Bombus ruderarius

Bombus ruderarius (Müller, 1776) Conservation status EN Distribution Bombus ruderarius has a wide distribution in the West Palearctic (Rasmont et al. 2021). In Belgium, the species has been strongly regressing over the last century (Folschweiller 2020) although its status is less critical than that of B. humilis. The subsisting locations are largely concentrated east of the Sambre and Meuse valley and in the coastal area of north-western Flanders. 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 21 st century (Rasmont et al. 2015, Ghisbain et al. 2024). Notes Bombus ruderarius, as many other species of the genus Thoracobombus, is mostly associated with open flower-rich grasslands. Its habitats of predilection also include coastal dunes, heaths and moors, woodland edge and clearings in woodlands. The diet of the species includes a large quantity of pollen from the Fabaceae family (Folschweiller 2020, Rasmont et al. 2021, Wood et al. 2021). Of the three individuals collected, two were foraging on plants from the Fabaceae family, and one on a plant from the Asteraceae family, all from mesic grasslands. Diagnosis In Belgium, the females of B. ruderarius (Fig. 7) can be confused with other species of black bumblebees with a ‘ red tail’, such as B. lapidarius, B. rupestris, B. soroeensis and B. cullumanus (although the latter is thought to be fully extinct in the country). However, females of B. ruderarius can be separated from all other concolour bumblebees by the combined presence of: (i) the mid-basitarsus with the distal posterior corner extended to form a sharp angle and (ii) orange fringes of hairs on the corbicula. The males of B. ruderarius 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).