Chalcidoidea

The Chalcidoidea are among the most difficult groups of Hymenoptera to identify because of their small size and the lack of adequate keys to the North American species. Several species vie for the distinction of being the smallest insect (about 0.2 mm long), and most species are less than 3-4 mm long. The characters used to distinguish chalcidoids from other Hymenoptera include the presence of a prepectus, failure of the pronotum to meet the tegula, 13 or fewer segments comprising the geniculate antenna, and drastically reduced wing venation. ~Ashmead (1904) recognized 14 families and provided the first comprehensive modern classification of the Chalcidoidea. Despite its many errors, this was a prodigious work and remarkable considering the primitive optical equipment and state of knowledge about the Chalcidoidea at that time. Nikol'skaya (1952) elevated the number of families to 24, and Boucek and Hoffer (1957) (subsequently translated by Peck, 1964) recognized 18 families. In this catalog Burks has reduced the number of families to eleven. Casual thought may lead one to wonder why there is so much inconsistency among workers regarding higher classification of chalcidoids. These classifications are based on external morphology, and the chalcidoids are exceedingly plastic morphologically. This plasticity generates differences of opinion over the limits of higher taxa because workers weight characters differently. ~Chalcidoids are found in all zoogeographical regions, in all terrestrial habitats, and all families are found in each zoogeographic region. Despite their omnipresence, chalcidoids remain one of the poorest known superfamilies. Taxonomically, the western Palearctic fauna is best known, followed by the Nearctic. The remainder of the zoogeographical regions (Neotropical, Ethiopian, Australian, and Oriental) are almost completely unknown with respect to their endemic faunas. Much of our knowledge of chalcidoids stems from species which are associated with agriculture. ~The body size and searching habits of chalcidoids make them suitable for fossilization in resinous amber, but fossil records of the Chalcidoidea are incomplete. Fewer than 50 species are known, and these belong to less than half of the chalcidoid families. The most comprehensive accounts of fossil chalcidoids are by Brues (1910), Doutt (1973), and Yoshimoto (1975). The last study provides a summary of knowledge about fossil chalcidoids. Yoshimoto (1975) reports that mymarids, trichogrammatids, and tetracampids are referable to the Cretaceous Period (70-90 million years before present). ~Owing to the paucity of knowledge about fossil chalcidoids and their morphological plasticity, the relationship of this superfamily to other parasitic Hymenoptera has not been conclusively established. We are not certain that the Chalcidoidea are monophyletic in the Hennigian sense, although some investigators have that opinion. An interpretation of chalcidoid phylogeny based on the known fossil record is provided by Yoshimoto (1975). ~The actual numerical dimension of the Chalcidoidea can only be speculated. The ichneumonid specialist Henry Townes (1969) has estimated that there are 60,000 species of Ichneumonidae. DeBach (1974) has estimated that somewhere between 70-90 percent of the parasitic Hymenoptera remain to be described. I believe that the Chalcidoidea will ultimately be recognized as larger than the Ichneumonidae. There are some who would disagree with this estimate, but their estimates are based on impressions developed from examining species that repeatedly have been submitted for identification. These species mostly are associated with the agroecosystem and represent only a portion of the total chalcidoid fauna. My primary interest in making these assertions is to stimulate research on the Chalcidoidea because they are a fertile area for investigations in biology, behavior, ecology, and systematics. ~Chalcidoids have diverse and frequently specialized feeding habits. Most species of chalcidoids are parasitic, but phytophagy probably has evolved several times in the Chalcidoidea because it is found in several distantly related taxa and many unrelated species of plants serve as hosts. Phytophagy is found most frequently in association with gall-forming habits, but the evolutionary significance of this observation remains unknown. ~Agaonids demonstrate the most intimate expression of phytophagy in the Chalcidoidea. This group is poorly represented in North America because all agaonids develop in fig seeds (Ficus spp.), and these plants occur naturally only in tropical and subtropical climates. All figs are dependent on agaonids for pollination, and agaonids can only develop within the receptacles of Ficus. Host specificity seems to be the trend in agaonids with each species of fig having its own agaonid for pollination (Ramirez, 1970 a,b; Grandi, 1961). Numerous other chalcidoids are associated with Ficus as inquilines (Hill, 1967 a,b). ~Other taxa of chalcidoids with phytophagous species include the Eurytomidae, Torymidae, brachyscelidiphagine Pteromalidae, and Tanaostigmatidae. ~Some ecologists prefer to use the term parasitoid to characterize parasitic insects. Protelean parasite is a phrase often used to distinguish between typical parasites and insects that are parasitic in the larval stage only (Askew, 1971). ~One definition of parasitism for all parasitic organisms is impractical because animal species are parasitic in many different ways. The parasitological definition of parasitism in the sense of parasitic worms and protozoa is unsuitable in the present context because parasitic chalcidoids do not behave in a manner consistent with that definition. Therefore it seems more appropriate to list some of the biological attributes of parasitic chalcidoids. Parasitic chalcidoids are characterized as follows: (1) they are obligate parasites in the larval stage only; (2) they require only one host to complete development; (3) they attack related taxa (other arthropods and usually insects); (4) if the parasite completes development, the host invariably dies; (5) the ratio of size between the parasite and host approximates unity (except in some cases where the parasitic larvae are gregarious or polyembryonic delevopment occurs). ~Adults of some species host feed, but the significance of this behavior is not always clear. Host feeding may provide nutrients necessary for ovary or egg development, or it may be a convenient source of nutrients necessary for sustaining life (Flanders, 1953; Doutt, 1964; Quezada et al., 1973). ~Parasitism by insects reaches its most elaborate development in the Chalcidoidea. Primary parasitism (larval development on a phytophagous host) is the most common type of parasitism by chalcidoids. Hyperaparasitism (a parasite attacking another species of parasite) is found almost exclusively in the Hymenoptera, and reaches its most extensive development in the Chalcidoidea as indicated by the fact that most families have hyperparasitic species. Further evidence of the extensiveness of hyperparasitism in this superfamily is found in the fact that several types of hyperparasitism have evolved in the group. These include secondary (a parasite attacking a primary parasite), tertiary (a parasite attacking a secondary parasite) and quaternary (a parasite attacking a tertiary parasite). Hyperparasitism probably evolves out of primary parasitism in situations involving strong interspecific competition. ~An unusual type of hyperparasitism occurs in Coccophagoides utilis Doutt and various related genera such as Coccophagus, Encarsia, and Prospaltella. Female larvae develop as primary parasites of armored-scale insects, and the male larvae develop as hyperparasites of their own females (Broodryk and Doutt, 1966). This phenomenon is called adelphoparasitism or autoparasitism and appears restricted to the aphelinines (Zinna, 1961; Flanders, 1959, 1967). ~Parasitic chalcidoids can be categorized on the basis of where the egg is deposited and how the larva feeds. Most species attack the host directly, but adult female eucharitids and perilampine pteromalids oviposit on vegetation and the first-instar larva (planidium) searches for the host (Smith, 1912; Clausen, 1940 a,b). Species in which the adult female directly attacks the host lay their eggs on the host's body and the larvae develop externally, or deposit their eggs inside the host's body and the larvae develop internally. There is a tendency for parasites that attack exposed hosts to develop internally (exception: elachertine Eulophidae), and parasites that attack concealed hosts to develop externally. ~The intra- and interspecific relationships among parasitic chalcidoids vary. Some species are solitary (one parasite per host), and others are gregarious (several parasites per host). When more than one parasite species develops on a host simultaneously, the condition is termed multiple parasitism. When more eggs of one parasite species are laid on a host than can develop to maturity, the condition is termed superparasitism. Supernumerary individuals are eliminated through larval combat or physiological suppression (Salt, 1961). ~the distinction between parasitism and predation sometimes fails, and some chalcidoids could be called predators. A prime distinction between parasites and predators is that predators frequently consume several prey, but parasites consume only one host per individual. The eunotine pteromalids and some mymarids could be regarded as egg predators because their larvae feed externally on scale-insect eggs in the "brood chamber" after they are oviposited by the female scale-insect (Clausen, 1940 a). ~Parasitic species that attack many species of hosts are called polyphagous; parasitic species that attack only a few species of hosts are called stenophagous; and parasitic species that attack only one species of host are monophagous. Complete host specificity is difficult to establish because it is based essentially on negative evidence. The fact that a parasite will not attack a host under some conditions does not constitute proof that it will not parasitize that species. Nevertheless, there is a tendency towards specialization in the Chalcidoidea, and this is reflected by: (1) repeated recovery of a parasite from a host species over a large area, but not from related host species that occur sympatrically; (2) demonstrated preference for a host species when a choice is available; (3) superior reproductive capability on a host species; and (4) physical limitations that prevent a parasite from attacking a potential host. ~Some polyphagous chalcidoids appear to prefer habitats rather than a taxonomically cohesive group of hosts. For example, Zagrammosoma species parasitize leaf-mining insects whether they are Lepidoptera, Diptera, or perhaps Hymenoptera. In contrast, related Diglyphus species parasitize only leaf-mining agromyzid Diptera. Other chalcidoids are extremely polyphagous. Dibrachys cavus (Walker) is an example. This species, like several others, has an exceedingly long host list that includes representatives of several orders. It usually develops as a primary parasite, but frequently also acts as a facultative hyperparasite (Graham, 1969). No explanation has been provided as to why one species should be so polyphagous and a closely related, morphologically similar species should be stenophagous or even monophagous. ~Likewise, there are associations between host stage attacked and the taxonomic assignment of the parasite. For instance, the Trichogrammatidae and Mymaridae exclusively develop on the egg stage of other insects and the spalangine pteromalids are pupal parasites (Annecke and Doutt, 1961; Boucek, 1963; Doutt and Viggiani, 1968). ~Chalcidoids parasitize more hosts in more different taxonomic categories than any other group of parasitic insects. This spectrum extends from spider eggs (Desantisca) to aculeate Hymenoptera (Melittobia, Leucospidae). A detailed account of the biology of chalcidoids requires more space than is available here. However, a short summary of some interesting host relationships is provided. ~A bizarre host association is found in Ixodiphagus and Hunterellus (Encyrtidae) whose species are primary, internal parasites of tick larvae and nymphs. These genera are cosmopolitan and may prove to be beneficial insects in tick control (Cooley and Kohls, 1934; Cole, 1965; Doube and Heath, 1975). ~The mymarid Caraphractus cinctus Walker is unusual in that it parasitizes dytiscid beetle eggs that are submerged beneath the surface of the water. The female parasite swims in the water by vibrating her wings and oviposits in the host's eggs. Females have considerable discriminative ability, and can detect eggs that have been parasitized (Jackson, 1958, 1966). ~The Eucharitidae are parasitic on Formicidae. The association apparently is an old one, and eucharitids oviposit on vegetation visited by worker ants. The eggs hatch, and the triungulin larvae are phoretically transported to the ant nest. Inside the nest the triungulin larvae eventually move into the brood chamber where they parasitize immature ants (Clausen, 1923; 1940 b,c). ~Other information about host association of chalcidoids is limited by a lack of knowledge about the immature stages of many groups of potential hosts. However, the higher taxonomic categories that include the most host species for chalcidoids include Lepidoptera, Homoptera, Diptera, Coleoptera, and Hymenoptera. Chalcidoids generally have failed to adapt to the nymphal stage of paurometabolous insects. The host spectrum of chalcidoids is being expanded constantly by more comprehensive biological studies of other insects. Given the diversity of habits, host associations, and stages attacked, it seems reasonable to conclude that any insect potentially includes several niches where a chalcidoid can develop. ~All known Hymenoptera develop parthenogenetically and chalcidoids demonstrate three types: arrhenotoky, thelytoky, and deuterotoky. Arrhenotoky is the most common type of parthenogenesis among chalcidoids. Uninseminated arrhenotokous females deposit haploid eggs that develop into hemizygous males. Inseminated arrhenotokous females produce female offspring from fertilized eggs and males from unfertilized eggs. Arrhenotoky is a mechanism whereby lethal and deliterious genes can be relatively rapidly eliminated from a population and superior genotypes can be relatively rapidly selected. ~Thelytoky is parthenogenesis in which males are unknown or rare and females produce females by various asexual mechanisms. Cytologically, diploidy is maintained by apomixsis and automixsis. Apomixsis (ameiotic thelytoky) is characterized by an absence of meiosis, and chromosome number is not reduced. Automixis (meiotic thelytoky) has reduction divisions, and diploidy is maintained in several ways. Rossler and DeBach (1973) review the methods of maintaining a constant chromosome number. ~Thelytoky is common among parasitic Hymenoptera, but the extent of thelytoky in the Chalcidoidea is not known because our knowledge of their biology is limited. Many species are known from the original description only, and many species have been described from the female sex only. Thelytoky may be more common than now realized. In the rather well known genus Aphytis, DeBach (1969) records that about 30 of the species are thelytokous. ~The evolutionary significance of thelytoky is an issue of debate. Traditional views hold that thelytoky is an "evolutionary blind alley". However, Rossler and DeBach (1972) have shown that at least one species of thelytokous chalcidoid has females that are capable of sexual reproduction. ~Deuterotoky is parthenogenesis in which unfertilized eggs develop into both sexes. The cytological mechanism of deuterotoky has not been examined in chalcidoids. This form of parthenogenesis is common in some other animals, and has been reported in some species of chalcidoids (Doutt, 1959). ~The cytogenetics of the Hymenoptera have been reviewed by Crozier (1975). That paper points to a lack of knowledge developed about chalcidoid karyotypes and cytological phenomena. ~Hymenoptera are haplodiploid and this has been confused with sex determination. The correlation between males being haploid and females being diploid is positive and strong, but haploidy and diploidy in themselves do not determine sex. Diploid males are known to occur (Whiting, 1945). Several theories have been advanced to explain sex determination in the Hymenoptera, but in no instance has one theory proven adequate to explain determination in all groups (Whiting, 1940, 1943; daCunha and Kerr, 1957; Slobodchikoff and Daly, 1971). Crozier (1975) suggests that any general theory should accommodate the multiple allele case with as little modification as possible. ~Polyembryony is a cytological phenomenon in which a single egg develops into many individual progeny. Among Hymenoptera the process occurs in the Platygastridae (Proctotrupoidea) and copidosomatine Encyrtidae (Silvestri, 1906; Leiby, 1922, 1926). ~Sex ratio in many species of animals approximates unity. In arrhenotokous chalcidoids the sex ratio usually is female biased and fluctuates between 60 and 80 percent female. Numerous factors have been implicated in the determination of sex ratio including size, stage, or species of host (Abdelrahman, 1974 a,b; Avidov and Podoler, 1968; Clausen, 1940 a), rate of oviposition (Abdelrahman, 1974 b), egg orientation (King, 1961), genetic factors (Wilkes, 1964), differential mortality (Roberts, 1933; Flanders, 1937; Abdelrahman, 1974 a), density fluctuations (Flanders, 1956), nutrition (Flanders, 1965; Moran et al., 1969), and many others. This list could be lengthened substantially and its only limitation now is lack of research. ~Mayr (1969) defines sibling species as "pairs or groups of closely related species which are reproductively isolated but morphologically identical or nearly so." Recent studies of chalcidoids have demonstrated that this group has many sibling species complexes (Hafez and Doutt, 1954; Claridge and Askew, 1960; DeBach, 1959, 1960, 1969; Khasimuddin and DeBach, 1976 a,b,c,; Rao and DeBach, 1969 a,b,c). These complexes suggest that chalcidoids are in an active state of evolution and speciating rapidly. Factors of chalcidoid biology that promote rapid speciation include: (1) short generation time; (2) several generations per season; (3) intensive inbreeding via sib mating; (4) microgeographic isolation; and (5) host preference. ~Chalcidoids are the most important group in applied biological control. Other taxa (Tachinidae, Ichneumonidae, Braconidae, Proctotrupoidea) are used extensively in biological control, but species-for-species chalcidoids have been used more successfully. DeBach (1964) lists 25 pest species with which complete biological control was achieved. Chalcidoids are responsible for control in 13 of these programs, a number far greater than any other taxonomic group. Agricultural pests in these control programs include many Homoptera, but some Coleoptera have also been controlled by chalcidoids (Taylor, 1937; Tooke, 1953; Williams et al., 1951). ~It is a pleasure to acknowledge the comments and suggestions on the preceding account made by the following individuals: Kenneth Cooper, Paul DeBach, Eric Grissell, Peter Price, and David Rosen.