The Phaeogenini: Widdle Icky Newmans

Female Diadromus collaris on a pupa of a diamondback moth Plutella xylostella, from here.

The ichneumons are perhaps the best-known family of parasitic wasps. Most people will have come across a description of the classic ichneumon lifestyle at at least some point: a female lays an egg in the larva of another insect, which then hatches into a wasp larva that eats out its hosts insides before emerging at maturity, leaving an empty husk behind. It is easy to see why ichneumons have become the poster children for parasitoid wasps everywhere: not only are they one of the most diverse wasp families, they can often be dramatic in appearance, growing to remarkable sizes. However, not all ichneumons are giants.

Female Eparces quadriceps, copyright Tom Murray.

The tribe Phaeogenini includes some of the smallest ichneumons, with some species being only a few millimetres long (Rousse et al. 2013). They belong the the subfamily Ichneumoninae, within which they are distinguished from most other tribes by their possession of round rather than elongate spiracles on the petiole. They are otherwise quite diverse in appearance, and Gauld (1984) suggested that they may be a polyphyletic assemblage of species that had convergently evolved their common features as a result of their small size. However, molecular phylogenetic analyses have largely supported the monophyly of the Phaeogenini (e.g. Quicke et al. 2009). One genus, Lusius, tends to be placed elsewhere among the ichneumons, but this is probably due to its having an anomalous 28S rDNA sequence with a number of deletions; Quicke et al. (2009) implied that they thought it more likely to still be a true phaeogenin. Some authors have suggested a relationship between phaeogenins and another unsual small ichneumon genus Alomya (in which case, due to the vagaries of priority, the name of this tribe becomes the Alomyini), but molecular analysis does not support this association.

Dirophanes fulvitarsis encounters a smaller wasp (perhaps a figitid?). Copyright J. K. Lindsey.

Like other members of the Ichneumoninae, the Phaeogenini are parasitoids of Lepidoptera: specifically, in accord with their small size, micro-lepidoptera. However, identification of the hosts of phaeogenins can be difficult, as they tend not to attack them until after the host has formed a cocoon (Diller & Shaw 2014). Where hosts are known, they are often borers in plant stems or leaves. The phaeogenin Diadromus collaris attacks the diamondback moth Plutella xylostella, a significant pest on brassicas and related plants. As such, it has been widely introduced around the world to help in the control of this pest.

REFERENCES

Diller, E., & M. R. Shaw. 2014. Western Palaearctic Oedicephalini and Phaeogenini (Hymenoptera: Ichneumonidae, Ichneumoninae) in the National Museums of Scotland, with distributional data including 28 species new to Britain, rearing records, and descriptions of two new species of Aethecerus Wesmael and one of Diadromus Wesmael. Entomologist's Gazette 65: 109–129.

Gauld, I. D. 1984. An Introduction to the Ichneumonidae of Australia. British Museum (Natural History).

Quicke, D. L. J., N. M. Laurenne, M. G. Fitton & G. R. Broad. 2009. A thousand and one wasps: a 28S rDNA and morphological phylogeny of the Ichneumonidae (Insecta: Hymenoptera) with an investigation into alignment parameter space and elision. Journal of Natural History 43 (23–24): 1305–1421.

Rousse, P., S. van Noort & E. Diller. 2013. Revision of the Afrotropical Phaeogenini (Ichneumonidae, Ichneumoninae), with description of a new genus and twelve new species. ZooKeys 354: 1–85.