Bumblebees

Since bumblebees are a group associated with cool climates, Britain supports a large proportion (~10%) of the world's bumblebee fauna. However, three of our 25 species have become extinct, and seven species are Biodiversity Action Plan (BAP) listed, a higher proportion than for any other insect group. Declines are primarily driven by habitat loss and declines in floral abundance resulting from agricultural intensification, notably the loss of ~97% of all species-rich grasslands (haymeadows, calcareous grasslands) in the last 60 years. The decline in the abundance of Red Clover, once a common fodder and ley crop and a major source of pollen and nectar for many bumblebee species, is likely to have had a significant impact. Effects of habitat degradation and fragmentation are compounded by the social nature of bumblebees and by their largely monogamous breeding system, which means that they have a very low effective population size (most bumblebees are sterile workers). Hence, populations are susceptible to chance extinction events and inbreeding. Given the importance of bumblebees as pollinators of crops and wildflowers, their declines have broad ecological and economic significance. Suggested measures for their conservation include tight regulation of commercial bumblebee use and targeted use of agrienvironment schemes to enhance floristic diversity in agricultural landscapes

Quantifying the attractiveness of garden flowers for pollinators

There is great interest in planting urban areas to benefit pollinating insects, with the potential that urban areas and gardens could act as an extensive network of pollinator-friendly habitats. However, there are a great many different plant cultivars available to the gardener, and a paucity of evidence-based advice as to which plants are truly most attractive to flower-visiting insects. Here, we report insect visitation to metre square plots of 111 different ornamental plant cultivars at a site in central UK. Data were collected over 5 years, and comprise over 9000 insect observations, which were identified to species (for honeybees and bumblebees) or as ‘solitary bees’, Syrphidae, Lepidoptera and ‘others’. Unlike some previous studies, we found no difference in numbers of insects attracted to native or non-native species, or according to whether plants were annuals, biennials or perennials, but we did find that native plants attracted a significantly higher diversity of flower-visiting insects. Overall, the most-visited plants were Calamintha nepeta, Helenium autumnale and Geranium rozanne. However, patterns of visitation were quite different for every insect taxa examined. For example, different species of short-tongued bumblebees showed little overlap in their most-preferred plant cultivars. Interestingly, very similar plant cultivars often attracted different insect communities; for example, 72% of visitors to Aster novi belgii were honeybees or bumblebees, while the related Anthemis tinctoria, which also has daisy-like flowers, did not attract a single honeybee or bumblebee but was popular with solitary bees, hoverflies, and ‘other’ pollinators. Some plant cultivars such as Eryngium planum and Myosotis arvensis were attractive to a broad range of insects, while others attracted only a few species but sometimes in large numbers, such as Veronicastrum virginicum and Helenium autumnale which were both visited predominantly by honey bees. It is clear that we do not yet fully understand what factors drive insect flower preferences. Recommendations are made as to which flower cultivars could be combined to provide forage for a diversity of pollinator groups over the season from early spring to autumn, though it must be born in mind that some plants are likely to perform differently when grown in different environmental conditions

Combined stress from parasites, pesticides and lack of flowers drives bee declines

Bees are subject to numerous pressures in the modern world. The abundance and diversity of flowers has declined, bees are chronically exposed to cocktails of agrochemicals, and they are simultaneously exposed to novel parasites accidentally spread by humans. Climate change is likely to exacerbate these problems in the future. Stressors do not act in isolation; for example pesticide exposure can impair both detoxification mechanisms and immune responses, rendering bees more susceptible to parasites. It seems certain that chronic exposure to multiple, interacting stressors is driving honey bee colony losses and declines of wild pollinators, but such interactions are not addressed by current regulatory procedures and studying these interactions experimentally poses a major challenge. In the meantime, taking steps to reduce stress on bees would seem prudent; incorporating flower-rich habitat into farmland, reducing pesticide use through adopting more sustainable farming methods, and enforcing effective quarantine measures on bee movements are all practical measures that should be adopted. Effective monitoring of wild pollinator populations is urgently needed to inform management strategies into the future

Effects of field-relevant concentrations of clothianidin on larval development of the butterfly Polyommatus icarus (Lepidoptera, Lycaenidae)

Arable field margins are often sown with wild flowers to encourage pollinators and other beneficial or desirable insects such as bees and butterflies. Concern has been raised that these margins may be contaminated with systemic pesticides such as neonicotinoids used on the adjacent crop, and that this may negatively impact on beneficial insects. The use of neonicotinoids has been linked to butterfly declines, and species such as the common blue butterfly (Polyommatus icarus) that feed upon legumes commonly sown in arable field margins, may be exposed to such toxins. Here, we demonstrate that the larval foodplants of P. icarus growing in an arable field margin adjacent to a wheat crop treated with the neonicotinoid clothianidin, not only contain the pesticide at concentrations comparable to and sometimes higher than those found in foliage of treated crops (range 0.2 to 48 ppb), but remain detectable at these levels for up to 21 months after sowing the crop. Overall, our study demonstrates that non-target herbivorous organisms in arable field margins are likely to be chronically exposed to neonicotinoids. Under laboratory conditions, exposure to clothianidin at 15ppb (a field-realistic dose) or above reduced larval growth for the first 9 days of the experiment. Although there was evidence of clothianidin inducing mortality in larvae, with highest survival in control groups, the dose-response relationship was unclear. Our study suggests that larvae of this butterfly exhibit some deleterious sublethal and sometimes lethal impacts of exposure to clothianidin, but many larvae survive to adulthood even when exposed to high doses

Evidence for competition between honeybees and bumblebees; effects on bumblebee worker size

Numerous studies suggest that honeybees may compete with native pollinators where introduced as non-native insects. Here we examine evidence for competition between honeybees and four bumblebee species in Scotland, a region that may be within the natural range of honeybees, but where domestication greatly increases the honeybee population. We examined mean thorax widths (a reliable measure of body size) of workers of Bombus pascuorum, B. lucorum, B. lapidarius and B. terrestris at sites with and without honeybees. Workers of all four species were significantly smaller in areas with honeybees. We suggest that reduced worker size is likely to have implications for bumblebee colony success. These results imply that, for conservation purposes, some restrictions should be considered with regard to placing honeybee hives in or near areas where populations of rare bumblebee species persist

Location of bumblebee nests is predicted by counts of nest-searching queens

1. Bumblebee nests are difficult to find in sufficient numbers for well replicated studies. Counts of nest-searching queens in spring and early summer have been used as an indication of preferred nesting habitat, but this relationship has not yet been validated; high densities of nest-searching queens may indicate habitat with few nesting opportunities (meaning that queens have to spend longer looking for them). 2. From mid April 2010, queen bumblebees were counted along twenty transects in grassland and woodland habitats in Central Scotland, UK. The number of inflorescences of suitable forage plants were also estimated at each transect visit. The area surrounding each transect was searched for nests in the summer. 3. In total 173 queen bumblebees were recorded on transects, and of these 149 were engaged in nest-searching. Searches subsequently revealed 33 bumblebee nests. 4. The number of nest-searching queens on transects was significantly, positively related to the number of nests subsequently found. Estimated floral abundance along the transect did not correlate with numbers of nest-searching queens or the number of nests found, suggesting that queens do not target their searching to areas locally high in spring forage. 5. The data suggest that counts of nest-searching queens do provide a useful positive indication of good nesting habitat, and hence where bumblebee nests are likely to be found later in the year