This section is from the book "The Gardener's Monthly And Horticulturist V23", by Thomas Meehan. See also: Four-Season Harvest: Organic Vegetables from Your Home Garden All Year Long.
One season the writer of this noted innumerable humble-bees, dead under the flowers of a Wistaria, and concluded they had been poisoned by the flowers of this plant; but the next and succeeding years he had seen bees in thousands collecting honey and pollen from the Wistaria without any injury whatever. He thought of this on reading the following, credited to the British Bee Journal: -
"Last summer I had a large quantity of Foxglove plants, cultivated varieties, and very beautiful in bloom, at the same time a quantity of Canterbury Bells growing near them, and in flower. I was much struck to find that a great number of the working bees were lying dead in the beds of the Canterbury Bell. Day after day I watched with much interest the movements of the bees, and found that after they had fed for some time on the flowers of the Foxglove they became stupid, and after leaving the Foxglove they went into the flowers of the Canterbury Bell, and, as a rule, died shortly after. Query - Did they die from the poison got in the Foxglove alone, or did it depend on their coming in contact with the flowers of the Canterbury Bell? I could not find any dead bees on any other plant or on the ground near by. - A. Paterson, M. D."
The fact is bees are often found dead in numbers under plants. Instead of poison it is more likely natural death should be the verdict. Bees live but one season in any event, and some are dying all through the summer. It is more likely that a lot of bees, whose time has nearly come get belated or overworked, and remaining in the flowers all night, meet with cold or dew, and die from old age or exhaustion.