Previous studies of the obligately eusocial sweat bee Halictus ligatus Say, at a nesting aggregation in Victoria, southern Ontario, Canada, showed how changes in local environmental conditions led to marked variation in demography, body size, and social patterns. Under the harsher weather conditions of 1990, mortality was higher, colony sizes were lower, brood body sizes were smaller, and surviving colonies were more strongly eusocial than under the milder conditions of 1991. These patterns suggested that the link between environmental conditions and colony social organization might be mediated by foraging effort, especially by queens during late spring and early summer when colonies were most susceptible to environmental constraints. Social parameters such as differences in reproductive effort between queens and workers or between haplometrotic and pleometrotic colonies, might also have a basis in differences in foraging effort. In this paper, I examine annual, seasonal, and social sources of variation in foraging effort of female H. ligatus from Victoria. Foraging effort of queens and workers was described in terms of seven different measures: number of foraging trips per day, flight time per trip, handling time per trip, average round trip time, total flight time per day, total handling time per day, and total provisioning time per day. These parameters were measured by observing individually marked queens and workers during their respective foraging periods in 1990 and 1991. Previously, it had been shown that in 1990 there were fewer days with weather suitable for flying, but observations of foraging effort show that the relatively large queens of 1990 spent about as much time foraging as the relatively small queens of 1991, and still produced much smaller brood. This suggests that a previously unsuspected effect of the poor weather in 1990 colonies was a scarcity of pollen, at least during the queen foraging period. Both queens and workers appear able to adjust their foraging effort to current conditions. As a result, the expected correlation between increased degree of eusociality (reproductive skew more strongly favouring queens) and stronger division of labour (workers doing a greater share of colony parental investment) did not occur, as workers apparently worked harder when their own reproductive opportunities increased. This may be a general pattern in social halictids.