Mechanisms of resistance and tolerance in African and European honeybees Apis mellifera L., against Varroa destructo

Abstract

The honeybee, Apis mellifera L., is indispensable to global food security, poverty alleviation and natural biodiversity conservation. However, the ecto-parasitic mite Varroa destructor and its associated pathogens are one of the most serious threats to the health of honeybees, especially both wild and managed European honeybees found in Europe and North America. In contrast to European honeybees, their African counterparts appear to be minimally affected by these stressors. However, the underlying mechanisms that contribute to their survival against the mites remain mostly unknown. To test the hypothesis that resistant defence behavioural mechanisms are responsible for the survival of A. m. scutellata in Kenya, grooming and hygienic behaviours in this honeybee subspecies with those of A. mellifera hybrids of European origin found in the USA against the mite were compared in chapter two. The description of two newly damage patterns inflicted on mites by honeybees in both African and European honeybee colonies is highlighted. Additionally, the potential role of grooming behaviour as a tolerant defence mechanism that could reduce the detrimental effects of the mites in the savannah honeybee colonies was underscored,though the expression levels of hygienic behaviour were similar in both honeybee subspecies. However, both hygienic and grooming behaviours could not explain the lower mite-infestation levels recorded in A. m. scutellata colonies. To explain the low mite numbers recorded in A. m. scutellata colonies, chapter three explored the involvement of other potential resistant mechanisms including suppression of mite reproduction in worker brood cells of this subspecies. Low fertility, fecundity and numbers of mated female offspring were identified as adaptive resistance processes of reduced Varroa mite reproductive success in A. m. scutellata colonies, which explained the slow mite population growth in colonies of this subspecies. Furthermore, mite offspring mortality in both sexes and absence of male offspring were identified as key factors to account for the low numbers of mated daughter mites produced in A. m.scutellata colonies. The relationship between Varroa mite-infestation levels on adult worker honeybees, grooming behaviour and titres of the insect juvenile hormone III (JH III) and that of its immediate biosynthetic precursor, methyl farnesoate (MF), MF + JH III, ratio of JH III to MF in the haemolymph of the African and European honeybees was explored in chapter four. Here, the results suggest that these hormones may not regulate these traits in the honeybee subspecies due to the absence of a significant correlation between them. Overall, this study has revealed the behavioural mechanisms that partly confer survival strategies in this specific A. m. scutellata population against the mite without requiring any miticide treatment. The study has also revealed that JH III, MF, MF + JH III or ratio of JH III to MF may not be considered as potential biomarkers for some behavioural traits studied herein in honeybees. Nevertheless, additional studies are necessary to help shed more light in this interesting area.