The butterflies emerge in late June or early July, and fly until late August or early September when they enter hibernation. During the winter months they sometimes awaken and fly on sunny days, but do not become fully active again until late March or early April. Both sexes remain on the wing until June, sometimes overlapping with the newly emerged summer adults. The Brimstone is the longest-lived butterfly species in Europe, and amongst the longest-lived on Earth.

In April and May they lay their pale greenish-white skittle-shaped eggs singly on the buds or young leaves of buckthorn bushes, chiefly alder buckthorn Frangula alnus on acid soils such as the New Forest heaths and woodlands; or purging buckthorn Rhamnus catharticus on chalk or limestone.

Brimstones will oviposit on almost any buckthorn, regardless of it's size or situation - I've found eggs on everything from tiny isolated saplings on open heathland to tall bushes in thickets on neglected chalk grassland. In woodland, semi-shaded bushes growing deep in the understorey seem to be used as often as those in sunlight along ride edges. The eggs can be laid at any height between 0.5 and 3 metres.

The eggs are laid singly but I have counted up to 19 on a single leaf, and up to 100 on a single small bush, laid over several days during repeat visits by one or more females. Eggs are laid over a protracted period - I have observed Brimstones ovipositing on the buds of tiny buckthorn plants in early April, on developing buds and young leaves in late April, and on the fully developed leaves of tall buckthorn bushes in late May or early June.

Both sexes emerge simultaneously in late June - or early July if the season is late. In the summer they visit woodland flowers including bramble blossom, marsh thistle, spear thistle and burdock. They are also particularly attracted to teasel, and often 3 or 4 Brimstones can be found on a single flower head. Most other butterfly species are unable to take advantage of this rich nectar source, but Brimstones have exceptionally long proboscises, enabling them to reach deep into the flowers.

At grassland sites by far the most highly favoured nectar source is wild basil, but knapweeds, small scabious, devil's bit scabious, musk thistle, spreading bellflower, St John's wort, hawkbit, ragwort, marjoram and buddleia are also visited.

Brimstones enter hibernation in August or early September. Most entomological books quote ivy as the commonest hibernation site, and indeed over the years I have found several individuals tucked beneath ivy leaves - usually at a height of about 3 metres where ivy densely covers the trunks of old oaks. I have more frequently however found them beneath bramble leaves, usually less than a metre above ground level. Even more commonly I have found them in tussocks of pendulous sedge, where they settle in the angle formed by the drooping leaves.

Males normally awaken from hibernation several days before the females, on the first sunny and warm day of Spring. This is typically in March, but on the morning of 21st February 2009, I counted no less than 19 males in Crab Wood, Hampshire. All were actively searching for females amongst brambles and ivy. Unusually, on 12th January 2008, I saw a female in flight in Stansted Forest - a full month before the appearance of the first males.

In the spring Brimstones nectar at primrose, bugle, bluebell, common violet, blackthorn, dandelion, cowslip, buttercups, daisies, wood anemone, vetches and sallow catkins.

In March 2007, also at Crab Wood, I witnessed an example of how Brimstones can detect changes in humidity and air pressure.  Shortly before midday, I watched 5 males actively investigating bramble bushes in a sunny glade. At first I thought they were patrolling in search of females, but after a while it became clear that they had detected an imminent change in the weather and were looking for somewhere to shelter. One by one they settled under bramble leaves to roost. Within a few minutes the sunshine had gone, clouds had rolled in, and rain was beginning to fall. An hour later, the sun had re-appeared, but the butterflies remained at roost.

Brimstones tend to go to roost earlier than most other butterflies - at about 3pm ( or a little later in mid summer ) they stop nectaring, even if the weather remains sunny, and all the Brimstones at a given site begin to settle one by one in ivy clumps, or beneath leaves of bramble, oak, hazel, dock or pendulous sedge. Within about 15 minutes or so every Brimstone has gone to roost, and even the reappearance of warm sunshine later in the day fails to arouse them.

In spring I've often seen hoverflies Episyrphus, and Muscid flies Helina, settled on the closed wings of Brimstones - possibly mistaking the butterfly for a leaf, or perhaps just taking advantage of the heat reflected from its wings.

Courtship takes place on sunny mornings in early spring, and females are probably mated within minutes of their first post-hibernation flight, in March or April.

If a male intercepts a female that has already mated, she immediately settles, inverts her wings, and raises her abdomen as a rejection signal. At this point, if the female is on the ground, the male will often walk onto her open wings, in which case the female "freezes" with her abdomen pointing vertically upwards to prevent the male from copulating. Males usually persist for several minutes however, and the female often tries to escape by flying deep into herbage in an attempt to lose her unwanted suitor.

When a male intercepts a virgin female, they usually fly a short distance to settle on a nearby bush. The male then attempts to copulate but rarely succeeds at the first attempt. The female plays "catch me if you can" escalating rapidly to a height of 20-30 metres, with the male in hot pursuit. Contact is then usually broken, and the pair return separately to ground level. Invariably within 2 or 3 minutes they meet again, and another similar chase takes place. Eventually, if the female is receptive to the males advances, she flies to a nearby leaf, either in a bush or among low herbage, settling beneath it with her wings closed. The male is then able to move alongside and copulate. He then adjusts his position until the pair face opposite directions.

It is normal for Brimstones to remain paired for several days, presumably because the metabolic rate is very slow during the cool days of early spring, when frosts and snow commonly occur :

On 30th March 2008 I found a copulated pair in Crab Wood, Hampshire. A week later on 5th April they were still in copula beneath the same leaf of dog's mercury. I recorded an even longer duration in April 1986 when I found a pair of mating Brimstones under a bramble leaf at Tugley Wood in Surrey - a place which I then visited on almost a daily basis. I marked the spot, and revisited the site the following day, finding them still copulated. Out of curiosity I returned several times during the next few days, during which the weather remained cool. They remained copulated for an amazing 17 days, finally separating and flying off when warmer weather returned at the end of the month.

Brimstones tend to go to roost earlier than most other butterflies - at about 3pm ( or a little later in mid summer ) they stop nectaring, even if the weather remains sunny, and all the Brimstones at a given site begin to settle one by one in ivy clumps, or beneath leaves of bramble, oak, hazel, dock or pendulous sedge. Within about 15 minutes or so every Brimstone has gone to roost, and even the reappearance of warm sunshine later in the day fails to arouse them.