Tuesday, 14 October 2008

An exceptional occurrence

By all accounts it has been something of a poor year for butterflies, with many species present in lower numbers than those usually seen. The poor weather over much of the summer contributed to difficult times for butterflies and there were several species that I either failed to catch up with entirely or of which I saw very few individuals. As such, a recent trip to Holkham proved to be a rather exceptional event thanks to the presence of a single White Admiral butterfly. I had gone to Holkham to look for a Radde’s Warbler that had been seen there earlier in the day. While I failed to catch up with the Radde’s Warbler, the presence of a fresh-looking White Admiral, close to where the warbler had previously been seen, more than made up for my disappointment.

This is a species of butterfly that I normally see much earlier in the year, with Knettishall Heath a favoured local site providing sightings of newly emerged individuals throughout June and July. The White Admiral is univoltine, which means that it has just a single generation each year. The graceful adults are on the wing from June to August, depositing eggs on the leaves of honeysuckle from which emerge the caterpillars. Each caterpillar begins its journey towards adulthood in late summer but then hibernates through the winter to resume its growth the following year. Given that the single generation of adult White Admirals is over by the end of August, my sighting of a newly emerged adult at Holkham posed an interesting question – what was it doing on the wing so late in the year? Was this an adult that had simply emerged spectacularly late or had a caterpillar completed its growth and transformation within a single season.

Searching through my numerous books on butterflies finally revealed the answer; I discovered that a small second generation of adults might very occasionally occur in those years when the first generation is on the wing exceptionally early in the season. It seems that a handful of individuals must have been on the wing in late May or early June, with some of the resulting caterpillars completing their stage of the life cycle without the winter break. If any eggs were to be laid as a result of these very late season adults then the resulting caterpillars would presumably enter winter hibernation at a much smaller size than normal, something that might reduce their chances of surviving through to next spring. Whatever the fate of this particular late-season individual I was glad to have seen it at a time when the butterfly season is just about coming to its end.

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