Bombus ruderatus

There are many similarities between B. ruderatus and B. hortorum, making it difficult to distinguish between the two species.[1] Due to numerous physical similarities, many scientists have suggested reassessing their current status as two distinct species.[5] Both bees are similar sizes, are black with two yellow bands, and the drones have similar genitalia.[1] Although they may be hard to assess at first glance, there are minor physical differences. For example, the width of two yellow bands on the scutellum and pronotum are relatively equal in B. ruderatus, while in B. hortorum, the band on the scutellum tends to be narrower than the band on the pronotum.[1] Physical variations may exist within a species as well, making it important to use other sources to distinguish between the two species.[1][5] Studies of mitochondrial DNA has shown a 6.2% divergence for COII and a 9.2% divergence for cytochrome b.[1] These percentages are higher than the divergences between other distinct species, affirming that these bees are in fact two distinct species.[1]

Most Bombus ruderatus nests tend to be subterranean. Queens have been seen to return to their maternal nest site, but will assess it indiscriminately to determine if it is suitable for colonization.[8]

In New Zealand, colonies started by natural queens of Bombus ruderatus tended to have a male-biased sex ratio.[11] Additionally, the two sexes emerged at different times, with males usually emerging before new queens.[11] Male-biased sex ratios tend to occur when queens have multiple mates.

In Bombus ruderatus, worker oviposition has been seen in association with oophagy, which may include the queen eating eggs deposited by workers.[12] Larvae ejection was also a common process done by workers and sometimes queens, and involved pulling live larvae out of cells and depositing them outside of the nest.[12] Analysis of ejected and survivor larvae showed no morphological difference so the reason for ejection is unknown, but female ejection occurred more frequently than male ejection.[12]

Bumblebees are important pollinators of crops and flowers, some of which are important for economic and agricultural reasons.[1] Bumblebees tend to be more effective pollinators than honeybees since they can fly at lower temperatures.[7] Honeybee pollination has been seen to decrease as the season progresses due to decreasing temperatures, while pollination of bumblebees usually remains relatively constant.[7] In 1982, Bombus ruderatus was brought to Chile to be used as a pollinator of the red clover, which is a small legume used for its seeds.[6] Chile is an important exporter in red clover seeds and thus their agricultural and economic success may depend on the effectiveness of bee pollination.[13] Most pollination of red clover is done by honeybees, but their pollination decreases in late summer to early fall, leading to less seed recovery.[13] B. ruderatus was introduced in order to compensate for late season decreases in pollination. Although there have been positive agricultural benefits from increased seed production, negative consequences, in terms of decreased native populations have been observed as well.[14]

Invasive species may be very harmful to native species by outcompeting them for resources or by passing illnesses through pathogen spillover.[10][14] For example, the introduction of Bombus ruderatus and Bombus terrestris in Argentina have been shown to decrease the population of the native species, Bombus dahlbomii.[14] This can be seen in the comparison of the visitation frequency of the two species. Since the introduction of B. ruderatus, the visitation frequency of Bombus dahlbomii to certain plants has decreased, while it has increased in B. ruderatus.[14] Although a large part of the decline of Bombus dahlbomii populations in southern South America could be due to the introduction of Apicystis bombi by B. terrestris, resource and foraging competition with B. ruderatus has further facilitated the decline of the native Bombus dahlbomii.[10][14] In general, B. dahlbomii tends to be more successful in areas where B. ruderatus or B. terrestris has not yet colonized.[14]

In recent years, many countries have seen decreases in bee populations, specifically bumblebees. Some possible explanations could include habitat fragmentation, climate change, disease, agrochemicals, and other factors.[14] As stated previously, B. ruderatus has seen a population decline in Britain and has disappeared completely from many of its previous sites.[5] Declines in bee populations can lead to other ecological issues, such as declines in the success of plant species that rely on their pollination.[4]

Recently, the European Union has introduced agri-environment plans that can be used to pay farmers to manage their land in a way that benefits bee species.[4] Britain adopted the Environmental Stewardship scheme with the goal of enhancing the supply of pollen and nectar sources for bumblebees.[4] These plans hope to counteract some of the issues that commercial developments have caused. For example, intense crop management with decreased mixed farming has led to “botanical simplification” of habitats, which may have contributed to decreased bumblebee populations by decreasing the quality and quantity of resources and habitats.[4]