The northern blue butterfly produces a single brood of young each year. Adult northern blue butterflies emerge in early June to July, with the males typically emerging before the females and flying within a metre of the ground as they patrol for a mate (2) (3) (4) (5). Mating usually occurs close to patches of dwarf bilberry (Vaccinium caespitosum), a small shrub which serves as a host plant for the northern blue butterfly’s larvae (2) (4) (5).
After mating, the female northern blue butterfly lays eggs singly on the stems of the host plant or on nearby vegetation. The eggs of this species remain dormant over the winter, not hatching until the following spring, when the caterpillar begins feeding on the host plant (2) (3) (4) (5). In some areas, the northern blue butterfly has also been observed using other species as host plants for its caterpillars, including black crowberry (Empetrum nigrum), Labrador tea (Ledum groenlandicum) and sheep laurel (Kalmia angustifolia) (2).
The caterpillars and pupae of the northern blue butterfly are tended by Formica ants, which probably help to protect the caterpillars from predators and parasitoids (4) (5). The caterpillar has specialised organs that produce a secretion that the ants feed upon, and probably also mimics ant pheromones to further manipulate the ants’ behaviour (5). The northern blue butterfly caterpillar usually pupates by mid-June to early July, forming a green pupa that gradually turns yellow and then black before the adult emerges about ten days later (4) (5).
The adult northern blue butterfly feeds on the nectar of a variety of native and non-native flowers (2) (3) (4) (5), but is nearly always found near to patches of the caterpillar’s host plant (5). Although the adult may live for up to three weeks after emerging, the average adult survival is usually no more than a week (5).