No matter how long you have been interested in butterflies and moths, they always retain the ability to surprise us.
It might be an unexpected species at a well-recorded site, the appearance of a scarce immigrant in your back garden or an unusual aberration with strikingly different wing patterns.
Another way in which these incredible insects never cease to amaze, is the appearance of adults at unexpected times of the year, and this autumn has provided plenty of such surprises. White Admirals, for example, have been reported from woods in Dorset, the Isle of Wight, Essex and Suffolk during September, well outside their normal mid-June to early August flight period. Other species normally found only in the summer have also been seen recently, including Pine Hawk-moth, Lime Hawk-moth, Elephant Hawk-moth, Riband Wave, Swallow-tailed Moth, Brussel’s Lace, Yellow-tail and Rosy Footman.
There is a tendency to consider the number of generations that a particular butterfly or moth species has each year as a fixed characteristic, akin to the division of plants into annual, biennial and perennial species. Field guides and enthusiasts talk of single-brooded (univoltine) species (such as Meadow Brown and Merveille du Jour), double-brooded (bivoltine) ones (such as Adonis Blue and Maiden’s Blush) and those that can achieve multiple generations each year (such as Speckled Wood).
But many butterflies and moths are much more flexible than this. Numerous species have more generations each year in the southern parts of their range than in the north, simply as a response to the climate. Common Blue, for example, and many moths (including Willow Beauty, Green Carpet, Common Wainscot and Snout), are typically bivoltine in southern Britain but univoltine in the north, with the precise boundary between these varying over time with the changing climate.
Other species seem to hedge their bets by having only a partial second generation (and sometimes only in certain years). In other words, some of the offspring of the first generation develop rapidly and emerge as a second generation, while others develop more slowly and don’t emerge until the following year. Examples include the Dingy Skipper, Small Blue, Small Pearl-bordered Fritillary, Peach Blossom and Small Fan-footed Wave.
It is into this latter category that this autumn’s unusual sightings probably fit. In very warm years, as we’ve experienced in 2014, there is the scope for some individuals to develop more rapidly and emerge within the same year rather than waiting for the following summer. We’d expect this sort of phenomenon to increase with climate change but such unexpected emergences, as we have seen with White Admirals and various moths in recent weeks, are unlikely to leave any offspring; in some cases because they cannot find a mate or, if they can, because there then isn’t sufficient time for the developing offspring to reach the necessary stage for successful overwintering before the cold weather arrives.
While such individuals may be doomed not to leave any descendents in today’s climate, the flexibility of species to respond to climate change (natural or human-induced) in this way is clearly an evolutionary advantage in an uncertain future.
Richard Fox
Surveys Manager
Follow me on Twitter: @RichardFoxBC