Photoperiod is an important cue that entrains and synchronizes main events in the life cycle of the temperate bumble bee Bombus impatiens

I examined the role of photoperiod as an environmental regulator of important events in the colony life cycle of the Common Eastern Bumble Bee Bombus impatiens. Temperate primitively eusocial bumble bees (Bombus) exhibit seasonal changes in colony development and behavior that closely track associated environmental changes. However, very little is known regarding the role of environmental signals in synchronizing the internal changes of the colony life cycle with the external environment. To investigate the effect of photoperiod on the colony life cycle I exposed young colonies to one of eight different photoperiod treatments and followed the life cycle until the queen’s death, where all other variables such as temperature, humidity and food were held constant. I found that colonies exposed to photoperiods with a continuously increasing day length produced larger colonies and synchronized their production of males and new queens to the final stages of the colony’s life cycle. Conversely, colonies exposed to constant conditions such as constant darkness, constant light or constant 12L:12D produced smaller colonies and there was no synchronization of the production of males and new queens. I also found that colony daily patterns of activity and rest can be entrained by changes in photoperiod, providing evidence of a colony circadian clock. In conclusion, I have shown that photoperiod is sufficient but not necessary to synchronize key stages in the annual life cycle of this species.