The flight of the Bumblebee

Image Credit: Sinéad Mohan

Bumblebees are a key pollinator group whose long-term decline has been associated with changes in crop fertilisation that could impact food cultivation. Ellen Nugent discusses UCD’s Dr Dara Stanley’s study, ecology, and what they need to survive.

The humble bumblebee is a staple of art, media and children’s literature. Bumblebees, Genus Bombas, encompass around 250 species distributed throughout the Northern hemisphere and are considered one of the most important and efficient pollinating insects. Ireland is home to 15 species of True Bumblebees and 6 species of Cuckoo Bumblebees. These species have an interesting dynamic as the cuckoo bumblebee species take over a true bumblebee’s hive, kill the queen, and enslave her workers to care for the cuckoo bumblebee’s young. 

Bumblebees are, unfortunately, a declining species – climate change, intensive agricultural practices, and land development have all affected bumblebee numbers. Fortunately, not all the genus of bumblebees are critically endangered yet, as each species declines at a different rate. These rates are affected by a wide host of factors; from nesting behaviour to the length of their tongues!

The Great Yellow Bumblebee (Bombus Distinguendus) is the rarest of Ireland’s true bumblebees. It’s a rather mysterious bee with very little known about its habits and habitats. UCD lecturer Dr Dara Stanley has recently authored a paper researching and collecting information about the Great Yellow Bumblebee. The paper investigated different habitats, plants visited by the bees, and the place of Bombus Distinguendus among Ireland’s bumblebee populations. The study also collected information on these populations, which can be used in conservation strategies.

Bumblebees are seen throughout Ireland, but they live and thrive in specific sites, and need specific plants from which to harvest nectar and pollen. Stanley’s research investigated four primary types of habitat: coastland grassland areas, grasslands rich in flora and fauna (species-rich), urban environments, and roadside verges. Bombus distinguendus was found only in grassland sites, although it has inhabited many different habitat types in the past – it is thought that agricultural modification of habitats has resulted in their widespread decline. Bombus distinguendus also showed a preference for plants such as Red Clover, Common Knapweed and Kidney Vetch. 

These habitat and foraging preferences are common in Irish bumblebee species, with some small variations. This, unfortunately, can result in depleted resources for bumblebees – if all species inhabit the same areas and forage from the same plants, competition increases dramatically. Roadside verges and urban areas often support less bumblebee diversity, as they cannot provide for the specific requirements of rarer species.

Conservation of Ireland’s bumblebee species, and bumblebees as a whole, is critical. Bumblebees are highly efficient collectors of pollen and have been proven to assist in the pollination of both significant crop species, and wild fauna of Northern Hemisphere countries. Insect pollination of crop species within the EU has been valued at estimates of 14 million euros, with a significant percentage of these insects represented by bumblebees. Bumblebee species are also vital to Irish ecosystems, as pollinators, prey, and as hosts to parasites and microorganisms. Stanley’s paper discusses methods of conservation for the Great Yellow Bumblebee specifically, but these conservation methods can benefit bumblebees, and Irish ecosystems, as a whole.

The conservation strategy of rewilding and reducing land use is highly effective but is rarely a strategy as it is often economically and time-consuming. Rarer bumblebee species such as Bombus distinguendus thrive in species-rich, minimally processed grasslands. These grasslands, unfortunately, are not efficient for agricultural activities – bumblebees do not do well in single-crop fields or grazing land with limited numbers of grass species. Ireland has recently seen a move from the use of hay towards the use of silage for animal feed – this further reduces crop diversity, and results in a lack of forage species preferred and sometimes required by bumblebee species. The use of pesticides or herbicides reduces the possibility of these areas supporting bumblebee populations even more.

Urban and roadside habitats are more easily established and maintained than grassland habitats. Their success as a habitat is almost entirely dependent on human intervention: roadside habitats that are trimmed during foraging season cannot support bumblebees, but group-lead initiatives like the All-Ireland Pollinator Plan can lead to well-established and diverse habitat areas. Plans such as these are effective, but their usefulness to different species of bumblebees can be vastly improved if they are linked to even more diverse habitats such as grasslands, especially if the grassland is minimally processed for agriculture, which can facilitate the creation of ‘habitat corridors’. Bumblebees have a very small foraging radius, and will not generally fly more than 1km from their nest – this means that rarer species with specific habitat requirements are unable to benefit from urban or roadside habitats if they are not in immediate range of their habitat. Creating corridors would allow these bees to establish hives closer to more easily established habitats, and somewhat reduce the need for rewilding agricultural land.

The decline of bumblebee populations is worrying, amidst a global trend of species decline, but hope remains for the conservation and growth of these insects. Urban and roadside habitat creation and ensuring that grassland areas with rare bumblebee populations are protected is a vital first step in stopping the decline in bumblebee numbers. Conservation initiatives and Agri-Environment schemes will be vital for educating and increasing interest in Ireland's agricultural populations. Increasing bumblebee populations is entirely possible with the introduction of habitat corridors and restricted processing of land would do wonders for the survival of Ireland’s endangered bumblebees, and its bumblebee population alongside our populations of flora and fauna that are dependent on these habitats on the whole.