Seasonal movement of royal chambers: where are the kings and queens of temperate termites in winter?

Abstract

Overwintering is a critical part of the annual cycle for species that live in temperate, polar, and alpine regions. As a result, low-temperature biology is a key determinant of temperate species distribution. Termites distribute predominantly in tropical regions, and only a few species are found in the temperate zone. As with other social insects, termites are characterized by the division of labor between reproductive and non-reproductive castes in which the survival of reproductives is crucial to maintaining their society. Here, in the termite Reticulitermes speratus, we report the discovery of an underground royal chamber that kings and queens use to survive the winter, which is separate from the one they use during the warmer breeding season. Our investigation of field colonies indicates that in the spring the royals are localized in decayed logs on the ground, then move to their underground royal chamber located in the roots of stumps in the fall. The winter minimum temperature measured in the royal chamber was higher than the ground surface temperature. In overwintering termites, the kings and queens had higher cold tolerance than workers and soldiers. The kings and queens were at risk of mortality from −8 °C, compared to the workers and soldiers at −4 °C. Air temperatures dropped below this critical temperature of −8 °C multiple times, as evidenced from the past 140 years of weather records in Kyoto. This suggests that the underground movement of the royal chamber may contribute to avoiding the risk of overwintering mortality. These results demonstrate the social strategies implemented to overcome the environment met while living at the latitudinal limits. This study sheds light on one of the most important aspects of the biology of termites in terms of predicting their geographic distribution and spread by climate change. This work also helps further the understanding of the termite’s social system, seasonal phenology, long-term survivorship, and life cycle, and contributes to the development of pest control strategies.