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Forewing mottled brown with a prominent white antemedian and postmedian patch at the costal margin; the AM patch sometimes extends as a band to the anal margin. Hindwing bright rust-orange with variably developed black median and marginal bands. Black patch extending from wing base to the median area below the discal cell. Sexes similar. Superficially similar to Dasyfidonia avuncularia, but the latter has two well-defined, continous black bands across the hindwing, not patchy and broken as in infans. Subspecies oregonensis, described from Port Orford, OR, is larger and lighter in colour and occurs in from southern BC southward. The hindwing dark markings are more extensive in populations from the southwest mountains of Alberta.

McGuffin (1988) details the immature stages. This is one of the first non-hibernating day-flying moths to emerge in the spring, having overwintered as pupae. The early flight period is reflected in the common name; Forbes (1948) called it the First-Born Geometer. Adults have rapid flight and are difficult to capture, but occasionally sip moist sand or mud on hot days; Song Sparrows have been observed to take advantage of this behaviour to prey on the moths (Newman & Donahue 1967).

Most reported larval hosts are in the Betulaceae, and include white birch (Betula papyrifera), dwarf birch (Betula glandulosa) and red alder (Alnus rubra). Because this species appears so early in the year, larvae of the European sister species A. parthenias first feed on catkins before leaves are available (Kimber 1999). A few rearings from willow and trembling aspen (Prentice 1963) may be accidental. Adults mud puddle and may visit animal dung (McGuffin 1988).