I blogged about the Holly blue butterfly (Celastrina argiolus) back in May but I’ve since read some really interesting info about this lovely little butterfly and have some new photos to share as well.
My information comes from the book I’m currently reading, which I highly recommend – it’s Wonderland: A year of Britain’s wildlife day by day by Brett Westwood and Stephen Moss. The entry from 13 August is about the Holly blue and answered a query I had about why I’ve recently been seeing so many Holly blues on Ivy plants rather than on Holly.
It turns out the Holly blue has two generations per year: as their name suggests, the females from the first generation lay their eggs on Holly plants and that’s what the first generation of caterpillars munch on. Then, once those caterpillars have pupated, they emerge as butterflies from around mid July, and the females from that second generation lay their eggs on Ivy, as that’s what the second generation caterpillars eat.
I’ve also been wondering why I seem to be seeing so many more Holly blues this year and Wonderland has the answer to that too:
These fluctuations [in population], over a cycle of five or six years or so, are caused by a small parasitic wasp called Listrodomus, which injects the caterpillars with a long sting-like ovipositor. The Listrodomus grub lives inside the caterpillar, but keeps it alive long enough to allow it to pupate, emerging later from the chrysalis. As wasp populations increase, they reduce the Holly blues. Fewer butterflies mean fewer opportunities for the wasps and so, in turn, wasp numbers fall too. This allows the butterflies to build up again, and that’s why over a span of several years our sightings of Holly blues go up and down.