**4. Conclusions**

This meta-analysis highlights the grea<sup>t</sup> diversity of techniques used by researchers in honey bee experimentations and the need to standardize the protocols. To do so, populations at risk, critical stages of development and the most relevant parameters to be measured should be identified. New standard tests should be developed, especially to better study the sub-lethal effects of stressors on bee health. In addition, greater importance should be given to the bee subspecies studied, to understanding the differences in their sensitivity to stress, and if possible, to identify one or two indicator subspecies. Moreover, this study highlighted the need to break through two important methodological and biological barriers that make risk assessment difficult: the technical difficulties encountered in field tests and the buffer effect of the colony. New technologies such as RFID chips or mathematical modeling could help to overcome these obstacles. This study also highlighted innovative research paths, particularly with regard to the impact of climate and habitat fragmentation, which, according to the few studies already carried out, could have significant deleterious consequences on bee colonies. Finally, as also pointed out by Benuszak et al. [20], efforts to strengthen the number of studies on the impact of co-exposures and metabolites should be continued. In order to develop standard protocols, the search for biomarkers as diagnostic tools seems to be an interesting route of exploration. This biomarker research should be facilitated by "omics" techniques such as genomics, transcriptomics, proteomics, and metabolomics.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/1424-2818/12/1/7/s1. Figure S1: Search string used to screen the SCOPUS and CAB ABSTRACT databases to select articles related to the impact of stressors on *Apis mellifera* published between 2007 and 2017, Figure S2: list of stressors considered as "anecdotal" to discard related articles from the review on the impact of stressors on *Apis mellifera* published between 2007 and 2017, Table S1: list of the 293 articles related to the impact of stressors on *Apis mellifera* published between 2007 and 2017 included in the analysis, Figure S3: Percentage of articles related to the impact of stressors on *Apis mellifera* (*n* = 293) included in the study and published in scientific journals between 2007 and 2017 (last access to database: March, 6th 2017), Figure S4: Number of publications (*n* = 293) related to the impact of stressors on *Apis mellifera* published between 2007 and 2017 studying the different bee categories according to year of publication, Figure S5: Proportion of the different methods used at the colony scale (a), at the individual scale in non-behavioral trials (b) and in behavioral trials (c), at the cellular (d), the molecular (e) and the genetic (f) scales. ("others": set of methods, each representing 2% or less. See Table S2 for details), Table S2: Detail of the "Others" sections of the Figure 4 and Figure S3.

**Author Contributions:** M.-P.C. had the original idea for the study. T.H. and M.-P.C. produced the methodology. T.H. realized the meta-analysis of papers, the database production, the data analysis and the initial version of the paper. M. -P.C. and M.L. reviewed the paper. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding to ANSES.

**Acknowledgments:** The authors wish to thank Druesne and Guitton for their help in bibliography management.

**Conflicts of Interest:** The authors declare no conflict of interest.
