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

Abstract

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 (CO 2), and humidity. Within five min after capture, the temperature reaches 46°C, and the CO 2 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, CO 2, and humidity on hornet mortality, we determined the lethal temperature of hornets exposed for 10 min to different humidity and CO 2/O 2 (oxygen) levels. In expiratory air (3.7% CO 2), the lethal temperature was ≥ 2° lower than that in normal air. The four hornet species used in this experiment died at 4446°C under these conditions. Hornet death at low temperatures results from an increase in CO 2 level in bee balls. Japanese honeybees generate heat by intense respiration, as an overwintering strategy, which produces a high CO 2 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. © 2012 Zoological Society of Japan.