Bee-balling is a defensive technique employed by Japanese honeybees, Apis cerana japonica, against the predatory Asian giant hornet, Vespa mandarinia. Upon recognition of the hornet intruder within the hive, hundreds of honeybees surround and restrain the hornet; forming a bee-ball. Subsequently the bee-ball experiences three distinct phases of temperature change (heating, heat retaining, and break up). The bees simultaneously elevate CO2 levels and temperature within the bee-ball, which jointly act to kill the hornet. To gain an improved mechanistic understanding of this process, a computational model of heat transfer and carbon dioxide transfer specifically examining the heating and heat retaining phase within the bee-ball was developed. The manipulation of model parameters to simulate different environmental conditions, bee arrangement and production rates provides insight into the process that would otherwise be difficult or near impossible to obtain through pure experimentation.

In this study, we considered the honeybees and the hornet to generate heat and CO2, while also exchanging heat to each other, and losing CO2 to the surroundings. To investigate the mechanism of the bee-balling behavior, we used COMSOL, a multiphysics finite element analysis and simulation software, to develop a simple geometry and replicate the heat and CO2 exchanging properties of the honeybees and the hornet during heating and heat retaining phase of the bee-balling process. The results of this study provide insight behind why bee-balls form, how the honeybees utilize heat and CO2 and modulate their movement, heat and CO2 production rate to effectively murder the murder hornet.