During copulation, males of Bombus terrestris fill the queen's sexual tract with a mating plug after transferring their sperm. The sticky secretion is produced by the male's accessory glands and disappears within a couple of days. Experiments now show that the primary function of the plug is to reduce the subsequent mating probability of the queen. The plug is not efficient in preventing sperm migration into the spermatheca. Due to its low energetic value, the plug is also unlikely to serve as a nuptial gift. This type of male interference with female mating propensity has so far not been found in social insects. This finding could, at least tentatively, explain why females of B. terrestris may not be able to take advantage of the demonstrated benefits of multiple mating. Furthermore, such male interference could be a more general phenomenon in social insects, with obvious ramifications for the evolution of polyandry in this group.