Ants are important harvesters of plant-derived sugars, but little is known about how anthropogenic disturbances influence this behaviour. We investigated factors related to honeydew harvesting by red wood ants, Formica aquilonia, in managed boreal forests. Ant activity was lowest in middle-age stands (30–40 years old), which had the fewest and smallest ant mounds. It was best predicted by a model containing an interaction among tree species, basal diameter and stand age. Individual workers collected similar masses of honeydew from the different aged stands, which suggests that colonies optimise foraging efforts by adjusting the number of active foragers to match resource availability. The rate of honeydew harvesting from recently clear-cut stands during the survey was similar to that in old stands, although significantly faster than in middle-aged stands. This may be a result of high aphid loads on clear-cuts and recent changes in forest management that improve the temporal continuity of forests for red wood ants. Anthropogenic alteration of habitats thus significantly alters energy use by ants, mainly as a result of changes in the abundance and size of ant colonies. This change is likely to have further consequences for ant-driven ecosystem functions.