Physiological and behavioural stress responses in the social honeybee, Apis mellifera

The honeybee Apis mellifera is the most important economic insect on the planet. It is also one the most important invertebrate models in neuroscience and social animal behaviour. A large part of honeybee research this past decade has been devoted to understanding the new sources of threats that bees are facing in the industrialised world in order to prevent the collapse of native and domesticated bee populations. More than ever we need to protect them and understand them better. This research project entitled "Physiological and behavioural stress responses in the social honeybee, Apis mellifera" explores specific responses to physical stressors but also considers stress responses in the social context of the bee hive that is vital for the individual honeybee to survive. In Chapter 1, the thesis is introduced by a broad literature review to define the terms and concept of stress in honeybees. This first part develops the following questions:what is "stress"? what can we call a "stress response"? How could we measure a response to stress in the honeybee? What are the areas of stress research in bees that need focus in the following years? This review helps to define the key research questions addressed in the thesis, especially concerning the measures of behavioural and physiological responses to stressors but also the eventual roles of stress sensitivity in the organisation of honeybee society. Chapter 2 reports the investigation of physiological responses (biogenic amines) and behavioural response (sting reflex) to physical stressors applied to bees for various durations. I found that dopamine and serotonin were increased after a long-term (three hours) stressor, but not after a short-term (up to twenty minutes) stressor. The nature of the stressor also affected differently the increase of octopamine and serotonin in the brain. These results were discussed in regard to the previously existing stress responses theories and mechanisms. Chapter 3 explored the role that stress sensitivity could have in the organisation of division of labour. We found that the first bees to react to a threat, the guards, were more sensitive to a stressor than the bees later recruited to a threat, the soldiers. Stress responsiveness also varied with age, and the distinctions in stress responsiveness between soldiers and guards were less pronounced in young bees. Differences in brain biogenic amine levels were found between castes, but brain biogenic amine levels did not correlate with stress responsiveness either between or within behavioural castes. This result supported the response threshold model of division of labour but is at odds with the defensive behavioural syndrome theory. In Chapter 4, I tested the effect of systemic cocaine treatment, and the systemic injection of neuropeptides of interest (corazonin, allatostatin A and alatostatin CC) on stress responses. Allatostatin A and Corazonin were not found to affect behavioural stress response but Allatostatin CC partially reduced sting responsiveness. Cocaine treatment increased sting responsiveness. This chapter along with chapter 2 clarified the hypothetical model of physiological stress responses in honeybees presented in Chapter1. Chapter 5 focused on how a social stress (long term loss of the queen) affected colony organisation and some worker physiological traits. It is well known that the absence of the queen changes the distribution of reproductive functions as workers develop their ovaries to lay male-eggs. But this chapter uncovered that the distribution of other tasks were also disturbed, and workers tended to become generalists in performing multiple tasks at the same time like nursing, foraging and laying eggs. This study revealed the importance of studying stress at a social level, and how the homeostasis of the social organisation is fragile. To conclude, Chapter 6 discusses generally how this body of work addresses the initial research questions about stress in honeybees. Indeed this thesis yields some clarity about: how "stress" can be defined in honeybees, What are the physiological component of stress responses in honeybees, and finally how can stress affect the social organisation of honeybees both at the individual level and the colony level. Also some suggestions for future research possibilities are addressed.