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- Name also: Common Brimstone
- Family: Whites and Yellows – Pieridae
- Subfamily: Sulphurs – Coliadinae
- Wing span: Medium-sized(–large), 45–60 mm (1.78–2.37 in.).
- Wing upper side: Male (lemon) yellow, female greenish white. Reddish central blotch on both forewing and hind wing.
- Wing underside: Like upper side.
- Habitat: Open forest land with an abundance of flora. Also culturally-influenced areas.
- Flying time: Mid-March–late June and second generation late July–October. Not seen in the middle of summer.
- Overwintering form: Adult butterfly.
- Larval foodplant: Alder buckthorn (Rhamnus frangula) and common buckthorn (R. catharticus).
The brimstone is easy to distinguish from its close relatives (if one can see the upper surface of the wings) because it is the only one of the Sulphurs that doesn’t have a dark border, and there are no Finnish species that it can conceivably be confused with. The sharp wing tips, its yellow or greenish white colour and reddish central blotches make the brimstone look quite unique. The brimstone always holds wings against each other when it is resting, so it is difficult to study the upper surface of the wing without catching it first.
The brimstone is one of the first butterflies to appear in the spring. Males become active weeks before females – often already in March – and fly around looking for females. The females usually begin to fly only around May, and after mating they lay their conical eggs one by one or in groups of several eggs among the leaves and buds of buckthorn plants, and on other buds if there are no leaves. The caterpillars are the same colour as the leaves and thus not so easy to spot, and they also make their cocoons among buckthorns.
The brimstone can be found in the southern half of Finland, as far north as Oulu – Kajaani, and it is one of Finland’s longest-lived butterflies. Not counting the caterpillar and cocoon stages (many caterpillars live for two seasons) it can reach a full year old, which is more than most other butterflies.
Of all the butterflies in Finland, only a few species overwinter as adults. The most common way to overwinter (around 70% of Finnish butterflies) is as a caterpillar, followed by as an egg, which 10% of species do. Species that normally overwinter as adults (and fly first in spring) are besides brimstone, comma, small tortoiseshell, peacock and Camberwell beauty.