Temperature and CO₂ concentration in honey bee hives exhibit circadian rhythms

Worker honey bees exhibit circadian rhythms with respect to locomotor activity but circadian analyses have seldom been applied to colony-level behavior. Circadian rhythms have been defined as having three main characteristics: a period of approximately 24 h maintained in the absence of external cues; a period maintained over a range of temperatures; and a phase fixed by external cues from the environment. In this study honey bee colonies were subjected to two kinds of light regimes at 5 °C in a cold storage unit (CSU): (1) constant darkness; and (2) after 6-12 d constant darkness, 12 h light exposure from 6PM to 6AM (i.e. a phase approximately 12 h offset from ambient conditions). Periodogram analyses of data from the 1st light regime showed that temperature and CO₂ concentration had stable 24 h periods after 20 d, as was observed for colonies in warmer temperatures in outside conditions. Period strength of temperature decreased over time in the CSU but not CO₂. Cosinor analyses of data from the 2nd light regime showed a temperature phase change of about 9 h 37 min between the end of the CSU period, after 28-33 d in light regime, and after 7-12 d in outdoor conditions in the post-CSU period. The same comparison for CO₂ concentration showed a phase change of about 11 h 55 min. These data indicated honey bee colonies produced circadian rhythms in hive temperature and CO₂ concentration with periods both present in the absence of external cues, and with phases that can be driven by light. Rhythms associated with CO₂ concentration changed with respect to light treatment more than rhythms associated with hive temperature. Based on data from longer-term (60 d) experiments, daily rhythm phases and day lengths differed significantly between hive temperature and CO₂ after 15 d in the 12 l:12D light regime, and remained so in outdoor conditions.