, 1967; Kasparyan, 1981), which apparently acts as an adaptation to retain purchase on a larval sawfly host during its penultimate molt. For example, Idiogramma species are koinobiont ectoparasitoids of xyelid sawfly larvae concealed in plant tissue, but idiogrammatines do not complete development until after the xyelid larva has vacated its mine in the host plant (immediately prior to which the xyelid molts) and has constructed a subterranean pupation retreat. Leaving the plant to pupate underground is advantageous for the host as it avoids being attacked as a prepupae or pupa by idiobiont ectoparasitoids. Molting prior to quitting the host-plant is selectively advantageous as it enables the xyelid to escape from primitive koinobiont ectoparasitoids. By circumventing the problem of ecdysis tryphonines have benefited from the first of their host's parasitoid avoidance strategies – they too escape the attentions of (facultatively hyperparasitic) idiobionts ovipositing through plant tissue.

Apart from avoiding being sloughed off during the host's penultimate molt, there are several other problems that koinobiont ectoparasitoids have had to overcome: vulnerability to attack by the host, and being subject to environmental factors such as desiccation. Tryphonines overcome the first problem by generally anchoring their eggs just behind the host's head, so the host cannot reach them with its mandibles (Baltensweiler & Moreau, 1957). In dry conditions the anchor stalk of the tryphonine egg may break (Morris et al., 1937), and so many tryphonines are restricted to attacking hosts in humid places or in partial concealment (Kasparyan, 1981). Some are nocturnal (e.g. Netelia spp.) and probably attack larvae that have completed feeding and are searching for pupation retreats (Vance, 1927; Schaff, 1972). The subfamily Lycorininae also has "stalk and anchor" eggs, and has avoided host attack and environmental factors by apparently attaching the egg to the cuticle inside the host's hind gut, using the ovipositor to probe up the anus (Shaw, 2004).

A unique and highly specialized form of ectoparasitoid idiobiosis occurs in the Polysphincta genus-group of the Pimplinae. The adult ichneumonid stings and temporarily immobilizes a spider on its web, and then carefully positions an egg on the immotile spider after first removing any other eggs that may be present. In many species, the ovipositor is only used for stinging the spider and the egg is extruded from the ovipositor's base. The spider recovers, continuing to feed and construct webs whilst the ichneumonid larva slowly grows, feeding through a minute integumentary lesion. The developing larva apparently produces a secretion that modifies the host's web-building behavior so that the spider constructs a web suitable for the wasp's pupation (Eberhard, 2000; Matsumoto, in press).

g. Endoparasitism and the problems with koinobiosis.
Many of the problems confronting koinobiont endoparasitoids can be circumvented if the parasitoid places its egg inside the host. In such a position, the egg, and subsequently the young larva, cannot be dislodged or shed during ecdysis, and is protected from direct environmental threats such as dehydration. Furthermore, the egg itself can absorb nutrients from the host, transferring some of the cost of egg-development from parasitoid to host, and thus facilitating the production of a larger number of eggs. Endoparasitoids are free to exploit the very large numbers of endopterygote larvae which feed in relatively exposed positions. Once freed from the constraints of ectoparasitic species, endoparasitoids may attack their hosts quite early in the host's developmental cycle. In some cases, e.g. in species of Collyriinae, some Diplazontinae and Stilbopinae and a few Campopleginae, they may even oviposit into the embryo in the host egg (Salt, 1931; Hummelen, 1974; Rotheray, 1981). Endoparasitoid koinobiosis occurs in the majority of ichneumonid subfamilies, including the most derived pimpliform taxa (Acaenitinae, Diplazontinae and the Orthocentrinae), all the ophioniform subfamilies (i.e., Anomaloninae, Banchinae, Campopleginae, Cremastinae, Ctenopelmatinae, Mesochorinae, Metopiinae, Ophioninae, Stilbopinae and Tersilochinae), and the Collyriinae. It also is found in many Ichneumoninae and a few Cryptinae.

As mentioned previously, endoparasitoid koinobiosis necessarily involves a close physiological interaction between the parasitoid larva and its host. The host immune system is a major evolutionary barrier that endoparasitoids have had to overcome. The parasitoid has to prevent the host defenses from destroying it, yet at the same time it must not incapacitate the host lest both perish. Selective evolutionary pressures on koinobiont endoparasitoids could be primarily physiological, perhaps explaining why there is often very little obvious structural difference in extremely speciose genera such as Enicospilus, Lissonota, Glypta, Mesochorus, Orthocentrus, Campoplex and Dusona - all of which are cosmopolitan and each likely to comprise more than 2,000 species worldwide.

Koinobiont endoparasitoids have a variety of mechanisms to circumvent the defenses of the hosts (Salt, 1968; Vinson & Iwantsch, 1980; Guzo & Stoltz, 1987), but a number of these only work well against a very few hosts (Salt, 1975; Vinson & Stoltz, 1986; Ohsaki & Sato, 1990). Consequently the host range of koinobiont endoparasitoids is generally narrow in comparison to those of an idiobiont (Sheehan & Hawkins, 1991). For example, the endoparasitic koinobiont Enicospilus americanus only parasitizes a few species of Saturniidae (Gauld, 1988b), whilst the closely related tropical species E. lebophagus is apparently restricted to just a single saturniid species, Rothschildia lebeau (Gauld, 1988c). Hyposoter PRO-3 has been reared on 114 of 115 occasions from a single species of Dyscophyllus (Hesperiidae) (Janzen & Hallwachs, 2008). In general, koinobiont endoparasitoids only attack hosts belonging to one or two families, or only a few species of one family.

The hosts of a great many koinobiont endoparasitic ichneumonids are the larvae of Lepidoptera (Banchinae, Ophioninae, most Anomaloninae, Stilbopinae, many Camplopleginae, Metopiinae, most Cremastinae, Ichneumoninae, and a few Ctenopelmatinae), and to a lesser extent, particularly in north temperate regions, the larvae of Symphyta (Collyriinae, most Ctenopelmatinae, some Campopleginae, a few Tersilochinae). Despite their huge diversity, the Coleoptera are attacked by only a relatively few specialized lineages of koinobiont ichneumonids: the Phrudinae, most Tersilochinae, a few Cremastinae, Anomaloninae and Acaenitinae, and a very few Campopleginae. Similarly the hyperdiverse order Diptera are attacked by only two koinobiont ichneumonid evolutionary lineages: the derived pimpliformes (Cylloceriinae, Orthocentrinae and Diplazontinae (Wahl, 1990)) - a lineage that only exploits the families Syrphidae, Tipulidae, Mycetophilidae and Sciaridae - and the stilpnine Cryptines which are larval/pupal parasitoids of cyclorrhaphous Diptera.

h. Gregariousness, secondary predation and hyperparasitism.
The overwhelming majority of Ichneumonidae are solitary parasitoids, that is to say, a single individual develops by consuming an entire host individual. Superparasitism occurs when more than one parasitoid is present on or in a host. In this case, the parasitoid larvae fight until only one remains. Fighting is not inevitable case and in some species multiple individuals do develop on a single host. Such gregarious parasitism is found mostly in idiobionts (such as Acropimpla, Agrothereutes, Gambrus, Ischnocerus and Sericopimpla (Shaw, 1999). It is less common amongst koinobionts where such behavior has evolved only in a few species of Adelognathinae, Tryphoninae (some Netelia), Banchinae (some Lissonota and Diradops) and Campopleginae (some Enytus and Olesicampe) (Shaw, 1999). The reverse situation, where a single parasitoid requires more than one host individual to complete its development, is found only in a few idiobionts. Examples are Aritranis spp. (Cryptinae) which attack a succession of aculeate pupae in adjacent nests in borings (Daly et al., 1967), Thrybius togashii (Cryptinae) which develops by consuming a large number of Tetramesa (Eurytomidae) larvae in internodal cavities of reeds (Phragmites spp.) (Matsumoto & Saigusa, 2001) and Tromatobia spp. (Pimplinae) which consume multiple eggs in a single egg-sac (Fitton et al., 1988). Strictly speaking such organisms are predators rather than parasitoids.

Whilst most ichneumonids develop on a single host as primary parasitoids, a few idiobionts may be facultative or even obligate hyperparasitoids. The tendency towards hyperparasitism is most pronounced in the endoparasitic koinobiont subfamily Mesochorinae where all species apparently develop as hyperparasitoids, usually of a braconid larva (the primary host) within a phytophagous larva (the secondary host).


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