Effects of genotype, environment, and their interactions on honey bee health in Europe Marina D Meixner 1 , Per Kryger 2 and Cecilia Costa 3 There are several reports of honey bee populations in Europe which survive without treatment for Varroa. However, when evaluated outside their native area, higher survival and resistance traits were not observed in colonies of a survivor population. Varroa infestation is strongly influenced by environmental factors, probably affecting threshold levels on a European scale. In a Europe-wide experiment colonies of local origin survived significantly longer than colonies of non- local origin, clearly indicating the presence of genotype– environment interactions. Transmission by Varroa selects for virulent strains of DWV, but it is currently unknown how these may interact with different genotypes of bees. The distribution of Nosema ceranae is significantly affected by environment, but there is at least one Nosema-resistant population. Addresses 1 LLH Bee Institute, Erlenstr. 9, 35274 Kirchhain, Germany 2 Department of Agroecology, Aarhus University, Forsøgsvej 1, 4200 Slagelse, Denmark 3 Consiglio per la ricerca in agricoltura e l’analisi dell’economia agraria - Unita ` di ricerca di apicoltura e bachicoltura, via di Saliceto 80, Bologna, Italy Corresponding author: Meixner, Marina D (marina.meixner@llh.hessen.de) Current Opinion in Insect Science 2015, 10:177–184 This review comes from a themed issue on Social insects Edited by Christina Grozinger and Jay Evans doi:10.1016/j.cois.2015.05.010 2214-5745/# 2015 Elsevier Ltd. All rights reserved. Introduction There is now agreement among scientists [1] that mul- tiple factors contribute to the frequently reported colo- ny losses of honey bees (Apis mellifera). Recently, increased efforts are noted to analyze this phenomenon using a holistic approach, by investigating interactions between the multiple factors, such as parasites and pesticides, that are known to affect honey bee health [2–4]. Indeed this approach is the most promising, because it reflects the complex situation of a honey bee colony which is closely linked to the environment it lives in [5]. However, one aspect relevant for colony health has re- ceived less attention than pathogens and parasites, or pesticides: the interactions between a colony’s genotype and its environment. It is worth noting that the natural range of Apis mellifera extends from Northern Europe to South Africa and from Iberia to Central Asia [6]. Within this large distribution range there is considerable envi- ronmental variation, both in relation to weather patterns and flowering seasons, and to occurrence and variation of parasites and pathogens. Ten of the approximately 26 sub- species are of European origin, including A. m. carnica and A. m. ligustica, which are preferred by beekeepers world- wide [7–9]. Breeding activities have focused on commer- cially desirable traits, often using inter-subspecies crosses and mass reproduction from limited stock, leading to hybridization or replacement of the original honey bee population in many places [7,10–12]. Substantial numbers of queens are traded across the borders of Europe, espe- cially A. m. ligustica from Italy, A. m. carnica from Austria and Slovenia, and the Buckfast hybrid mainly from cen- tral European countries [7]. In consequence, the natural distribution of the autochthonous European honey bee populations has changed very much. Other human activi- ties, such as agriculture and urbanization have also affect- ed the distribution and status of European honey bee populations, negatively influencing genetic diversity and density of honey bee colonies [13]. The arrival of the invasive parasitic mite Varroa destructor in Western Eur- ope in the 1970s [14] all but wiped out the wild honey bee population [8,9]. Management practices changed to in- clude regular application of acaricides, thereby helping to secure the survival of colonies. In consequence, though, mite-susceptible colonies are given the chance to propa- gate and transmit their susceptibility traits to the next generation. In the long term, this practice leads to an increasing dependency on medication and prevents the establishment of mechanisms of mite-tolerance, as de- tailed in a recent study [15  ]. In spite of the availability and regular application of Varroa control measures, higher than usual colony losses were noticed in Europe from 2002 to 2003 onwards [16–18]. The question of adaptation of genotypes to rapidly chang- ing environments on honey bee health in Europe has only recently received scientific interest. In this paper, we review the impact of two invasive parasites, Varroa destructor and Nosema ceranae, known to affect honey bee health, under the aspect of environmental influence and genotypic variation, with a special focus on genotype– environment interactions. Available online at www.sciencedirect.com ScienceDirect www.sciencedirect.com Current Opinion in Insect Science 2015, 10:177–184