Bombus humilis Illiger 1806

Bombus humilis Illiger, 1806 Conservation status CR Distribution Bombus humilis has a large Palearctic distribution (Rasmont et al. 2021). In Belgium, however, the species has vanished from nearly all the localities where it was found a hundred years ago (Folschweiller 2020). The subsisting locations are largely concentrated in the south-easternmost part of the country, in the Belgian Lorraine, with a few scarce observations east of the Sambre and Meuse valley. Habitat degradation is considered as the most threatening factor for the species, but climate change is also expected to induce substantial reductions of its European range by the end of the century (Rasmont et al. 2015, Ghisbain et al. 2024). Notes The species is mostly associated with open flower-rich grasslands and mostly collects pollen on plants from the Fabaceae, Lamiaceae and Boraginaceae families (Rasmont 1988, Folschweiller 2020, Rasmont et al. 2021, Wood et al. 2021). We collected a single individual, a queen, foraging on tufted vetch (Vicia cracca, Fabaceae) within mesic grasslands. Folschweiller (2020) cite Bombus humilis as frequently in sympatry with B. sylvarum and B. veteranus, two strongly-threatened species at the Belgian scale. Diagnosis Bombus humilis is amongst the most polymorphic bumblebee species of Europe (Fig. 6). The females of the species can be mostly confused with local colour forms of other bumblebee species belonging to the subgenus Thoracobombus, especially B. pascuorum and B. muscorum. In the females of B. humilis, however, the area directly in front of the central ocellus is largely smooth, with only a few punctuations, while this area is much more punctured in B. pascuorum and B. muscorum. In addition, the hairs on tergite 6 of B. humilis females are black, thick and erect, while they are ginger, thin and more parallel to the cuticle in B. pascuorum. The males of B. humilis can be differentiated from the males of all other bumblebee species on the basis of the morphology of their genitalia (drawn in Rasmont et al. 2021).