[Thesis defence] 4 May 2023 - Amélie Noël: "Chemical ecology of the behavioural defence of the honey bee (Apis mellifera) against the parasite Varroa destructor", UR406 Bees & Environment Laboratory - INRAE PACA, Avignon

Date and place

4 May 2023 at 2pm
Avignon University, Jean-Henri Fabre Campus, CERI, Amphi Blaise

Discipline

Agricultural sciences

Laboratory

Laboratory UR406 Bees & Environment - INRAE PACA, Avignon

Management

Dr. Yves Le Conte & Dr. Fanny Mondet

Composition of the jury

• Pierre Giovenazzo, PR, Université de Laval, Québec - Rapporteur
• Anne-Geneviève Bagnères-Urbany, DR-HDR, CNRS, CEFE, Montpellier - Rapporteur
• Freddie-Jeanne Richard, MCF-HDR, CNRS, EBI, University of Poitiers, Poitiers - Examiner
• Jean-Christophe Sandoz, DR-HDR, CNRS, EGCE, Gif-sur-Yvette - Examiner
• Yves Le Conte, DR-HDR, INRAE, A&E, Avignon - Thesis supervisor
• Dr. Fanny Mondet, IPEF, INRAE, A&E, Avignon - Thesis co-supervisor

Summary of the thesis

Varroa destructor is a global threat to the honey bee (Apis mellifera) and beekeeping. If left untreated, varroa infestation of a colony can compromise its survival in just a few months. However, some colonies have developed resistance to the parasite and are able to regulate the growth of the varroa population within the colony. This resistance is based on various characteristics, including VSH behaviour (Varroa Sensitive Hygiene) and the re-operculation of brood cells. These two hygienic behaviours involve (i) the detection of parasitic brood cells, (ii) the opening and then (iii) the choice of what to do with these brood cells. Depending on the choice made, the brood is sacrificed and the cell emptied of its contents (VSH) or it is reoperculated, leaving the brood intact (reoperculation). The mechanisms underlying the choice between these behaviours remain relatively unknown, but they require the emission of a semiochemical message, the detection and neuronal processing of this message by the workers and the appropriate behavioural response. In the scientific literature, ten compounds have been identified as triggering VSH behaviour, and no compound has been identified as triggering re-operculation behaviour. Only two of these compounds are characterised as Volatile Organic Compounds (VOCs).

It is thought that compounds of different volatility (VOCs and low-volatility compounds) are involved in VSH and re-operculation behaviour. The VOCs are thought to propagate rapidly through the wax seal, signalling parasitism, while the less volatile compounds allow precise identification of the parasitized alveolus. The aim of this thesis is to learn more about the hygienic behaviour involved in the resistance of honey bee colonies to the parasite. Varroa destructorand in particular by helping to decipher VSH behaviour and re-operculation behaviour. To this end, VOC emissions were studied in the context of healthy brood development and varroa parasitism, and the behaviour of workers towards all the compounds identified as being characteristic of parasitised brood was tested. This thesis highlighted the emission of 32 VOCs during healthy brood development and nine VOCs characteristic of varroa parasitism, five of which were identified. The behavioural results highlight the importance of VOCs in the detection, opening and re-operculation of brood cells, while low-volatile compounds appear to play a particularly central role in pupal sacrifice. This study opens up new perspectives for understanding these two resistance behaviours of honey bees in the face of their main parasite. Varroa destructor.

Keywords : Varroa destructorchemical communication, volatile compounds, resistance, VSH, reoperculation.