Habitat loss has been implicated in the decline of forest-dwelling caribou (Rangifer tarandus caribou), but it is unknown how biting insects, potentially important components of boreal forest habitat for caribou, influence the activity of this threatened species. During summers in 2011 and 2012 in northern Ontario, Canada, we quantified the relative abundance of black flies, mosquitoes, and tabanids in boreal forest stands of different ages and related their abundance to caribou activity. We counted insects in young (25–35 yrs since forest harvesting), intermediate (36–69 yrs), and old (>70 yrs) stands using sweep nets and counts on human subjects. We related the daily variation in abundance of these insect families, along with daily maximum temperature, to the activity of female caribou, determined by accelerometers in global positioning system collars. We found higher insect abundance in young versus old stands. During the first 5 minutes in a forest stand, the rate of accumulation of mosquitoes and black flies on human subjects increased, but at a decelerating rate, whereas tabanid abundance declined over time. On days when tabanids were more numerous, female caribou were less active, possibly a response to reduce exposure and harassment. To a lesser extent, mosquitoes and black flies also tended to elicit lower activity of caribou. Our study reveals that biting flies can alter the behavior of female caribou in the boreal forest. Loss of old stands may accentuate the potential for insect harassment.