Effect of hornet disturbance on intruder acceptance by honeybee guards

Guarding behaviour is the mechanism used by Apis mellifera to protect its colony from potential intruders (i.e. robber foreign bees) and is fundamental for the maintenance of the colony integrity. Such behaviour is based on the nestmate recognition process and is performed, at the hive entrance, by the bee guards which can distinguish between nestmates and non-nestmates due to the differences in the intruders' cuticular chemical profile. The resident guards' response against non-nestmate intruders can realize in aggressive behaviours aimed to repel them. The aggressive response varies alongside the season due to nectar richness in the environment and it is inversely proportional with the amount of available provisions. In early autumn such response is particularly evident due to the scarce presence of resources and it is the best moment to observe and investigate this behaviour. In the present thesis, conducted at the Department of Biology of the University of Florence under the supervision of Rita Cervo (external relator) and Federico Cappa (correlator), we focused our attention on the effect that a further stimulus of disturbance for honeybee guards, in addition to the presence of a foreign conspecific, can affect the ability of the guards to detect and efficiently repel non-nestmates. We chose hornets as added stimulus because, as predators, they represent remarkably concerning factors for the colony. We carried out our experiment during September 2018 because, in this period of the year, the guards' response, as well as hornet predation activity, are maximum. We tested 41 different colonies belonging to 5 different apiaries in the Florence surroundings. Field bioassays consisted of the presentation to bee guards of a non-nestmate bee lure alongside three different disturbance factors one by one: two predators lures, respectively Vespa crabro and Vespa velutina, and a generic disturbance factor (a black piece of sponge). The two hornets were chosen to evaluate if two different predators, respectively a native and an alien species, could elicit a different response in the prey. The trials were video-recorded and the video recordings were analysed with BORIS software to examine differences in guards' response to bee lures among the treatments. Data analysis revealed an overall similarity of response across the four treatments, suggesting that honeybee guards are capable of efficiently recognize and repel foreign foragers despite the presence of external disturbance factors. This result shows how guarding behaviour has been shaped by natural selection to permit guards' multitasking performance and a global efficient defensive system in A. mellifera. Nevertheless, further studies would be necessary to assess if more alarming situations (i.e. the presence of more V. velutina predators at the same time as usually occurs in the field), could impact differently on guarding behaviour.