**1. Introduction**

Honey bees are important pollinators of most wild plants [1] and agricultural crops [2]. They are the most economically important group of pollinators worldwide [3] and are also crucial for maintaining biodiversity [4]. In particular, the economic contribution of the honey bee *Apis mellifera* to agriculture is estimated at USD 20 billion in the US and more than USD 200 billion worldwide [5].

Over the past decades, significant losses of wild and domestic bees have been reported in many parts of the world [6], threatening the ecosystem services they provide. Many hypotheses have been put forward to explain these losses, but the causes are not ye<sup>t</sup> clearly identified [3]. So far, no single factor appears to act as the main driver of bee decline [7,8] and this phenomenon is now widely regarded as multifactorial [6–10]. Among the factors involved, biological and chemical agents are at the forefront. Indeed, bees are chronically exposed to pesticide cocktails, but also to many parasitic and infectious agents (PIAs), some of which are still emerging as they are disseminated by humans and international transport [9]. In addition, other stressors such as habitat loss, beekeeping practices, climate change or decreased abundance and diversity of floral resources are likely to contribute to the decline of bees, making them more sensitive to other stressors [9]. This multiplicity of stressors makes

the diagnosis of bee decline all the more complicated given that bees are exposed simultaneously or successively to several stressors, resulting in many interactive and synergistic effects [11].

Although mass mortality is the most striking impact of stressors on bees, some non-lethal effects can also lead to important losses. For example, one hypothesis to explain recurrent weakening of honey bee colonies is that disoriented bees are no longer able to find their way home [12]. Thus, the effect of stressors on bee health is not limited to lethal effects but is also related to behavioral changes [13], impaired cognitive functions and sensory abilities [14,15], and physiological [16], molecular [17] and genetic changes [10]. At present, the effects of pesticides and other stressors on bee health are still poorly understood and are not evaluated by standard regulatory procedures for risk assessment [18]. The economic and ecological challenges represented by bee decline explain the current search for risk assessment procedures and methods to analyze the impact of stressors on bee health [19]. This meta-analysis was conducted to identify gaps in the current research and new potential directions for research.

The aims of this meta-analysis were (i) to carry out an inventory of the bee populations under study, the stressors studied and the methods used by the scientists, (ii) to investigate whether the stressors with the greatest impact on bees could be identified, and (iii) to explore whether the evidence for an impact varies according to the type of study or to the scale of study. This work follows a bibliographic study carried out in 2016 on the exposure of honey bees to plant protection products [20].

#### **2. Materials and Methods**

#### *2.1. Identification of the Key Concepts and the Relevant Keywords*

The population, exposure, outcomes (PEO) method was used to define the key concepts of the analysis. Three key concepts were then identified: the target population, the stressors studied and the methods used. Keywords were listed for each key concept after reading a subset of scientific papers related to the impact of stressors on *Apis mellifera*. A search was subsequently performed in Scopus and Cab Abstract databases with these keywords (see Figure S1 in the Supplementary Materials for details of the search string) and resulted in the selection of 3999 articles.

## *2.2. Literature Search*

The target population included in the study comprised subspecies of the honey bee *Apis mellifera* with the exception of *A. m. scutellata*, *A. m. capensis* and Africanized bees. All epidemiological units (colony, adults, brood) and development stages (eggs, larvae, pupa) were included in the analysis. The papers included in this survey were published during the last ten years (from 2007 to 2017, last access: 6 March, 2017). The articles were available in full text and written in English. The primary search in the Scopus and Cab Abstract databases resulted in the selection of 3999 articles; 1187 duplicates were removed (Figure 1), then 717 articles were excluded because they dealt with *A. m. scutellata* or *A. m. capensis* (82 articles), with other organisms (73 articles), with the efficiency of veterinary treatments (117 articles), with the presence of pesticides in bee matrices (26 articles) or because they were off topic (419 articles).

A new search was performed on the remaining 2095 articles to better focus on the impact of stress on honey bees. To be included in the analysis, the title of the articles had to contain the following words: ("honey bee" or "mellifera") and ("impact" or "affect" or "effect" or "influence" or "toxicity" or "impair" or "induce"). Following this procedure, 386 articles were selected. Reviews and articles dealing with stressors considered as "anecdotal" (i.e., stressors to which bees are rarely exposed for example caffeine, nanoparticles; see details in Figure S2, Supplementary Materials) were excluded from the list. After this selection process, 293 articles were included in the analysis (see Table S1 in Supplementary Materials for the references). Although our paper selection was implemented thoroughly, we acknowledge that some references may have been omitted, however we believe that the number of these references is very small.

**Figure 1.** Preferred reporting items for systematic reviews and meta-analyses (PRISMA) flow diagram describing the meta-analysis of literature on the impact of stressors on honey bees' health—summary of how the systematic search was conducted and eligible studies were identified.
