Caste is carefully regulated in termites.  Indeed, the success of all social insects is reliant on their ability to maintain optimal levels of soldiers, workers and reproductives.  Over a 3-year period, we conducted bioassays each month (except June) with worker only groups placed in a 28°C incubator in total darkness and found that the number of days needed for soldier formation of the Formosan subterranean termite, Coptotermes formosanus Shiraki, varied depending on the time of the year (month).  In March, just prior to the major swarming exodus for alates of this species (April), 9 days was required before a worker molted to presoldier.  All other months took longer before such a molt occurred, with the longest required time to molt being in December (30 d); a time of reduced egg laying and caste-dependent duties for most natural colonies. The number of days required for soldier formation was closely aligned with the formation time of presoldiers.  The length of time that termites were kept in the laboratory before being used for the bioassays (7 d – 1 yr) did not affect this annual rhythm.  This is the first demonstrated evidence of an environment-independent circannual clock in a social insect.