Differences in heat sensitivity between Japanese honeybees and hornets under high carbon dioxide and humidity conditions inside bee balls.

Upon capture in a bee ball (i.e., a dense cluster of Japanese honeybees forms in response to a predatory attack), an Asian giant hornet causes a rapid increase in temperature, carbon dioxide (CO₂), and humidity. Within five min after capture, the temperature reaches 46°C, and the CO₂ concentration reaches 4%. Relative humidity gradually rises to 90% or above in 3 to 4 min. The hornet dies within 10 min of its capture in the bee ball. To investigate the effect of temperature, CO₂, and humidity on hornet mortality, we determined the lethal temperature of hornets exposed for 10 min to different humidity and CO₂/O₂ (oxygen) levels. In expiratory air (3.7% CO₂), the lethal temperature was ≥ 2° lower than that in normal air. The four hornet species used in this experiment died at 44-46°C under these conditions. Hornet death at low temperatures results from an increase in CO₂ level in bee balls. Japanese honeybees generate heat by intense respiration, as an overwintering strategy, which produces a high CO₂ and humidity environment and maintains a tighter bee ball. European honeybees are usually killed in the habitat of hornets. In contrast, Japanese honeybees kill hornets without sacrificing themselves by using heat and respiration by-products and forming tight bee balls.