Bourdon menteur

Results from climate and land use change (such as grazing) modelling suggest that Bombus mendax is expected to lose a significant part of climatically suitable territories by the end of the century. Under the most severe climate change scenario modelled by Rasmont et al. (2015) (GRAS), the species could lose up to 79% of climatically suitable areas by 2100 (under a no dispersal model, and up to 55% under a full dispersal model). The latter authors placed the species into the category “High climate change risk”, reflecting the potential risk of climate change on this species. Projections of ecological niche models by Ghisbain et al. (2023), considering both land use change and climate change, also predict reductions in ecologically suitable areas for B. mendax in Europe by 2061-2080 (up to 30% under the SSP5-8.5 scenario). In addition, the potential for B. mendax to successfully disperse in the areas that will remain climatically and/or ecologically suitable is probably very low, given that it is already trapped in high altitudes. Preliminary results by Biella et al. (submitted) suggest, however, that the species has shifted towards higher altitudes in the last decades.

It remains unpredictable whether the areas that are projected to appear “suitable” for the species in the future will remain clear of additional anthropogenic stressors not considered in the models of Rasmont et al. (2015) and Ghisbain et al. (2023). These additional (and likely synergistic) pressures include infrastructure development (for tourism and transport), pesticide use, pollution (by nitrogen and its subsequent impact on nitrophobic flowering plants, or by heavy metals), the spread of exotic invasive plants, the invasion of managed species and subsequent pathogen spillover, among other potential stressors known to substantially impact bumblebee populations.

It is possible that climatic events not considered in the models of Rasmont et al. (2015) and Ghisbain et al. (2023), such as the increased occurrence of droughts, heat waves, or storms will further modify the future range of this species. The resistance of B. mendax to acute heat stress is unknown, but previous works on another bumblebee model have demonstrated impaired cognition of these bees at elevated temperature (Gérard et al. 2022), as well as reduced fertility (Martinet et al. 2020), and research is needed as to whether B. mendax would be similarly threatened by repeated exposures to such temperatures.

Whether the flowering plants the species is most strongly associated with will remain sufficiently abundant in the future is also unknown. Wood et al. (2021) showed that the diet of B. mendax is strongly associated with Fabaceae. The decline of this resource could continue to negatively affect the populations of B. mendax in Europe.

The increased relative abundance of other species such as B. lapidarius and B. terrestris in sympatry with B. mendax could pose a threat to the latter in the Pyrenees (P. Rasmont pers. comm. 2023).

This species is included in the National Red Lists of Germany (Not Threatened; Westrich et al. 2011), Italy (Data Deficient/Least Concern; Quaranta et al. 2018) and Slovenia (Rare; Anonymous 2002). It occurs in protected areas.

General efforts aimed at pollinator conservation may contribute to maintain B. mendax populations. At the microscale, the development of tourist infrastructure within the area inhabited by the species should be carefully monitored. At the macroscale, suitable efforts include strengthened regulations about greenhouse gas emissions to mitigate the ongoing climate change and its subsequent impact on this species and other insects.