Table 2.

Hypotheses explaining bee deaths on Tilia.

hypothesis	prediction	supporting evidence	opposing evidence	research need
1. toxic Tilia metabolites	toxic metabolites in Tilia nectar or pollen with lethal or sub-lethal effects on bees	affected bees appear paralysed before dying [12]; suggestion of mannose (toxic to bees) in Tilia nectar based on limited paper-chromatographic investigations [12,36]	no detection of mannose by gas chromatography in T. tomentosa nectar or dead bees; no experimental evidence for toxicity of T. tomentosa nectar [35,37]	detailed chemical analysis of Tilia pollen and nectar metabolites, experimental tests of toxicity
2. insecticides	insecticide (e.g. neonicotid) application to Tilia trees killing bee foragers	prior application of neonicotinoids to Tilia recorded in isolated cases [28,38]	phenomenon existed before use of neonicotinoids [12,14,21], most cases without known previous insecticide application (table 1)	persistence of neonicotinoids in Tilia and exposure of bees from Tilia pollen and nectar when neonicotinoids are applied outside flowering period
3. natural causes: predators/old age	dead bees owing to background mortality from e.g. predators and old age	T. tomentosa flowers during the end of the colony cycle of some bumblebee species; birds and wasps observed preying on bees on flowering Tilia [15]	majority of dead bees are not old, bee deaths also occur without predator attacks [15]	additional quantification of background mortality from predation or old age of bees foraging on Tilia
4. starvation	dead bees owing to insufficient nectar resources during T. tomentosa flowering period causing starvation	most deaths occur at end of Tilia flowering period when nectar production is very limited [13,17,39], foragers on T. tomentosa have depleted body sugar reserves [35], dying bees can recover when fed Tilia nectar [40], scarcity of alternative nectar resources during T. tomentosa flowering suggested [17,35]	bee deaths can occur when alternative food sources are available [18,19]	comparison of bumblebee mortality on T. tomentosa and nearby plants flowering simultaneously; comparison of colony resource intake and mortality in comparable landscapes with and without T. tomentosa
5. chemical deception	chemical deception (e.g. by volatiles, caffeine) causes overvaluation of Tilia as resource and increased foraging persistence once nectar is depleted, leading to starvation	the presence of caffeine in Tilia honey [41], caffeine modulates bee foraging, increasing persistent return to depleted food sources and causing overvaluation of sugar rewards [42–44], the presence of volatile compounds in Tilia flower scent that act as foraging recruitment pheromones in bumblebees [41,45,46]	known Tilia flower volatiles are common in plants not associated with bee deaths [47]	analysis of volatiles from T. tomentosa flowers; exposure of bees to caffeine on Tilia and effects on foraging behaviour; interaction of T. tomentosa volatiles and caffeine in reward association learning
6. interactive effects	bee deaths owing to interaction of factors in hypotheses 1–5	plausible, but not investigated	not investigated	interactions between factors in hypotheses 1–5 should be studied