Though I’ve been in Wales almost 10 years and was fascinated by British butterflies even before I settled here, I only saw my first Large white caterpillars in June last year. So, when I spotted several more on the large advertising signage fencing off a local building site, I was delighted … until I looked more closely (and just a warning, this is a bit gruesome!).

The larvae were surrounded by small yellow cocoons and, when I looked even closer, I could see small yellowish larvae squirming between the bodies of the caterpillars and the cocoons. It is highly likely that the Large whites have been parasitised by the wasp Cotesia glomerata. In Life Cycles of British & Irish Butterflies, Peter Eeles writes that the wasp

deposits between 15 and 80 eggs inside each young larva that it parasitizes. The wasp larvae feed on the layer of fat beneath the skin of their host, thereby avoiding vital organs, and, when their host is fully grown, break through the skin and pupate within yellow cocoons on or near their host.

If this wasn’t gruesome enough for one morning’s finds, I then noticed a tiny wasp amongst the cocoons. At first, I thought it must be one of the Cotesia glomerata that had hatched, but no. When I checked at home later, a quick internet image search revealed that this did not resemble C. glomerata and, on Wikipedia, I found the information that C. glomerata is itself parasitised by a couple of other wasp species, Lysibia nana [it is listed as L. nanus in British taxonomic listings] and Gelis agilis. I’ve just been reading that the female G. agilis is wingless so I’m wondering if the wasp I saw is L. nanus. As this species is rarely recorded and there are no Welsh records, I’ve sent a query to the parasitic wasp expert at the Natural History Museum; I’ll update this post if/when I get an answer.

**UPDATE** This is the response I received from Dr Gavin Broad from the Natural History Museum: ‘You are correct, your wasps are most probably Lysibia nanus. There is a very similar species, Lysibia tenax, more rarely recorded but with much the same hosts. I don’t believe it is actually a separate species and L. nanus is certainly the usual suspect reared from Cotesia glomerata cocoons.’