Brimstones breed mainly in wooded areas where their foodplant is present, or in more open country where, especially on chalk, Buckthorn bushes grow in abundance. The species has been recorded in the Isle of Man where buckthorn is not known, so it will travel some distance. Brimstones are commonly found in gardens.
Alder buckthorn and purging buckthorn only.
The butterfly is the only one, outside the Nymphalidae, which hibernates in the adult stage. When resting with wings closed it has a remarkable resemblance to ivy leaves, and it is among such evergreens that the Brimstone hides for the winter. Early in spring, even as early as February, the butterflies may be tempted out and then return to hibernate until later. The eggs are usually laid in May, sometimes in April. The butterflies are long-lived and the same female might lay fertile eggs as late as July and be on the wing when the progeny of her first eggs have started to emerge from the chrysalis in July.
The male is easily distinguished from the female by his bright yellow colouring, though with wings closed, this difference is not obvious. The eggs are laid mainly on the very tips of terminal shoots, usually before the leaves have fully formed from the buds so they can be easy to find, usually on both the upper and undersides of the leaves. Young larvae live tightly pressed along the ribs of the leaf, but their eating holes betray them. Larger larvae strip leaves systematically from a branch, then retreat to a resting place amongst uneaten leaves and take up a curious raised, humped attitude which proves to be a remarkable camouflage. Pupae have curious bulging wing cases which gives them an acutely arched back. They are not formed on the foodplant, where their likeness to the leaves would be excellent camouflage, but some distance away on another plant. The adults are seen in greatest profusion in July and August after they emerge from the pupa. They spend most of their days feeding, with a distinct preference for purple flowers, those of thistle, knapweed, scabious, bramble and clover being their favourites.