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Adults begin flying in mid-March in the southern reaches of its habitat and remain active until the end of May in the north. There is likely only one generation per year as demonstrated by S. inornata, a closely related group, which are univoltine (Hodges 1974).

Semioscopis merriccella looks similar S. packardella with the overall colouration of the costal margin and most of the forewing being brown (Hodges 1974). The presence of a black band in the basal anterior region of the forewing distinguishes S. packardella from S. merriccella (Hodges 1974). If the band is present in S. merriccella it will be relatively narrow and the costal margin will be concave (Hodges 1974). Forewing length of S. merriccella is usually longer (11 to 15 mm in length) than S. packardella (Hodges 1974). Genitalic features can also be used to distinguish the two species from each other (Hodges 1974). Male S. merriccella have two cornuti on a narrow and curved aedeagus (Hodges 1974). In comparison, the aedeagus of S. packardella males are straight, taper to a point and have eight cornuti (Hodges 1974). Females are distinguished based on the number of inward directed projections on the signum (Hodges 1974). Female S. merriccella have several of these projections while S. packardella females have two (Hodges 1974).