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Volume 2012 (2012), Article ID 134746, 24 pages
Diversity of Species and Behavior of Hymenopteran Parasitoids of Ants: A Review
1Departamento de Entomología Tropical, El Colegio de la Frontera Sur, Avenida Centenario km 5.5, 77014 Chetumal, QRoo, Mexico
2Centre de Recherches sur la Cognition Animale, CNRS-UMR 5169, Université de Toulouse, UPS, 118 route de Narbonne, 31062 Toulouse Cedex 09, France
Received 3 October 2011; Accepted 28 October 2011
Academic Editor: Alain Lenoir
Copyright © 2012 Jean-Paul Lachaud and Gabriela Pérez-Lachaud. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Reports of hymenopterans associated with ants involve more than 500 species, but only a fraction unambiguously pertain to actual parasitoids. In this paper, we attempt to provide an overview of both the diversity of these parasitoid wasps and the diversity of the types of interactions they have formed with their ant hosts. The reliable list of parasitoid wasps using ants as primary hosts includes at least 138 species, reported between 1852 and 2011, distributed among 9 families from 3 superfamilies. These parasitoids exhibit a wide array of biologies and developmental strategies: ecto- or endoparasitism, solitary or gregarious, and idio- or koinobiosis. All castes of ants and all developmental stages, excepting eggs, are possible targets. Some species parasitize adult worker ants while foraging or performing other activities outside the nest; however, in most cases, parasitoids attack ant larvae either inside or outside their nests. Based on their abundance and success in attacking ants, some parasitoid wasps like diapriids and eucharitids seem excellent potential models to explore how parasitoids impact ant colony demography, population biology, and ant community structure. Despite a significant increase in our knowledge of hymenopteran parasitoids of ants, most of them remain to be discovered.
Ants are distributed all over the world, and their colonies provide both a stable food resource and numerous niches for thousands of other organisms, termed myrmecophiles, that exhibit a diverse array of relationships with their hosts [1–7]. Among myrmecophiles, numerous species of hymenopterans are associated with ants through predation, parasitism on the brood and/or adults, cleptoparasitism, parabiosis, mimetism, true symphily, or indirect parasitism through trophobionts and/or social parasites. However, in most cases, the precise nature of their relationship with their ant hosts remains obscure.
A review of the diversity of parasitoid wasps attacking ants has not been attempted since the work of Schmid-Hempel . In his extensive review of the parasites of social insects, he pointed out the wide variety of hymenopteran parasitoids that attack these insects but, with the exception of the family Eucharitidae (with 33 valid species really involved), his list provided very few other examples (only 10) of true parasitoidism, that is, cases where the attack of the wasp species on ants (adults or brood) has been reliably demonstrated.
Knowledge has increased greatly in the intervening years, and numerous cases of parasitic associations involving wasps and ants have been reported. Moreover, changes in nomenclature and phylogeny have been numerous in the last two decades (see, e.g., [8–15]), and many species names of both the parasitoids and their ant hosts required emendations.
In the present paper, we address only hymenopteran parasitoids and focus strictly on ant-parasitoid wasp associations in which parasitism has been established beyond any doubt, and where ants are proved to be the primary hosts. Therefore, no bethylid species are considered here even though various members of the genera Pseudisobrachium and Dissomphalus are strongly suspected of being parasitoids of ant brood [16–18]. Neither are any species of ceraphronid, dryinid, figitid, platygastrid, proctotrupid, or pteromalid wasps considered although several species belonging to the genera Ceraphron, Conostigmus, Gonatopus, Kleidotoma, Platygaster, Exallonyx, and Spalangia are known to be associated with ants, most of them probably as parasitoids [2, 19–24]. All of these species were omitted from the present paper because wasps have not been reliably reared from ants or their brood. Moreover, according to the definition of “parasitoid” which implies the killing of a single host, associations such as those involving numerous sphecid species, particularly those of the genera Aphilanthops, Clypeadon, and Tracheliodes, which are known to specialize with preying on and storing numerous adult ants (of the genus Formica, Pogonomyrmex, or Liometopum, resp.) [25–27] are not dealt with. Likewise, the highly interesting associations of ants with some braconid species such as Compsobraconoides sp.  and Trigastrotheca laikipiensis Quicke , which are known to consume various stages of their ant hosts (Azteca spp. and Crematogaster spp., resp.) during their development, are not covered in the present paper.
In spite of such restrictions, the list of hymenopteran species reliably involved in parasitic associations with ants remains impressive and represents more than a quarter of all of the hymenopteran species known to be associated with ants . Here, we attempt to provide an overview of both the diversity of the species of parasitoid wasps known to attack ants and the diversity of the interactions they have developed with their hosts. By so doing, we also call attention to this little known biodiversity.
2. Checklist of Hymenopteran Parasitoids of Ants
Records of associations of hymenopteran wasps with ants involve more than 500 wasp species , but only a fraction have unambiguously been reported as parasitoids. The term parasitoid applies to organisms whose juvenile stages are parasites of a single host individual, eventually sterilizing, killing, or even consuming their host, while the adult parasitoid is free living . With few exceptions, female parasitoid wasps oviposit on or inside the body of their host, typically another arthropod, and all stages of development of the host are susceptible to attack. After hatching, the parasitoid larva feeds on the host’s tissues, gradually killing it. A survey of the literature since 1852 and some of our own unpublished results have allowed us to identify at least 138 species (see Table 1 and Supplementary Material available online at http://dx.doi.org/10.1155/2012/134746) reported as primary endo- or ectoparasitoids of larvae, pupae, or adult ants. All of these species are included in 3 superfamilies: Chalcidoidea (with 6 families concerned), Ichneumonoidea (2 families), and Diaprioidea (only 1 family) (Table 1). In 2007, Sharkey  estimated that there were approximately 115,000 described species of Hymenoptera (perhaps up to 1,000,000 if undescribed species—especially species of parasitoid wasps—were included), and that Chalcidoidea and Ichneumonoidea were the most species-rich superfamilies among the parasitoid hymenopterans. So, it is not surprising that most of the parasitoid wasps attacking ants belong to these two superfamilies, especially the Chalcidoidea which alone includes more than 70% of all of wasp species parasitizing ants registered until now.
In the following text, we follow Sharkey  for the higher-level phylogeny of the order Hymenoptera (see also ). The taxonomic validity of the scientific names is in accordance with Bolton [8, 9] for ants, and with different databases available on the web for other hymenopterans: Hymenoptera Name Server (version 1.5) (http://osuc.biosci.ohio-state.edu/hymDB/nomenclator.home_page), Global Name Index (version 0.9.34) (http://gni.globalnames.org/name_strings), Universal Chalcidoidea Database  (http://www.nhm.ac.uk/chalcidoids), and Home of Ichneumonoidea (version 2011) (http://www.ichneumonoidea.name/index.php). Authors of all scientific names are given throughout the text only when they are not already reported in Table 1.
The superfamily Diaprioidea is a monophyletic group, with 4 recognized families , and accounts for more than 4000 species around the world in over 210 genera [8, 155–157], almost all in the family Diapriidae. Most diaprioids are primary endoparasitoids of dipterans (eggs, larvae, or pupae), but several species are known to attack Hymenoptera, Homoptera, or Coleoptera, and some are facultative or obligate hyperparasitoids. Some of the species attacking Diptera have been considered as potential biological control agents, but their efficiency has not been demonstrated [157, 158].
Despite their number, the members of this large family are relatively unknown and less than half of the 4000 species estimated to occur worldwide have been described [8, 156, 159]. Three subfamilies are currently recognized: Ambositrinae, Belytinae, and Diapriinae . Their biologies are diverse, but most species are primary parasitoids of puparia of Diptera [156–160].
Although some diapriids have only occasionally been found in ant nests, a number of species are closely associated with ants (all belonging to the Belytinae and Diapriinae subfamilies). However, there are few behavioral data on host-diapriid myrmecophile interactions (but see ). These symphyles are often highly adapted to their hosts, exhibiting morphological and behavioral adaptations to living with ants (extensive morphological mimicry of the host ants—coloration, ocellus regression, convergence in sculpture—, presence of appeasement substances in specialized structures and trichomes, trophallaxis, etc., [161–166]), which presumably aid them in avoiding detection and/or aggression by host ants . The adaptations can include secondary apterism in which the wings of the wasps are assumed to have been bitten off by either the parasite itself or its host (e.g., Mimopria, Bruchopria, Lepidopria, and Solenopsia, [156, 161, 164, 167, 168]). Most often, the presence of a diapriid in an ant nest is suspected to be just circumstantial  and related to its search for dipterous hosts, such as Tetramopria aurocincta Wasmann found in nests of Tetramorium caespitum (L.) . This wasp is in fact a parasitoid of the puparia of Compsilura concinnata Meigen (Diptera: Tachinidae), a primary parasite of the lepidopteran Hyphantria cunea (Drury) . Occasionally, diapriids enter ant nests for temporary shelter since some species hibernate in the host nest as do Solenopsia imitatrix Wasmann and Lepidopria pedestris Kieffer in the nests of Solenopsis fugax (Latr.) [37, 164].
Only a few diapriids are true parasitoids of ant brood. Ever since the pioneering work of Wasmann in 1899 , most diapriids found in ant nests were assumed either to parasitize insect myrmecophiles (dipteran or coleopteran) inside the host nest or, less frequently, to be primary parasitoids of ant larvae. However, the first record of a diapriid positively reared from ant brood was reported just in 1982 by Lachaud and Passera , who reared Plagiopria passerai from cocoons of queens of the formicine Plagiolepis pygmaea (Figures 1(a) and 1(b)). As far as known, diapriid parasitoids attacking ants develop as solitary or gregarious, koinobiont endoparasitoids of the host larvae [34, 36, 38, 169], and worker and/or reproductive immature stages can be parasitized [37, 169, 170]. Ramos-Lacau et al.  observed oviposition of Acanthopria sp. in young ant larvae under laboratory conditions. Late parasitized larvae are easily recognized by their dark coloration, compared to nonparasitized larvae, due to the developing wasp visible through the cuticle [35, 36, 38]. Worker ants do not discriminate between parasitized and nonparasitized larvae [35, 38, 169], but adult parasitoids are aggressively attacked by their hosts under laboratory conditions [35, 36].
From the 121 diapriine species in 34 genera that have been collected in association with ants , development of immature stages as parasitoids of ant larvae has been demonstrated for only 26 species in 7 genera, most of which are only known at the level of morphospecies (Table 1): 15 species of Acanthopria, 3 of Mimopriella, 1 of Oxypria, 1 of Plagiopria (P. passerai), 4 of Szelenyiopria, and 2 of Trichopria (T. formicans and Trichopria sp.) [34–38, 169, 170]. The ant hosts of these diapriines belong to 8 species in only 4 genera: the myrmicine fungus-growing ants Cyphomyrmex, Trachymyrmex, and Acromyrmex and the formicine Plagiolepis. Fifteen species of Belytinae belonging to 11 genera have also been reported from ant nests [30, 171–173], but none has been reliably reared from the ants, and their actual relationship with their hosts remains unknown.
In some cases, the rate of parasitism can reach high levels. Two recent studies have provided important details of the biology of diapriids and have also investigated their impact on ant-host populations. Fernández-Marín et al.  found that between 27 and 70% of the colonies of 2 species of Cyphomyrmex were parasitized by one species in Puerto Rico and by up to 4 concurrent morphospecies of diapriids in Panama. Similarly, the work of Pérez-Ortega et al.  showed that another fungus-growing ant, Trachymyrmex cf. zeteki, was attacked by a diverse community of diapriids in Panama, with a mean intensity of larval parasitism per ant colony of 33.9%, and a prevalence across all ant populations of 27.2% (global data for all 6 diapriid morphospecies present at the study site).
The superfamily Chalcidoidea is considered as one of the most abundant, species-rich, and biologically diverse groups of insects with 23,000 species described and a conservative estimation of about 400,000 to 500,000 species in over 2040 genera distributed in 19 families [32, 174–178]. Though some species are phytophagous, most Chalcidoidea are parasitoids of other insects, and numerous species are currently used as biological control agents against insect pests.
Chalcididae is a moderate-sized family with more than 1450 species and over 85 genera. Chalcids are primary parasitoids of Lepidoptera or, to a much lesser extent, of Coleoptera, Diptera, Hymenoptera, and Neuroptera, and various species are hyperparasitoids of other hymenopterous parasitoids . Most often they parasitize host larvae or pupae, but a few species can parasitize eggs.
Very few species, like Epitranus chilkaensis (Mani) (referred to as Anacryptus chilkaensis) found with the formicine Camponotus compressus (Fabr.) in the Barkuda Island (India) , are known to be associated with ants [179, 181], but true parasitoidism has rarely been documented. Only species of the genus Smicromorpha seem to be specialized as parasitoids of the larvae of the green ant, Oecophylla smaragdina. The only unquestionable (see ) record of parasitoidism is that of Dodd in the early 20th century, describing Smicromorpha doddi in North Queensland (Australia) parasitizing larvae of this weaver ant, “depositing eggs upon them when the workers are using their silk-spinning larvae for the purpose of binding the leaves together when building a new nest” . No other example of true parasitoidism has ever been quoted for the genus Smicromorpha but, more recently, adults of another species of this genus, S. masneri, were reported emerging from O. smaragdina nests collected in Vietnam and maintained in controlled green-house conditions in the USA, which strongly suggests that these wasps are also primary parasitoids of weaver ants . Moreover, two other species, S. keralensis  and S. minera , have been observed hovering over nests of O. smaragdina in India and Australia, respectively, a behavior likely to be related to parasitism of ants (see below under Braconidae and Ichneumonidae). For such reasons, all these members of the genus Smicromorpha can reasonably be suspected of being true parasitoids of the larvae of this ant host and were included in our list (Table 1).
Encyrtidae is a large family of parasitic wasps, currently including more than 460 genera and 3700 species, and is one of the key chalcidoid families for the biological control of insect pests [178, 182, 183]. Most encyrtids are primary endoparasitoids of immatures or, less commonly, adults of Coccidae and Pseudococcidae; others are hyperparasitic through other hymenopterous parasitoids, and some can attack insects in other orders, mites, ticks, or spiders [184, 185]. Some species are polyembryonic, a single egg multiplying clonally in the host, producing large numbers of identical adult wasps.
At least 25 species of encyrtid wasps representing 16 genera are known to be indirectly associated with ants through primary parasitism of the trophobionts they exploit and protect ; for example, the species Anagyrus ananatis Gahan is indirectly associated with the ant Pheidole megacephala through the trophobiotic Pseudococcidae present in their nest . However, very few encyrtids have been reported as directly associated with ants. Apart from Taftia prodeniae Ashmead, which was found to exhibit a phoretic association (wasps were found clinging to the ant’s antennae) with the dolichoderine ant Dolichoderus thoracicus (F. Smith) (referred to as D. bituberculatus (Mayr)) , and an unidentified species recently reported from a refuse deposit of the ecitonine ant Eciton burchellii , only Holcencyrtus wheeleri (Ashmead) (referred to as Pheidoloxenus wheeleri), found in nests of the myrmicine ants Pheidole tepicana Pergande (referred to as P. instabilis)  and P. ceres Wheeler (referred to as P. ceres var. tepaneca Wheeler) , has been suspected of being “probably also entoparasitic on these ants or their progeny during its larval stages” . However, the parasitic relationship was never proved. Only very recently a Neotropical, gregarious endoparasitoid species, Blanchardiscus sp. (?pollux) (determination by J. S. Noyes), was recorded from French Guiana attacking pupae of the ponerine ant Pachycondyla goeldii  and thus constitutes the first true case of parasitism on ants for this family. However, no information has yet been published, and the exact identification of the species still needs to be confirmed.
This is a small family but the largest and most diverse group of hymenopteran parasitoids attacking ants since all of its members, where the host is known, parasitize ant brood [11, 66, 72, 78, 83, 189–191]. Fifty-three genera and more than 470 species are currently described and distributed in three subfamilies: Oraseminae, Eucharitinae, and Gollumielinae.
All of the species have a highly modified life cycle [63, 66, 76, 83, 108]. Like the Perilampidae  and the ichneumonid species Euceros frigidus , but unlike most parasitic wasp species, eucharitid females deposit their eggs away from the host nest, in or on plant tissue (leaves and buds) [72, 189] (Figures 2(a) and 2(b)), and the very active, minute (less than 0.13 mm), strongly sclerotized first-instar larva is termed a “planidium” (Supplementary material 2 available online at http://dx.doi.org/10.1155/2012/134746). It is responsible for gaining access to the host ant brood by using various phoretic behaviors including either attachment to an intermediate host (as in some orasemine species [11, 72, 83, 86, 88, 93] and, possibly, in Gollumiella antennata (Gahan) ( but see ) or, more generally, to foraging ant workers. On occasions (as is apparently the case for Pseudochalcura gibbosa and Gollumiella longipetiolata), attractive substances are suspected to be present in or on the eggs [46, 72]. Within the nest, the planidium attaches itself to an ant larva (Figures 2(c) and 2(d)): Eucharitine planidia attach externally to the host larva, whereas orasemine and gollumielline planidia partially burrow into the host larva, in the thoracic region just posterior to the head capsule [11, 70, 72]. All of the Eucharitidae develop as koinobiont, larval-pupal ectoparasitoids. At molting of the host larva, the planidium migrates to the ventral region, just under the legs (Figure 2(e)), of the newly formed ant pupa for further development which is only completed when the host pupates [76, 83, 93, 189] (Supplementary material 3 available online at http://dx.doi.org/10.1155/2012/134746). In general, only one parasitoid develops per host but, occasionally, more than one adult eucharitid can develop in a single host (superparasitism) (Figure 2(f)) [72, 83], especially when larger brood (sexual brood) is parasitized [193, 194], and one exceptional case of multiparasitism involving two different species from two different eucharitid genera (Dilocantha lachaudii and Isomerala coronata) has even been reported from a single pupa of the ectatommine ant Ectatomma tuberculatum . In almost all of the cases, adults emerge among ant brood (but see ), and, even if in some cases they are well treated within the nest by their hosts (as is the case for Orasema coloradensis which is transported, cared for, and even fed by the workers of Pheidole bicarinata ), they have to leave the host nest to reproduce. Ants show only moderate aggression to newly emerged eucharitids [58, 70, 75, 106, 189, 195, 196], suggesting passive or active chemical mimicry of the host ants [58, 75, 195]. If the parasitoid wasps do not exit their host nest by themselves, ant workers transport them outside (Figure 2(g)) as if they were refuse [58, 77, 196], ultimately enhancing wasp dispersal. Parasitism is very variable and localized in time and space [106, 193, 194]. A very high local prevalence may lead to only a low impact at the regional scale, suggesting that these parasitoids do not have a major influence on the dynamics of their ant host population .
According to Heraty , the hypothesized phylogeny of Eucharitidae is highly correlated with the subfamilies of their ant hosts and responsible for differences in behavior related with egg placement, activity of the planidium, and access to the ant host. Oraseminae (Orasema, Orasemorpha, and Timioderus) primarily attack myrmicine ants (numerous species of Pheidole and Solenopsis, and some species of Diplorhoptrum, Monomorium, Temnothorax, Tetramorium, and Wasmannia, see Table 1), and exceptionally formicines (Formica subnitens and F. oreas comptula in the case of O. coloradensis, ) or ecitonines (Eciton quadriglume in the questionable case of O. rapo, [11, 83]). For Eucharitinae, the only two host records for the tribe Psilocharitini (Neolosbanus) concern the ponerine genus Hypoponera , while the numerous members of the tribe Eucharitini are essentially parasitic on medium to large ponerines (Pachycondyla, Odontomachus, and Dinoponera) and ectatommines (Ectatomma, Gnamptogenys, Typhlomyrmex, and Rhytidoponera), but also on myrmeciines (Myrmecia) and numerous formicines (Anoplolepis, Calomyrmex, Camponotus, Cataglyphis, Formica, Lasius, and Polyrhachis); without exception, all of the scarce records of associations of eucharitines with myrmicine ants (Messor, Myrmica, and Pogonomyrmex) are highly doubtful (Table 1). Finally, the only host record for the Gollumiellinae concerns a formicine (Paratrechina).
The hosts of most eucharitid genera seem to be restricted to only one or a few closely related ant genera and, for a long time, all species were considered as host-specific parasitoids, at least at the host genus level . However, recent results [76, 78, 79] raised questions concerning the degree of host specificity in eucharitids and about the factors that determine the association of these parasitoids and their hosts. Results in the guild of eucharitid parasitoids associated with ponerine ant species in southeastern Mexico and French Guiana suggest that some eucharitid wasps tend to be oligophagous in their host choice: some eucharitid species can attack different hosts from different genera and different subfamilies such as Kapala iridicolor, which parasitizes one species of Ectatomma, two of Gnamptogenys, and one of Pachycondyla [76, 78]. Furthermore, concurrent parasitism has been reported for Ectatomma tuberculatum, which is simultaneously parasitized by Dilocantha lachaudii, Isomerala coronata, and Kapala sp. , or for E. ruidum parasitized by two Kapala species, K. iridicolor, and K. izapa [76, 193].
The family Eulophidae is the largest of the Chalcidoidea with up to 4470 species in 297 genera. The majority of the species are primary parasitoids attacking a large variety of insects (mainly Lepidoptera and Coleoptera, but also Diptera, Thysanoptera, and Hymenoptera), and occasionally mites or spiders. Many species are facultative or obligate hyperparasitoids of other Hymenoptera, and some are even phytophagous. Entomophagous larvae can develop as koino- or idiobionts, gregarious or solitary, and ecto- or endoparasitoids, and according to the species, eulophids can attack eggs, larvae, pupae, or even the adults of their hosts .
Despite the large number of species in this family, parasitization of ants is uncommon among Eulophidae, and only few associations involving eulophid wasps and ant hosts have been reported to date. Almost all are from genera belonging to the subfamily Entedoninae. Three concern species indirectly associated with ants as they parasitize insects living in ant nests: Pediobius acraconae Kerrich which has been reported  from a last instar larva of the pyralid lepidopteran Acracona remipedalis Karsch found in a nest of Crematogaster depressa (Latr.) or C. africana Mayr in Nigeria, and both Microdonophagus woodleyi Schauff in Panama and Horismenus microdonophagus Hansson et al. in Mexico, which parasitize larvae of Microdon sp. syrphid flies living in nests of the dolichoderine Technomyrmex fulvus (Wheeler) (referred to as Tapinoma fulvum)  and of the formicine Camponotus sp. ca. textor , respectively. Three other species (two Entedoninae and a Tetrastichinae) have been reported associated with ant nests, but direct parasitism on the ant brood was not clearly established in any of these cases: Myrmobomyia malayana Gumovsky and Bouček with nests of an ant species of the genus Dolichoderus in Malaysia , an unidentified species of Horismenus from the bivouac and refuse deposits of the army ant Eciton burchellii , and an unidentified species of Tetrastichus from a nest of the formicine Myrmecocystus mexicanus Wesmael in Nevada .
In fact, only five species are known as true primary parasitoids of ants (Table 1). An unidentified gregarious parasitoid, apparently closely related to the genus Paracrias (according to Gahan in ), possibly Horismenus sp. , was recorded parasitizing larvae of the myrmicine Crematogaster acuta in Guyana, the prepupae of another unidentified species of Crematogaster were parasitized by Myrmokata diparoides  in Cameroon, Pediobius marjoriae was reared from cocoons of the formicine ant Lepisiota sp. in Uganda , and two species of Horismenus, H. floridensis and H. myrmecophagus, were found parasitizing the pupae of Camponotus atriceps and C. floridanus in Florida , and of the weaver ant Camponotus sp. ca. textor in Mexico , respectively. In the latter two cases, Horismenus larvae develop as gregarious endoparasitoids of the ant larvae (Figure 3), and large numbers of parasitoid individuals can develop from the same host: up to 21 for H. floridensis and between 4 and 12 for H. myrmecophagus. Finally, two other cases deserve to be added to this list since two other ant species have recently been found parasitized by eulophids: the ponerine ant Pachycondyla crenata (Roger) in Mexico and an unidentified species of Camponotus (Dendromyrmex) in French Guiana ; however, the identity of the parasitoids has not been confirmed yet.
Eurytomidae is a moderate-sized family with 90 genera and at least 1400 nominal species [13, 32, 200, 201]. Eurytomid wasps exhibit a wide range of biologies, but most of the larvae are endophytic either as seed or plant stem eaters or as parasitoids of gall formers or other phytophagous insects. Most species are primary or secondary parasitoids, attacking eggs, larvae, or pupae of various arthropods (Diptera, Coleoptera, Hymenoptera, Lepidoptera, Orthoptera, and Araneae).
A few species have been reported as indirectly associated with ants, like Eurytoma rosae Nees von Esenbeck found with Lasius flavus and Eurytoma sp. found with Formica (?) rufibarbis (misidentified as Polyergus rufibarbis) , but most probably these eurytomids only fed on the gall-forming cynipid larvae and/or on the gall tissue on Rosa spp. which are visited by these ant species, without any direct relationship with the ants. Recently, various adults of a new genus and species, Camponotophilus delvarei Gates, were found within nests of the weaver ant Camponotus sp. ca. textor , but the exact nature of their relationship with the ants remains unclear. As a matter of fact, only 3 or 4 species from the single genus Aximopsis (see Table 1) have been reported from Guatemala, Costa Rica, Guyana, Colombia, and Peru as parasitoids of queens of various species of dolichoderine ants (Azteca alfari, A. australis, A. constructor, A. pitieri, A. ovaticeps, and A. salti) and one formicine (Camponotus balzani), all of which colonize Cecropia spp. internode chambers by chewing a hole through a prostoma and entering the internode. The parasitoids attack only founding queens and feed on their host, while the internode chamber is sealed with parenchyma scraped from the internal stem walls [115, 116, 118]; there is never more than one wasp larva or pupa per foundress ant . Queen parasitization was thought to occur before they entered their dwellings (Bailey, in ); however, as suggested by Davidson and Fisher , the location of the ant host may occur through searching for host plants since female Aximopsis were observed to visit various seedlings, where they inspected newly sealed prostoma. This fact has been confirmed recently. A picture of an A. affinis female ovipositing through a prostoma into an Azteca queen at La Selva Biological Station, Costa Rica, was provided by Weng et al.  (their Figure 16). In this site, among the internodes that harbored Azteca ants, 43% contained dead queens, of which 13% contained A. affinis .
Perilampidae is a small family closely related to the Eucharitidae, composed of up to 270 species from 15 genera. A feature shared with Eucharitidae is that the first-instar larva, the “planidium”, is responsible for gaining access to the host, rather than the egg-laying female . Most species are hyperparasitoids on ichneumonid wasps or tachinid flies which are primary parasitoids of Hymenoptera or Lepidoptera, or parasitoids of wood-boring platypodid and anobiid beetles, and some species can attack Orthoptera, Neuroptera, or Hymenoptera [190, 204].
Association of perilampids with ants seems extremely casual. The only report deals with an unidentified species from Peru found parasitizing cocoons of the ponerine ant Pachycondyla luteola, inhabiting internode chambers of a Cecropia, with as many as nine perilampid wasps emerging from a single pupa of this ant . However, no other details were ever published, and the species apparently remained undescribed.
The superfamily Ichneumonoidea, with only two extant families, accounts for more than 40,000 species around the world, and there are estimated to be approximately 100,000 species [205–207]. Most are primary ecto- or endoparasitoids, idio- or koinobionts, especially attacking immature stages of a wide variety of insects and arachnids, and more occasionally adults. Some members use many different insects as hosts, and others are very specific in host choice. Various ichneumonoids are successfully employed as biological control agents in controlling insect pests such as flies or beetles.
This is a very large family with 48 subfamilies, more than 1050 genera and about 17,600 described species worldwide and exhibiting a variety of biologies [207–209]. The total number of species is estimated to be 40–50,000. Many braconids parasitize nymphal stages of Hemiptera, Isoptera, and Psocoptera; a few genera also parasitize adult Coleoptera and Hymenoptera . Two major lineages occur within the Braconidae: (a) the cyclostome braconids, most of which are idiobiont ectoparasitoids of concealed Lepidoptera and Coleoptera larvae although many are koinobiont endoparasitoids of Diptera and Hemiptera, and (b) the noncyclostome braconids which are all endoparasitoids, and most generally koinobionts, typically attacking an early instar of their hosts (see  for a comprehensive overview of their biology).
Numerous braconid species have been reported in association with ants. Some, such as Compsobraconoides sp.  and Trigastrotheca laikipiensis , are predatory on several developmental stages of ants. Others, such as Aclitus sappaphis Takada and Shiga found in nests of Pheidole fervida Smith [211, 212], Paralipsis enervis (Nees von Esenbeck) found with Lasius niger , or P. eikoae (Yasumatsu) found with L. japonicus Santschi (referred to as L. niger (L.)) and L. sakagamii Yamauchi and Hayashida [212, 214], are in fact primary parasitoids of root aphids and can only be considered as indirectly associated with the aphid-attending ants; however, they have developed highly sophisticated relationships with their hosts involving chemical mimicry and chemical and tactile communication to obtain regurgitated food (trophallaxis).
For several other species, the exact nature of the association with the ant host has not been clearly established, but at least 15 euphorine species can be considered as true parasitoids of adult ants even if direct evidence of oviposition has been obtained for only 11 of them (see Table 1). All of these parasitoids are grouped in three extant genera, Elasmosoma, Kollasmosoma, and Neoneurus, and one fossil genus, Elasmosomites, all belonging to the tribe Neoneurini. Evidence from Eocene Baltic amber, as demonstrated from an individual of Elasmosomites primordialis emerging from the abdomen of a Lasius worker (Figure 4(a)), indicates that the parasitoid association between neoneurine braconids and ants has been in existence for at least 40 million years . Although oviposition into the abdomen of adult worker ants has been reported on several occasions [56, 120, 121, 126, 127, 140], detailed descriptions were rare and, until recently, restricted to only two species. In the case of N. mantis attacking Formica podzolica, Shaw [142, 143] gave interesting information both on the “perching” behavior displayed by the parasitoid females in their ambush strategy to locate their hosts and on the attack sequence which is completed in less than 1 s and is characterized by a reduction of the usual braconid oviposition sequence, the first two steps (antennation of the host and ovipositor probing) being entirely lost in favor of speed. For E. michaeli, Poinar  not only described the attack behavior, exclusively focused on major workers of Formica obscuriventris clivia (Figure 4(b)), but also provided invaluable information on the altered behavior of parasitized ants, on the development of the immature stages, and on cocoon formation and adult emergence. Immature stages of Neoneurini parasitoids attacking adult ants develop as koinobiont endoparasitoids in the abdomen of workers, and fully developed larvae leave the host to pupate in the soil .
Very recently, slow motion video recordings were used to describe the oviposition behavior in adult ants for 3 other species , and we refer the reader to their excellent films, which show the variability in oviposition behavior within the tribe. Neoneurini wasps parasitize worker ants in the vicinity of the nest entrance(s), or while foraging. Females of Elasmosoma luxemburgense hover over the nest entrance of Formica rufibarbis and attack workers from behind, grasping the ant abdomen with the three pairs of legs involved, and probably ovipositing through the anus. The whole behavioral sequence (alighting, grasping, ovipositor insertion, and takeoff) lasted a mean of 0.73 s. The ants were aware of these attacks, turning around and chasing the wasps with open mandibles ( doi: 10.3897/zookeys.125.1754.app1). Females of Kollasmosoma sentum attack workers of Cataglyphis iberica in the vicinity of nest entrances, or when carrying prey and walking more slowly than usual. Attacks usually occurred during the brief stops characterizing Cataglyphis workers walks. The wasps were extremely fast and attacked the ants from behind. Oviposition took place in both the dorsal and ventral surfaces of the ant’s gaster, likely through intersegmental membranes. Wasps adjusted their alighting strategies according to the direction of their own approach to the targeted ant, and to the position of the ant’s gaster (horizontal or vertical position, distinctive for the genus Cataglyphis), and accomplished extraordinary pirouettes. The whole oviposition behavior lasted only 0.05 s on average. The ants were often aware of the presence of the parasitoids, aggressively turning around with open mandibles, or extending their hind or middle legs to hit them ( doi: 10.3897/zookeys.125.1754.app2). Finally, N. vesculus females alight and probably oviposit in the mesosoma of Formica cunicularia workers. As for N. mantis [142, 143], they were observed ambushing or hovering over the nest entrance. Females preferentially attacked ants while at a vertical position (going up a tree trunk, e.g.). The wasps approached the ants from behind, alighted, held the ant’s thorax with their raptorial fore legs, bent their abdomen towards the postero-lower part of the ant’s thorax, and oviposited. The ovipositor is thought to be inserted near the posterior coxal cavities. The whole oviposition behavior lasted a mean of 2.02 s ( doi: 10.3897/zookeys.125.1754.app3).
With few exceptions, neoneurine wasps have been found in association with formicine ants [129, 207, 215, 216]. It is thought that formic acid used by these ants could serve also as a kairomonal stimulant to host-seeking hymenopterous parasitoids [120, 127, 129]. Far less is known about the fate of parasitized ants. According to Poinar , Formica ants parasitized by E. michaeli form an assembly along the edge of their superficial nest when the parasitoid larvae are about to leave the host to pupate. This behavioral modification is thought to increase the survival of adult wasps.
Several morphological and behavioral adaptations, apart from rapidity of attack, contribute to the success of these wasps in parasitizing aggressive adult ants: for example, the vestigial tarsal claws and enlarged pulvilli (suction like disks, [130, 131, 217]) of Elasmosoma spp., or the raptorial fore legs of Neoneurus spp., enable wasps to grasp and hold the ant firmly while ovipositing. Likewise, the peculiar ventral spine of K. sentum females, located on the fifth sternite, could help to fix the wasp’s position during oviposition, when the body of the wasp goes back tending to the vertical position, and fore legs detach from the ant’s cuticle. Finally, the longitudinal disposition of K. sentum females’s tarsi on the ant metasoma, one over the other, enables the necessary rotation of the body to adjust itself to the position of the ant’s gaster, before oviposition. The wasp rotates counterclockwise if the right tarsus is placed over the left one; and if the left tarsus is placed over the right one, the rotation is clockwise.
Ichneumonidae is the largest family in the Hymenoptera with about 23,330 described species worldwide in 46 subfamilies and 1207 genera; the total number of species is estimated to be more than 60,000 [207, 218, 219]. Most of the members of this large family are parasites of holometabolous insects, but a few species parasitize spiders (egg sacs, spiderlings, or adults) or egg sacs of pseudoscorpions. Many ichneumonids are hyperparasitoids of other ichneumonoids or of tachinid flies, and some species are egg-larval parasitoids, laying an egg in the host egg but consuming the host in its larval stage [218, 219].
Various species of the genus Gelis (all of them initially referred to as Pezomachus) and a few others of the genera Agrothereutes, Aptesis, Pleolophus, and Thaumatogelis have been reported by various authors to be associated with ants of the genera Lasius, Formica, Myrmica, Temnothorax, and Solenopsis [24, 56, 220–222]. However, no information is available on the exact relationship with their ant host, except that in some cases (such as Pleolopus micropterus (Gravenhost) (referred to as Pezomachus micropterus) and T. vulpinus (Gravenhorst) (referred to as Pezomachus vulpinus)), they were clearly reported as “found in the nest of Formica rufa, not reared from cocoons” . Until now, true ichneumonid parasitism on ants has been demonstrated only for 3 species, all belonging to the subfamily Hybrizontinae and very likely to the same tribe Hybrizontini. The most ancient report dates back to 1852  and concerns Eurypterna cremieri described as hovering over a nest of Lasius fuliginosus in Germany. This behavior, suspected to be related to the search of an appropriate host, was later confirmed by different authors not only for the same host species in France and Italy [146–148] but also for three other species of ants in the genera Lasius and Formica in France, England, and Japan [123, 144, 149]. In the early 20th century, Cobeli  described how four females of E. cremieri were hovering over trails of L. fuliginosus, while ants were moving their nest to another nest site, inspecting each ant worker that was transporting a larva. The female parasitoids quickly drew closer to the larva, and folding up the abdomen touched it, presumably depositing an egg. Such behavior was only observed with ants transporting a larva and did not trigger any reaction from the workers. In spite of the interesting information supplied, this report passed more or less unnoticed until 2010 when the parasitic nature of this behavior could be confirmed (and even photographed) concerning Lasius nipponensis transporting brood between two nests . Only workers carrying something in their mandibles were tracked by E. cremieri females hovering about 2 cm above them. And only those carrying a larva were attacked after a sudden dive of the wasp which gripped the targeted larva with the tarsi of its fore and middle legs, bent its abdomen down, exerted its ovipositor, and oviposited in the larva before flying away in search of a new host. The complete sequence lasted less than 1 s and elicited some brief excitement from the worker ant. Dissection of a stung ant larva showed that a wasp egg was present in the somatic cavity. Another undescribed Hybrizontinae species (gen. nov. sp. nov.) was similarly reported by the same authors as hovering over workers of the slow moving ant Myrmica kotokui which were holding something in their mandibles. As for E. cremieri, only those carrying a larva were more closely inspected and were attacked in a similar manner as previously described, but in that case, the complete attack sequence lasted longer (3-4 s), and oviposition itself took at least 1 s. A third case of ant larval parasitism has very recently been confirmed and involves Hybrizon buccatus females. This species had been frequently reported in association with (or hovering over) different ant species from various genera (Myrmica, Lasius, Formica, and Tapinoma, see Table 1) [24, 140, 144, 146, 152, 153] and was reared from nests of Lasius alienus where the ichneumonid naked pupae had been found among ant-host cocoons . But it was not until 2011 that the oviposition into larvae transported by Lasius grandis workers could be observed and filmed during brood transfer between two nest entrances . Only final instar larvae were attacked, in a very similar way to that previously described for E. cremieri, and the complete sequence lasted between 0.40 and 0.58 s. Chemical and/or visual cues are likely to be involved in the location of the ants’ trail since H. buccatus females have been observed continuously hovering over the trail for a period of time, even in the absence of ants. Finally, considering both the hovering behavior as a reliable evidence of parasitism and the fact that all three ichneumonid parasitoids known until now to attack ants are restricted to the Hybrizontinae, two other cases are likely to be added to our list: Ghilaromma fuliginosi and H. rileyi which have been reported swarming and hovering over the nests of Lasius fuliginosus [150, 151] or attracted to a disturbed nest of L. alienus , respectively. However, in both cases, direct oviposition into ant larvae or adults needs to be confirmed.
Since the last paper on parasites of social insects by Schmid-Hempel , the number of reliable records of parasitoid wasps attacking ants and their brood has grown dramatically from about 43 species to at least 138 belonging to 9 hymenopteran families. Furthermore, the knowledge of the biology and behavior of those wasps and the nature of their interactions with ants has significantly progressed, though many gaps still remain. Most likely, hymenopterous parasitoids of ants are more abundant than suggested by our list of reliable records, and future studies focusing on the immature stages of ants under close scrutiny would certainly increase this list substantially.
All castes of ants and all developmental stages, excepting eggs, are the target of parasitoid wasps. For example, neoneurine braconids parasitize adult worker ants while foraging or performing other activities outside the nest [129, 131, 143], while eurytomids of the genus Aximopsis attack adult queens at the very moment of nest foundation [115, 116, 118, 119]. However, in most cases, ant larvae are the target of parasitoid attacks, either inside or outside their nests. Larvae can be parasitized outside the protective walls of the nest during transportation when ants move from one nest to another as for some euphorine braconids and hybrizontine ichneumonids [129, 149], or while being employed to fix or build a new nest as occurred for the green weaver ant larvae attacked by the chalcidid Smicromorpha . Most often, ant larvae are attacked inside the nest, notwithstanding the pugnacious character of ants. For eucharitid and perilampid wasps, planidia are transported by phoresis into the targeted nest where they actively search for a larval host. The extremely small size of the planidia is assumed to facilitate both entrance into the host colony and initial parasitization , but in most other parasitoid wasps (diapriids, encyrtids, entedonine eulophids, and some eurytomids), it has been assumed that it is the female that searches for a host nest, enters it, and oviposits on or in the larval host. So far, however, how the females gain access into the ant nest and complete the oviposition process has never been described, and the initial stages of development of these parasitoids are in most cases unknown (but see [35, 131]).
Hymenopterous parasitoids attacking ants exhibit a wide array of biologies and developmental strategies: ecto- or endoparasitism, solitary or gregarious, and idio- or koinobiosis. Besides, the behavioral strategies evolved to cope with ant aggression or to exploit the communication system of ants are also impressive. Most of these parasitoids belong to families with species using a wide range of insects or arthropods as primary hosts, and in many cases of recorded associations between parasitic wasps and ants [20, 23, 112, 114, 128, 160, 186, 198], the primary host of the parasitoids is not the ant but another insect species present in the ant nests. Such indirect association through parasitism of trophobionts or other myrmecophiles suggests that a possible path to the parasitization of ants by hymenopterous parasitoids could have evolved as a shift from the initial primary host (Diptera, Coleoptera, or other insect myrmecophiles) to the ant host larvae through a gradual process of association and integration with the ant hosts. Such a hypothesis proposed for diapriids by Huggert and Masner  and widened by Hanson et al.  to hymenopterous parasitoids in general might apply for numerous families, and a supporting example has recently been suggested among eulophids . However, other evolutionary paths are likely to be involved in the case of eucharitids and perilampids and those species that attack adult ants and deserve further study.
Despite a significant increase in our knowledge of hymenopterous parasitoids of ants in the last 15 years, the remark of Schmid-Hempel  concerning parasitism in social insects in general: “the existing knowledge is bound to be a massive underestimation, since the true abundance and distribution of parasites remain to be discovered” is still, more than ever, a topical subject. Most hymenopterous parasitoids attacking ants remain to be discovered. Moreover, despite the presumed importance of some of them as natural enemies of ants, few quantitative data are available on the impact of these natural enemies on their hosts (see ). Based on their abundance and success in attacking ant hosts [36, 83, 193, 194], some parasitoid wasps like, for example, diapriids and eucharitids, seem excellent potential models to explore how parasitoids impact ant colony demography, population biology, and ant community structure, and further studies focusing on these issues will certainly contribute to deepening our knowledge on this important group of parasites.
The authors thank M. W. Gates, C. Hansson, J. M. Heraty, M. Loiácono, and T. C. Narendran for making available various bibliographic references, and L. Passera and G. Poinar Jr. for kindly supplying their original pictures of ant attack by Plagiopria passerai, Elasmosomites primordialis, and Elasmosoma michaeli. They are also indebted to T. C. Narendran, G. Poinar Jr., and an anonymous reviewer for constructive comments and suggestions on a previous version of this paper, and to Peter Winterton for grammatical improvement.
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