The population levels of the small tortoiseshell (Aglais urticae) seem to vary year on year, almost cyclical. I can clearly remember in 2002 counting fifteen on ice plants in our garden at one go, all jostling for space and a chance to get at the nectar. However, by 2012 I wrote that I was concerned by the almost non-existence of small tortoiseshells anywhere. Last year and this they seem to me to be one of our most frequently seen butterfly species again.
The reason for these ups and downs in numbers seems to be something of a mystery and Oxford University Zoology department are investigating what the reason(s) might be. One theory is that it is linked to the arrival from the continent of a small parasitic fly, Sturmia bella, in the late 1990's. The small tortoiseshell has a close relationship with the common nettle (hence the 'urticae' of the scientific name) and its caterpillars thrive on them. The fly lays its eggs on nettle leaves and the caterpillars consume them and the parasite then eats the inside of the caterpillar. This is now the most frequently recorded parasite of small tortoiseshell caterpillars killing 60% of them where present.
It may be that as small tortoiseshell adult numbers fall because of the parasitic fly there are less hosts for the fly and so the fly numbers fall allowing more caterpillars to survive to adulthood and so the process continues.