In article , David Hill
writes
Was going to get rid of some nettles earlier this week only to find one
clump swarming with caterpillars (Probably painted lady) several hundred and
all about 1/2 inch.
Late, but they may well miss the birds that so often have them with me
earlier in the year.
a few years ago I was going to spray nettles in the spring, only to find
they were all swarming with caterpillars, so I left them.
Within a week not 1 caterpillar left, watched Blue tits gathering them to
feed their chicks, so some wary nice blue tits but few butterflies.

If they're on nettles they are probably Small Tortoiseshells, or
possibly Peacocks, and this isn't really late, this is the second
breeding of the summer. They normally have two life cycles a summer, the
over-wintering adults producing eggs in April-May, which hatch into
caterpillars in May-June and, after pupating, produce adults by late
June and in July, and then these in turn lay eggs to produce
caterpillars right now in July-August. There's plenty of time for these
to pupate and then hatch into butterflies in time for them to
over-winter and round we go again.

--
Malcolm