Massive bee mortality associated with linden trees: Bee diversity and susceptibility to nectar sugars

Recent massive bumble bee deaths associated with linden in Oregon were linked to neonicotinoids used for aphid control. However, mortality of bumble bee foragers on linden in Europe decades before neonicotinoids were developed was believed to result from toxicity of mannose in nectar speculated to be due to competition with glucose for hexokinase during glycolysis. Negative impacts of mannose on bumble bees were, however, never documented. Our objectives were to: 1) Assess pollinator diversity and abundance on linden to determine which species besides bumble bees were likely affected by neonicotinoid sprays in Oregon; 2) Determine impacts of mannose and other sugars on bees. In a survey of three linden species, honey bees (66%) were most abundant followed by syrphids (21%), bumble bees (10%) and other insects (9%). In laboratory bioassays, significant (p < 0.05) mortality was observed when honey bees (> 90%) and bumble bees (>30%) were presented with mannose (MAN) and galactose (GAL) compared with glucose (GLU) (<10%). Even the lowest dose (0.5 M) of mannose and galactose was highly toxic (>90% mortality) to honey bees compared to glucose. However, when combined with glucose, honey bee mortality was lower (20%) with low ratios (90:10 GLU: MAN and GLU:GAL) compared to higher ratios (50:50 and 10:90; GLU:MAN and GLU:GAL) (>90%). Based on the results, mannose and galactose may cause bee mortality by disruption of glucose metabolism. Further research is needed for estimation of toxic sugars in linden nectar, and the biochemical basis for the toxicity.