A case for a free-running circannual rhythm in soldier developmental time of Formosan subterranean termites

The success of all insect societies relies on their ability to maintain optimal levels of different castes. Here we report on an apparent free-running circannual rhythm that optimizes the developmental time of the soldier caste of Coptotermes formosanus Shiraki. Over a 3 year period, bioassays were conducted each month (except June) with groups of 100 termite workers in a 28°C incubator in total darkness. The number of days needed for C. formosanus soldiers to develop varied depending on the time of the year (month). In March, just prior to the major swarming exodus for alates (April to June), 9 days were required before a worker molted to a presoldier. Longer times were required for such a molt in all other months, with an increasing trend from April to December (from 13 to 30 days) and a decreasing trend from January to February (from 25 to 12 days). Colony origin or the length of time that termites were kept in the laboratory under constant conditions (26 – 28°C, 70 – 80% RH) before testing (7 days – 1 year) did not affect this rhythm. This is the first demonstrated evidence of a free-running circannual rhythm in a social insect. Received 23 July 2007; revised 9 and 21 August 2007; accepted 23 August 2007.