Dear Bee Watchers:
For an introduction to nectar glands, nectar components and toxic components please consider reading Chapter 7 in Bernhardt, P. (1993) "Natural Affairs; A Botanist Looks at the Attachments Between Plants and People" (Villard Books, 1993) and/or look up the appropriate papers by the late, Irene Baker. There are various reasons why the nectars of some plants are toxic to certain animals.
1) Yeasts invade converting sugars into ethanol.
2) Some contain trace amounts of weird sugars (e.g. lactose) that may be toxic to animals with metabolisms that mistake them for sucrose. Indeed some animals can't even digest sucrose. NEVER feed a canary table sugar.
3) In odd cases, alkaloids made by the plant get into the nectar. This includes the nectars of most rhododendrons and mountain laurels (Kalmia), death camas (Zigadenus), creeping bellflower (C. rapunculoides), yellow flag iris (Iris pseudacorus) etc..
Now here's the really interesting thing. Molecules toxic to some animals have absolutely no effect on other species. Honeybees die when forced to forage on death camas in a bad year but seem to be unaffected by alkaloids in rhododendrons and mountain laurels. However, when humans eat too much rhododendron or mountain laurel honey they can become very sick. The golden rain tree sounds like a new and interesting story. Does its nectar snuff Bombus because it's making ethanol, an indigestible sugar or is there an alkaloid, steroid or some other secondary molecule fatal to these bees?