Anthidium (Anthidium) formosum are dark brown to black with yellow maculations (Gonzalez and Griswold 2013). Females have yellow to ferruginous pubescence, sometimes with white pubescence on the sides of the thorax and abdomen, and have a body length of 11.5–13.5 mm (Gonzalez and Griswold 2013). Males have white pubescence on their head and thorax, and range in length from 12.8–15.4 mm (Gonzalez and Griswold 2013).
(modified from Gonzalez and Griswold 2013)
Anthidium formosum may be confused with A. illustre based on the large body size, female T6 distal margin with a deep median emargination, and elongate male penis valves (Gonzalez and Griswold 2013). Both sexes of Anthidium formosum can be differentiated from A. illustre by the lack of preapical projections on the labrum (Gonzalez and Griswold 2013). Male A. formosum can also be differentiated from A. illustre by the shape of S4 and S6 (Gonzalez and Griswold 2013).
Anthidium formosum adults have been recorded in flight from May to September, with peak activity occurring from the last half of June to the first half of August (Gonzalez and Griswold 2013).
Anthidium formosum is a generalist that has been observed visiting a variety of species within Asteraceae, Boraginaceae, Fabaceae, Lamiaceae, Orobanchaceae, and Ranunculaceae (Gonzalez and Griswold 2013).
Anthidium formosum nesting biology was determined from trap nests (Horning 1969; Parker 1987). Nests plugs are constructed out of a variety of materials, ranging from only trichomes to trichomes with pebbles, small pieces of wood, masticated plant material, or lizard dung (Krombein 1967). Horning (1969) also observed resin in the nest plugs of three A. formosum nests in Idaho.
Anthidium formosum occur throughout California, Oregon, Washington, Montana, Wyoming, and Colorado. They are found primarily in the Great Basin, Snake-Columbia, and Wyoming Basin shrub steppe; Colorado Plateau shrublands; Wasatch, Uinta, and Great Basin montane forests; and eastern Cascades and Sierra Nevada forests (Gonzalez and Griswold 2013).