This study investigated the effects of alarm pheromone components on the heat production rates of hornets (Vespa crabro) by means of direct calorimetry. In a flow-through system, pheromones from hornets, honeybees (Apis mellifera) and yellowjackets (Vespula vulgaris) were sucked through a measuring vessel containing a group of hornet workers. The locomotive reaction of hornet workers was recorded as an increase of the heat production rate. Hornets exhibited a strong response to their own alarm pheromone components, mainly 2-methyl-3-butene-2-ol (MBO). They also reacted intensively upon the main alarm pheromone component of the honey bee, isopentylactetate (IPA), but less pronounced to alarm pheromone components of yellowjackets. The metabolic response of hornets to MBO was dose-dependent. The heat production rates of provoked hornet workers were similar to those of flying hornets. (z)-9-Pentacosene, a Substance which is believed to be a thermoregulative brood pheromone, induced no metabolic reaction. (C) 2003 Elsevier B.V All rights reserved.