Bum-bursting mummy-wasps

Yes, you know it from the title - it's parasite time. If I ever feel that tiny beetles are too easy to identify and I fancy a challenge, the parasitic Hymenoptera are the group of choice - huge numbers of species, tiny differences between them, and few accessible keys. Yay. To make sure I don't get lazy, I have a hatchery where pupae (for example those I dislodge cutting firewood, mending the garden fence etc) are kept to see what they turn out to be as adults, identified and maybe even released. Sometimes other things turn up, for example this mummified larva of the knot-grass moth Acronicta rumicis on a bramble stem.

Mummified larva of Acronicta rumicis

It's worth noting that this is not a pupa - it is the moth larva's empty skin stuck to the stem by the dark brown sticky substance you can see just behind the head. This 'glue' is released by the fully grown wasp larva by cutting a slit in the underside of its host. I have had the 'mummy' in a hatchery for several weeks, but this morning I found an exit hole at the rear of the dead moth larva, and a lively adult wasp scuttling and flying inside the container.

Acronicta rumicis 'mummy' showing the parasite's emergence hole

The parasitic wasp cooled down and quiet. 1 square = 5mm.

So, time for identification. It's a wasp of some sort within the superfamily Ichneumonoidea. However, the abdomen isn't on a thin stalk and the wing venation indicates it is of the family Braconidae rather than Ichneumonidae.

Braconid wasp showing wing venation

For comparison, the wing venation of the braconid Aleiodes praetor, from Huddleston & Gauld (1988)

Now, it's worth noting that there is no guide to British braconid genera or species. Shaw & Huddleston (1991) gives a key to the subfamilies, but beyond that, identification requires a variety of more-or-less obscure journal articles in most cases, and the taxonomy has undergone a lot of revision. Fortunately however, this is (for a braconid) a relatively straightforward specimen. Firstly, the subfamily key takes it to Rogadinae and secondly, the wing diagram above matches the specimen closely because (handily) they are the same - Aleiodes. In the most recent checklist (Broad et al. 2012) there are 37 species of this genus in Britain and all are believed to be solitary (unlike many braconids where many parasites develop in a single host, the best known probably being Cotesia glomerata AKA Apanteles glomeratus). Although species identification is challenging, there are some clues. For example, the first two abdominal segments (blue arrows in the photo below) have longitudinal ridges running along the middle of their upper surfaces and this is typical of the common species A. praetor.

Aleiodes sp. showing ridges on the first two abdominal segments (blue arrows)

I couldn't check this tentative ID myself - at least not without accumulating some articles I don't have ready access to and/or visiting a museum collection (even online there is very little in the way of images, keys and so on). So, I passed this onto some braconid specialists who have confirmed it isn't A. praetor (not orange enough, though there are more technical ID features required too!) and I hope I'll get a definite species ID soon. Until then, I await whatever else appears in the hatchery...

Ventral view of Aleiodes showing orange legs with some black areas, and orange mouthparts.

References

Broad, G. R., Shaw, M.R. & Godfray, H.C.J. (2012). Checklist of British and Irish Braconidae (Hymenoptera) [30th April 2012 version]. Free download here.
Huddleston, T. & Gauld, I. (1988). Parasitic wasps (Ichneumonoidea) in British light-traps. The Entomologist 107(2): 134-154.
Shaw, M.R. & Huddleston, T. (1991). Classification and Biology of braconid wasps (Hymenoptera: Braconidae). RES Handbooks for the Identification of British Insects 7(11): 1-126. Free download here.