Case study on forage plants of the heath bumblebee (Bombus jonellus) in southwest Iceland

INTRODUCTION Bumblebees (Bombus spp.) are pollinators of especially high conservation interest. They have behaviours (pollen-gathering, buzz pollination), morphological structures (branched body hairs well-adapted for retaining pollen), and endothermic capabilities, that make them welladapted for transporting large amounts of pollen in subarctic regions (Heinrich & Vogt 1993, De Luca & Vallejo-Marín 2013). Recent findings indicate that 23.6% of bumblebee species in Europe are threatened with extinction, and that 45.6% of Europe’s bumblebee species are in decline (Nieto et al. 2014). These declines are likely due to multiple threats acting synergistically, but the primary threat is the loss and fragmentation of foraging and nesting resources (Kosior et al. 2007, Potts et al. 2010, Goulson et al. 2015). In Iceland, the aggressive spread of invasive non-native plant species such as Nootka lupine (Lupinus nootkatensis Donn) and cow parsley (Anthriscus sylvestris L.) (Magnússon 2011, Icelandic Institute of Natural History 2016) represents a serious threat to native forage-plant communities. Recent research suggests that Iceland’s only native bee, the heath bumblebee (B. jonellus Kirby), is at risk of serious declines in Iceland due to the spread of invasive plant populations (Willow 2016). To protect B. jonellus in Iceland, we need to not only manage invasive plant species, but also improve our knowledge of the native food plants that B. jonellus visits (Willow 2016). A range of flowering plant species used by B. jonellus in Iceland is given in Prŷs-Jones et al. (1981, 2016), with estimates of their significance as forage resources. However, further systematic observations of foraging preferences are required, as the plant-pollinator network throughout Iceland is undergoing changes, particularly due to the spread of invasive plant populations and the introduction of exotic bumblebee species (Magnússon 2011, Icelandic Institute of Natural History 2016, Prŷs-Jones et al. 2016). The primary aim of this study was to determine the significance of various plant species, across the forage season, as forage for B. jonellus in relatively natural environments in south-west Iceland. The importance of each forage plant species was estimated from the number of observed B. jonellus visits.