Bombus muscorum

B. muscorum, like many bees, exhibit haplodiploidy. The diploid queens produce diploid workers and new queens from fertilized eggs. Haploid males are produced from unfertilized eggs. Workers are also able to lay unfertilized eggs that develop into males.[15] Haploid males produce identical haploid sperm while diploid females produce genetically variant haploid eggs through meiosis. Sex is determined under a single-locus complementary sex determination (sl-CSD) system, where multiple alleles at a single locus determine the sex of an individual. Sex locus heterozygotes develop as females, while hemizygous and homozygous eggs develop as haploid and diploid males.[19]

B. muscorum is thought to be a monandrous species; the queen mates only once with a single male to start a new colony.[15] This monandrous behavior decreases the amount of genetic variation present in a single colony relative to that of a polygynous or polyandrous species. As a result, B. muscorum has an increased susceptibility to the effects of inbreeding.[18] Queens may possess a distinctive odor that signals relatedness and prevents interbreeding.[15] Studies have shown, however, that mating appears to be random.[18]

Male mating strategies of bumblebees can be grouped into four broad categories: patrolling, racing, territorial, and cruising. Males of B. muscorum exhibit territorial strategies for mating. In this strategy, males choose a perch outside of a mature nest and pursue potential mates that it spots. Males compete with one another for preferred perches, each attempting to find a spot close to the nest entrance. When another bee flies too close to a perched male, the male sets off in pursuit for several seconds, seeking to either chase away a potential competitor or mount a potential mate. After the chase, one of the males will land back at the perch close to the nest.[15]

As a result of small population sizes and haplodiploidy, B. muscorum has an increased susceptibility to inbreeding. As sex is determined at a single locus, the overall fitness of a population is directly related to the number of different alleles at the sex locus, which in turn is related to the size and isolation of the population.[7] When males mate with related queens, diploid males with reduced fertility and immune response are produced.[18] 10 out of 14 B. muscorum populations in the Hebrides showed significant signs of recent bottlenecking. In recent years, diploid males have come to represent a considerable proportion of males within populations in the British Isles, suggesting an increase in inbreeding among populations.[7]

B. muscorum exhibit small foraging ranges, having been recorded going no further than 500 m from their nest to forage. Foraging, however, often takes place within 100 m of the nest. Individuals are often observed foraging at the same spots between 50 and 200 m away from the nest. This variation in individual flight range is attributed to an individual's body size.[20] B. muscorum has been described as doorstep foragers, due to the tendency of workers to use food resources near the nest more frequently than the workers of other species. This restricted radius may be an important factor in the decline of the species.[21]

B. muscorum build its nest on or just under the ground. It cards together collected moss and dry grass to cover the nest. It is this behavior that gives B. muscorum the name moss carder bee.[10][22] It rarely, if ever, crosses sea barriers greater than 10 km to establish a nesting site. Once a nest is established, the bee is notoriously aggressive, readily attacking intruders that are too close to the nest, which they bite and sting simultaneously.[18]

B. muscorum may contract infections of Crithidia bombi, a trypanosome parasite, from infected nest mates or from others while foraging on contaminated flowers.[23] Infections have been linked to reduced individual and colony fitness, but the specific relationship between Crithidia bombi and B. muscorum is complex.[24] The presence of Crithidia bombi has been found to be higher in populations with lower genetic diversity. As B. muscorum populations continue to lose heterozygosity, the impact of parasitism increases, pushing already threatened populations closer to extinction.[25]

Due to recent decades of agricultural intensification in Europe, the natural habitat of B. muscorum has been largely diminished. Many of the permanent flower-rich areas where the bee commonly forages have been destroyed by increased commercial farming.[26] B. muscorum has poor dispersal ability compared to other species of bumblebee, making it more vulnerable to habitat loss. As a result, populations have decreased dramatically in recent decades.[27]

B. muscorum is currently listed as vulnerable in Europe by the IUCN.[4] Populations are diminishing in Britain and the species is currently in a species recovery program. Decreasing populations are evident in multiple European countries. Estonia lists the species as vulnerable. Germany and the Netherlands consider the species to be endangered.[28]

B. muscorum is often found on sea wall flood defenses managed by the Environment Agency. Nests can be found on the flat grassy area between the sea and the borrowdyke, a brackish pond behind the sea wall. As grasslands on sea walls are predominantly unimproved, they have become an important habitat for rare insect species such as B. muscorum. Current research is examining how these sea walls can be maintained to preserve rare populations.[29]