- Bees provide critical pollination services to diverse agricultural crops, native plants and trees. Globally, there are reports of bee declines which have been attributed to diseases, exposure to pesticides and changes in land use that are believed to have led to a reduction in foraging resources and nesting habitats for native bees. Other factors such as low nectar production caused by water stress in plants or toxins present in nectar may also be responsible but have received little attention. Risks associated with foraging behaviors are particularly critical as bees spend considerable time seeking food resources. Linden, a common ornamental tree in urban areas, produces an abundance of nectar and pollen, and thus benefits bees. However, in the late 1970's, dead bees were observed under linden in Europe when environmental conditions were dry. European researchers speculated that the causal factor was the presence of the sugar mannose in linden nectar under drought stress. Mannose is similar in structure to glucose which is used by bees as a carbohydrate source. The toxicity of mannose was believed to be due to disruption of glucose metabolism resulting from competition between mannose and glucose for the enzyme hexokinase during the glycolysis cycle that provides energy for bees. In laboratory studies mannose and galactose were shown to be toxic to honey bees. Their impacts on bumble bees were, however, not determined. There is little information available about the associations of bees with linden in the USA. Occasionally dead
bumble bees have been observed under linden trees in western Oregon in the west coast of the USA. The current study was conducted to: 1) Examine bloom and nectar production in linden, correlate nectar production with environmental conditions, and with diversity and abundance of foragers; and 2) Determine the impacts of mannose and galactose on honey bees and bumble bees.
The study was conducted in 2014 and 2015 in the city of Corvallis in western Oregon. Honey bees, five species of bumble bees, solitary bees (Halictus spp.), yellow jackets and dipterans (primarily syrphids), visited four species of linden surveyed during bloom. Honey bees were the dominant foragers, and accounted for 69% of foragers in 2014 and 84% in 2015. Nectar production in linden flowers was highest in the morning, and was positively correlated with relative humidity and negatively with temperature. However, there was no correlation between nectar production and the abundance of foragers over both years. A preliminary HPLC analysis of linden nectar samples collected from three linden trees showed a peak with the same retention time as a mannose standard. Further analyses are needed for confirmation of the presence of mannose in the nectar of linden. In a laboratory bioassay, mannose and galactose were toxic to both honey bees and bumble bees. However, when the toxic sugars were presented to honey bees and bumble bees in combination with the non-toxic glucose, the toxic impact was significantly lower (p< 0.05) if the proportion of glucose was high (90%) compared to combinations with lower proportions (10% or 50%) of glucose. These results provide support for the hypothesis that mortality of bees when exposed to mannose is due to competition with glucose for the hexokinase enzyme during glycolysis. However, it is still not known why higher bumble bees have been reported to die after foraging on linden when honey bees are the dominant foragers on linden, and are susceptible to the toxicity of mannose. It is possible that honey bees and bumble bees differ in their ability to assess the presence of toxins in nectar or that other factors are involved. Further research is needed for determining differences, if any, in the foraging behaviors of honey bees and bumble bees on linden trees, and for detecting other nectar compounds in linden that may differ in their impacts on different species of bees.