*3.2.WhatKindofPopulations wereStudiedintheArticles?*

2017 and included

Worker bees were by far the most studied category of bees (67%, *n* = 230) and of these, foragers were widely represented (17% of the total population) (Figure 3a). Brood and colonies were studied in 13% of cases (*n* = 43), while drones (male bees) and queens were rarely studied (2% and 5% of the publications, respectively). Queens have been studied since 2011 whereas drones have only been included in papers since 2013 (see Figure S4 in the Supplementary Materials). Workers, and especially foragers, are the first to suffer from abnormally high mortality rates and seem to be the age class most exposed to stressors. Moreover, due to their number they constitute the greater part of the colony and probably represent the simplest biological material to study in hives. Drones and queens are key elements for colony survival due to their role in reproduction and recent studies have revealed that their reproductive capacities are altered by stressors [21–28]. In addition, nurses play a decisive role in larvae development due to the quality of the food they produce. However, bee nurses together with queens and drones are poorly studied when compared to bee workers.

More than half of the authors did not specify which *A. mellifera* subspecies was used in their experiments (Figure 3b), probably because there is numerous inter-subspecific cross-breeding in the field that makes identification difficult. However, stress sensitivity may differ between two subspecies [29,30]. Therefore, the subspecies is an important parameter to take into account and should be documented. When specified, the most studied subspecies were *A. m. carnica* (13%) and *A. m. ligustica* (9%) followed by Buckfast bees (5%) and *A. m. mellifera* (2%).

Therefore, it seems important to identify the populations most at risk or the critical developmental stages, to identify the differences in stress sensitivity between subspecies and potentially to define one or two indicative subspecies.

**Figure 3.** *Apis mellifera* categories (**a**) and subspecies (**b**) studied in the 293 articles related to the impact of stressors on *Apis mellifera* published between 2007 and 2017. "Others" refers to *A. m. Anatoliaca, A. m. caucasica, A. m. iberiensis, A. m. intermissa, A. m. jemenatica, A. m. macedonica, A. m. adansonii, A. m. andersonii, A. m. bandasii, A. m. carpathica, A. m. meda*, Kona bees (Hawaii) and Russian bees.

#### *3.3. What Stressors Were Studied and Did They Have an Impact on Bee Health?*

Biotic stressors were very seldom studied (11%) compared to abiotic ones (89%).

## 3.3.1. Biotic Stressors

Publications on biotic stressors (mostly parasitic and infectious agents (PIAs)) mainly concerned the parasitic mite *Varroa destructor* (Mesostigmata: Varroidae), the fungal agen<sup>t</sup> *Nosema spp.* (33% and 32%, respectively) and viruses (17%). These three categories represent the most widespread parasitic and infectious agents in bee colonies (Figure 4a). Predators and the small hive beetle *Aethina tumida* (Coleoptera: Nitidilidae) were very little studied. However, the recent detection of the latter in Europe and the Philippines [31] and the expansion of the Asian hornet *Vespa velutina* (Hymenoptera: Vespidae) could reverse this trend.

## 3.3.2. Abiotic Stressors

The most studied abiotic stressors were pesticides (61%, Figure 4b). Insecticides were the most tested (half of them were neonicotinoids (Figure 4d)) while fungicides and herbicides were under-studied. Beekeeping practices were relatively highly studied (29%). Three quarters of the beekeeping practices under study were PIA control systems, whether they used chemicals or not (Figure 4c). Bee nutrition was relatively well investigated (17%), while queen management, wintering methods and hive transfers were little studied (≤4%). Among the PIA control methods, "hard" chemical treatment methods [32] were more studied (60%) than "soft" methods. Indeed, these products are often acaricides or fungicides, potentially harmful for bees. Essential oils and organic acids, considered as "soft" methods [32], were studied in 23% and 12% of cases, respectively. Furthermore, Jacques et al. [33] have shown during the EPILOBEE surveillance project that poor beekeeping practices and the lack of expertise of some beekeepers represented one of the major causes of colony loss in Europe. It should be noted that the present meta-analysis only takes into account veterinary products (mostly acaricide treatments) and techniques currently used in beekeeping. Experiments studying any other active ingredients (e.g., toxicity of essential oils not used in beekeeping) were discarded from the analysis.

Other stressors like GMOs, metals (aluminum, lead, cadmium, arsenic and iron through oral exposure), climate and habitat fragmentation were poorly studied (≤4%).

**Figure 4.** Proportion of the different biotic (**a**) and abiotic (**b**) stressors, beekeeping practices (**c**) and insecticides (**d**), identified in the articles related to the impact of stressors on *Apis mellifera* published between 2007 and 2017. "Other pesticides" (Figure b) refers to adjuvants, inert ingredients, pesticide residues in fields, and wood preservatives. The "other" sections are detailed in Supplementary Materials, Table S2.
