Females, are most commonly seen and can be startling to unsuspecting observers. They're bluish-black, somewhat larger, more flattened and less fuzzy than bumble bees. Although they're not normally aggressive they will sting if annoyed.
Males are seclusive, blond to tan in color, somewhat smaller and harmless.
Nesting females burrow finger-size holes into dead tree limbs and trunks, fireplace wood, poles or unfinished beams or rafters of buildings to nest.