Bees and bumblebees have fallen on hard times, which has raised alarm that these vital pollinators might disappear from gardens, fields and meadows in the UK and across much of Europe. Excessive pesticide use, habitat loss and climate change have  have all been blamed.

Scientists have now pinpointed another reason why honey bees and bumblebees may find it tougher: a lack of plant diversity in monocultured agricultural areas.

Experts at the Laboratory of Apiculture and Social Insects at the University of Sussex studied 22 species of flower in southern England and analyzing how the behavior of more than 1,000 bees was impacted as they visited their favorite flowers to collect pollen.

Energy efficiency, it turns out, is a major factor for bees and bumblebees in deciding which plants to visit and where. In other words, a clear link exists between flower morphology and energy efficiency, affecting the foraging preferences of the insects, the researchers say.

This helps ensure that bees and bumblebees can successfully coexist in certain areas with various plants.

“While they forage on the same flowers, frequently we find that bumble bees will outnumber honey bees on a particular flower species, while the reverse will be true on a different species growing nearby,” explains Francis Ratnieks, a professor of apiculture at the University of Sussex.

“What was remarkable was that differences in foraging energy efficiency explained almost fully why bumble bees predominated on some flower species and honey bees on others,” Ratnieks adds.

The researchers have found that the bodyweight of bees and the frequency of their visits to specific flowers correlate with their level of energy efficiency with a larger bee needing more energy to visit flowers by flying to them.

“The rate at which a bee visits flowers, the number of flowers per minute, determines how much nectar, and therefore energy, it collects. Together, the ratio of these factors determines bee foraging energy efficiency,” the researchers note.

Because bumblebees are larger than honey bees, they can outcompete honey bees in some ways, but they also lose out in other ways.

The researchers, who published their findings in a study, used a portable electronic balance to weight insects and found that  bumble bees are almost twice as heavy as the honey bees, which means they use almost twice as much energy as honey bees to visit flowers during the same time. With a stopwatch the scientists determined that bumblebees visit flowers at twice the rate of honey bees.

“In essence, bumble bees have an advantage over honey bees in being faster at visiting flowers, so can gather more nectar (energy), but a disadvantage in being larger, and so using more of the nectar energy to power their foraging. On some flower species this gave an overall advantage to bumble bees, but on others to honey bees,” Ratnieks says.

Bumblebees dominated on some species of flower such as lavender, visiting them at almost three times the rate of honeybees. “Bumble bees have a foraging advantage on some plants, and predominate on them, while honey bees have an advantage on others and predominate on these,” the scientist says.

The shape of flowers can have a bearing on the foraging behavior of bees and bumblebees. Ling heather, a plant with a mass of small flowers, was better suited to the needs of nimbler honey bees whereas Erica heather, with its large bell shaped flowers, was better suited to bumble bees, the scientists say.

This finding highlights the need for a diversity of flowering plants in areas populated by honeybees and bumblebees. “Bee conservation therefore benefits from flower diversity, so that should certainly be a focus on bee conservation efforts. But fortunately, flowering plants are diverse,” Ratnieks stresses.

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