This has been discussed here many times, as far back as 1995:
Subject: Re: Lethal nectar?
From: Ian Stuart McLean <[log in to unmask]>
Date: Mon, 13 Mar 1995 19:30:33 -0500
Bumbles get into trouble under some species of lime. Tilia Sp
The problem is with nectars which are high in the sugar Mannose. Bumbles
cannot digest this, they don't have the right enzymes, wheras honeybees
can. I've never heard of trouble with Thistles either, perhaps the thistles
were under or near Tillia petiolaris, T. tomentosa, T. orbicularis. These
are the main ones to cause trouble in the UK.
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