Brimstone (Gonepteryx rhamni)
Brimstones are often the first butterfly of spring as adults emerge from hibernation on mild days in March and, indeed, may briefly wake and fly on any warm winter day before returning to hibernation.
Females are much paler and can look almost white but lack the black markings that other “whites” share. After mating and egg-laying the over-wintered adults die off, the next generation appearing in August to feed up before entering hibernation.
Images L-R: male drying wings after emergence; caterpillar
Flight period: early spring to end of May and then the second brood August to about October