I suspect the number is inflated - it's unlikely (but not impossible) that anyone counted all the bees. Estimating that number by eye could be off by an order of magnitude.
Probably the tree and surrounding vegetation were flowering abundantly, attracting many insects from the area. I would say thousands of insects on a large tree is not unfeasible at all, perhaps even tens of thousands. Whether they were all bees, and whether the bees were really all bumble bees, is not clear.
You're right that 25,000 bumble bees in one place does sound logistically difficult to arrange. We can back-of-the-envelope it...
(edit: increased foraging distance with data from reference, which slightly changes the conclusion)
It would be ~150 colonies within a several kilometer radius. If we assume mean foraging distance of 10km [1], thats (a rather beautiful) 314km^2 of source area. We should probably also assume that the vast majority of bees in the area are not visiting those two trees, as there are likely other good sources in the area too. If we assume 1-5% of the local bees might be visiting the trees, there would have to be a colony density of ~10-50 colonies per km^2. That is high, but not impossible. Previous measurements of bumble bee colony density have estimated in the region of 0.5 to 20 per km^2 [1].
So, one of the following is probable: the number of bumble bees is exaggerated in the story, or they weren't all bumble bees, or some of my estimates are wrong. I would arrogantly assert the first two are more likely :)
I know how ants stop going to a food source (e.g. the pheremone trail stops getting refreshed on the return trip) but how do bees stop? I know they transmit pathfinding information with dancing... do the bees just stop dancing on return or something?
The reason I ask is the 1-5% figure you mention. I assume bees must have some mechanism to prevent sending bees to fatal sites. For ants, no one comes back, so no one keeps going. Ant traps short-circuit this by providing food with a slow-acting poison - the ants make it back with "food," causing more ants to be sent, and soon a significant portion of the stockpile is poison, drastically reducing numbers in the colony. Could that have happened here? It sounds like that's probably not the case, given that the bees appear to have died practically on contact with the linden trees.
Probably the tree and surrounding vegetation were flowering abundantly, attracting many insects from the area. I would say thousands of insects on a large tree is not unfeasible at all, perhaps even tens of thousands. Whether they were all bees, and whether the bees were really all bumble bees, is not clear.
You're right that 25,000 bumble bees in one place does sound logistically difficult to arrange. We can back-of-the-envelope it...
(edit: increased foraging distance with data from reference, which slightly changes the conclusion)
It would be ~150 colonies within a several kilometer radius. If we assume mean foraging distance of 10km [1], thats (a rather beautiful) 314km^2 of source area. We should probably also assume that the vast majority of bees in the area are not visiting those two trees, as there are likely other good sources in the area too. If we assume 1-5% of the local bees might be visiting the trees, there would have to be a colony density of ~10-50 colonies per km^2. That is high, but not impossible. Previous measurements of bumble bee colony density have estimated in the region of 0.5 to 20 per km^2 [1].
So, one of the following is probable: the number of bumble bees is exaggerated in the story, or they weren't all bumble bees, or some of my estimates are wrong. I would arrogantly assert the first two are more likely :)
1. https://ir.library.oregonstate.edu/xmlui/handle/1957/35082