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Rich cream-yellow with maroon brown markings, consisting of two to three wedge shaped spots along the forewing costa, prominent discal spots, and variable PM and submarginal lines. The forewing PM band is usually light and spotty, but always widens to a patch at the anal margin. Fringe entirely maroon-brown, unlike the similar M. occiduaria which has the dark forewing fringe restricted to the apex. Sexually dimorphic, females with slightly reduced wings, although not to the same extent as andersoni, simplex and occiduaria. Ferguson (1953) named the subspecies amboflava from western Canada (type locality Calgary) to describe our yellow populations, which are, according to Ferguson, quite distinct from the smaller, greyish eastern sulphurea.

The life history has not been recorded in detail. Mature larvae are light green with a slightly paler head and a cream dorsal and lateral stripe (Franklin 1907, cited in McGuffin 1972). In eastern North america this is a forest species associated with blueberry (Vaccinium) and sweet gale (Myrica gale). The western race prefers drier habitats, and is not found in association with these plants. The differences in adult size, colouration and habitat preferences between eastern and western populations suggests that they may be separate species.