It won't be the lily which is causing a problem to the bees. They may be suffering from something when they arrive and are too ill to leave.

Drunk (literally) on the nectar. The nectar produced by lots of plants and trees has a narcotic effect on bees. Lime trees are a well documented case

Is that true of all lilies, worthy? I had (to my relief, having seen very few bees up till then) lots of bees on my Lilium regale earlier on this year, and they didn't seem to have a problem coming and going.

Thankyou !

Well this has turned out to be a very interesting research project, so thanks for the question, Savethebees. There is a general belief that bees can get drunk on nectar, but there's no evidence to back it up. Despite the presence of some yeasts in some nectar/pollen, it does not appear to cause fermentation. It seems the commonest cause of 'drunk' behaviour in bees is actually exposure to pollutants of some kind, as in certain pesticides or those present in the air generally. Ethanol is often present in over ripe fruit, and might be present in nectar, if someone could prove that nectar does ferment within a flower. In America there is a fly which can infect bees causing apparent drunkenness, leading to the bees being called 'zombie' bees, but as far as I'm aware, this is not a problem in the UK. There is a theory that previously pollinated flowers which still have nectar present may be in a state of ferment due to the introduction of various microbes (brought during visits from other insects) interacting with any yeast present, but again, this is only a theory. Seaburngirl, another member, might be able to give more info on the possibility of 'drunk' bees.