I believe I've told you about the last 3 years in Switzerland, when, each year, one of my larvae in captivity [out of about 20] developed rapidly, pupated and produced an imagine in sept/oct; astoundingly, this happened last year in the UK[but not in any of the previous years in the UK, 2003-2008] , when 2 out 4 larvae did this, although the development was slower than in Switzerland and the larvae perished [in november!] before pupating.
Of course, my conditions are slightly warmer than in nature because my sallow is sleeved in a net, so the wind chill factor is reduced, but not that much, surely? The sallows are always outside.
Anyway, the best comments I have had on this were from Giles Carron [sadly deceased] in 2000.
I think it makes very interestiung reading, and I want to share it with you
I would agree with Giles' comments. I think it is easy to under-estimate the degree of 'forcing' caused by retention of your larvae in sleeves. If Brown Hairstreak caterpillars are sleeved outdoors, even when kept in shady conditions, emergence times are approximately 2-3 weeks early. Larval growth rate in betulae is slow - it can be much more rapid (pro-rata considering ultimate size) in iris, and the degree of potential 'forcing' of the latter is probably greater. I've little doubt that the odd wild cat will grow super-fast (and usually perish), but these are the individuals that might assist an evolutionary switch to bivoltinism in the dim and distant future, should climatic and other conditions ever allow it. I'm sure that sleeving will encourage a much higher incidence of super-growth than is ever seen in the wild.
Best Wishes, Neil