In the north this species tends to only have one brood but in the south it can have up to three. Adults emerge around the beginning of April to May and if there is a summer brood it emerges from the end of July and lasts until September.
The female lays her eggs at the base of flower buds. The eggs are small but visible, pale discs and they hatch after about two weeks. The larvae then develop through four instars and can appear in at least three forms; most are plain green but some have either purple dorsal and lateral stripes or pink dorsal and lateral stripes. Larvae first eat the flower buds and later the insides of the developing fruit in which they leave a trail of holes.
One unusual feature of the species is that no-one has ever reported seeing the pupal stage in the wild; perhaps because the pupa are high up in trees or shrubs, or low down, possibly underground, where they may be looked after by ants.
The parasitic ichneumon wasp Listrodomus nycthemerus appears to play a major part in the butterfly's life cycle, causing the cyclical boom and bust pattern in its populations. The wasp injects an egg into a first instar larva, which then develops inside the larva without killing its host until it emerges as a wasp from the pupal shell. The Holly Blue spends winter as a pupa and it is assumed that wasp eggs laid on second or third brood larvae also overwinter inside the pupa, which is not killed until just before the wasp emerges the following year. A few days before this event the pupa becomes discoloured in patches.
We know nothing about the success of this wasp in Yorkshire but since the butterfly arrived in 1990 it has gone through the typical population cycles that are known in the south and thought to be associated with the burden of the wasp. The wasp builds up its population with that of the butterfly until the wasp is prolific enough to kill lots of the butterflies and reduce the wasp’s own numbers as a result. The butterfly then breeds again in peace for a few years and build its numbers up again until the wasp catches up and the cycle starts again.