Diachasma Foerster, 1862

The ferrugineum species group of Diachasma consists of the Nearctic species Diachasma alloeum (Muesebeck), Diachasma ferrugineum (Gahan), and Diachasma muliebre (Muesebeck). All three of these species are parasitoids of fruit-infesting Tephritidae. Although currently placed in the genus Diachasma because of the shortened clypeus (and resulting exposed labrum), these three species share many morphological and biological similarities with the New World species of Diachasmimorpha (Wharton 1997). All three species of Diachasma that occur in the New World were originally described in the genus Opius, during a period when nearly all 1000+ species of the subfamily Opiinae were placed in Opius.

The three species of Diachasma that attack fruit-infesting Tephritidae are known only from North America. Diachasma muliebre is a western species, and has been reared from the western cherry fruit fly, Rhagoletis indifferens Curran. Diachasma alloeum was for many years known only from the East, where it attacks Rhagoletis pomonella (Walsh). It was subsequently recorded from the state of Washington (see also Forbes et al. 2009). Diachasma ferrugineum is essentially from the northeastern US and adjacent parts of Canada but there is also a record from Florida. It attacks the cherry fruit flies Rhagoletis cingulata (Loew) and Rhagoletis fausta (Osten Sacken). No other members of the genus Diachasma are known to attack fruit-infesting Tephritidae, though at least three of the European species have been reared from hosts representing other fly families.

A key for distinguishing the three North American species reared from Tephritidae is provided by Wharton and Marsh (1978). In addition to separation by hosts and distribution, Diachasma muliebre is known only from females and appears to be thelytokous. The ovipositor is distinctly longer than the body in Diachasma alloeum, and about equal in length to the body in the other two species. Diachasmimorpha mellea , which also attacks the same host (Rhagoletis pomonella) in the same geographical region as D. alloeum, can be readily distinguished from D. alloeum by differences in the clypeus, which is short (broadly exposing the labrum) in D. alloeum.

Species excluded. A specimen in HNHM labeled as the lectotype male of Diachasma rufipes Szépligeti, 1905, is a member of the genus Notiopambolus Quicke and van Achterberg. The valid combination is Notiopambolus rufipes (Szépligeti) (Shirley et al. 2014). Several species of Notiopambolus have been described from eastern Australia (Quicke and van Achterberg 1990, Belokobylskij 1992). It is likely that rufipes is a senior synonym of one of these previously described species, but we are unable to place it at this time

1. D. alloeum face: anterior...
2. D. alloeum mesothorax: dors...
3. D. alloeum abdomen: lateral...
4.D. alloeum habitus: lateral
Taxonomic History / Nomenclature
Diachasma Foerster, 1862: 259. Type species: Opius fulgidus Haliday, 1837. Monobasic and original designation.

Type locality of the type species: Isle of Wight; holotype in Dublin, probably female (van Achterberg 1997).
Valid genus

Basic information on Diachasma and its included species can be found in the catalog by Yu et al. (2012). Shirley et al. (2014) provide additional information.

Diachasma has traditionally been characterized by the combination of a short second submarginal cell, short clypeus with broadly exposed labrum, occipital carina that is broadly absent dorsal-medially, and relatively complete fore wing venation (Fig. 11). Diachasma lacks the carapace-like metasoma and clypeal adornments that characterize the few other opiines with these same features. The genus is not demonstrably monophyletic (Tobias 1977, Wharton 1988, 1997) but resolution of this problem will require dense sampling across the subfamily Opiinae to uncover relationships of the various, seemingly disparate, elements currently residing within Diachasma. The genus is based on D. fulgidum (Haliday), a West Palaearctic species (Figs 1-13).
1. Metasoma, D. fulgidum...
2. Dorsal habitus, D. fulgidum...
3. face and gena, D. fulgid...
4. mandibles, D. fulgidum...
5. Head, dorsal, D. fulgidu...
6. Head, lateral, D. fulgi...
7. Mesosoma, lateral, D. fulg...
8. Mesonotum, D. fulgidum...
9.Petiole, D. fulgidum
10. Propodeum, D. fulgidum...
11. Metasoma, D. fulgidum...
12. Face, D. fulgidum...
Diachasma Foerster is a morphologically diverse group containing 34 valid, extant species. The geographic range is primarily Holarctic, but the distribution is disjunct, with several species endemic either to eastern Australia or the island of New Guinea.
No referenced distribution records have been added to the database for this OTU.
Biology / Hosts
As is true of all opiines, members of the genus Diachasma are parasitoids of cyclorrhaphous Diptera. Three host groups are known, corresponding to three morphologically distinct groups of species. Diachasma fulgidum and two other morphologically similar species are parasitoids of leaf-mining flies in the genus Pegomya Robineau-Desvoidy (Anthomyiidae) (Fischer 1972, Jimenez et al. 1992), some of which are notable pests. Diachasma striatum (Foerster) and D. wichmanni (Fischer) have been reared from species of Chyliza Fallén (Psilidae) inhabiting twig galls on trees and shrubs (Fischer 1957, Grijpma and van Achterberg 1991). These two species are sometimes placed in Atoreuteus Foerster (Wharton 1988, Grijpma and van Achterberg 1991, Quicke et al. 1997), of which striatum is the type species. A further three species from the Nearctic Region are parasitoids of fruit-infesting Tephritidae (Muesebeck 1956, Wharton 1997). One of these, D. alloeum (Muesebeck), shows evidence of sequential sympatric speciation in association with its host Rhagoletis pomonella (Walsh) (Forbes et al. 2009). Wharton (1997) suggested that these three species may be more closely related to the Nearctic species of Diachasmimorpha Viereck that also attack fruit-infesting Tephritidae than to D. fulgidum. These three tephritid parasitoids are treated in more detail in the remarks section.