Females lay their small pale green eggs on the uppersides of young leaves of Common Nettle Urtica dioica and occasionally other related plant species. The eggs are laid singly and darken just before hatching, which is after one to five weeks depending on the weather.
Larvae are pale greenish-brown at first, turning to brown with black heads and occurring in pale and dark forms. Blackish spikes develop in the second instar. Larvae are parasitised by various wasps and flies and turn yellow if affected.
Larvae live in individual tents made by tying two leaves together with silk and the final tent is usually made by chewing part way through the top of the nettle stalk (about 15 cm down), causing it to flop over. The tent is pulled together with silk threads and the larva changes into a grey to pale brown pupa, blotched with varying amounts of gold. The pupal stage takes from two to four weeks.
There is still lots to learn about adult Red Admirals and how and when different generations breed and move north in the spring and how they survive winters. For example, despite arriving females appearing to be already mated, males set up temporary territories for a week or so at a time. However we have no evidence of courting or mating. Researchers don’t know if mating only happens at night, or high up in trees or not at all in Britain. From August to November Yorkshire Red Admirals head south, perhaps heading for the Mediterranean in winter. Do these adults breed when they get there and, if so, are these returnees important to the numbers? At Spurn point peak counts of hundreds of butterflies a day have sometimes been seen heading south in September.
Sometimes, instead of going south, Red Admirals try to hibernate, usually in the open, on the side of large trees in woodland, where they are almost impossible to see, or on sheds or houses.