Probably more bumblebees are
killed by parasites than by predators. This may be because the bumblebee
females are armed with a sting, but it is also due to the protection given by
their warning colouration.
Some crab spiders ambush bumblebees at flowers. These spiders do not spin webs, but sit and wait for their prey to come within reach. They are well camouflaged. In north America Misumenia vatia catches bumblebees as they land on flowers.
Robber flies catch their prey on the wing in their legs. And in north America have been found to prey on bumblebees.
A few species of bird can remove the sting
before eating the bumblebee, e.g. bee-eaters, spotted flycatchers, tits and shrikes. The Bombus lucorum queen below left was probably killed by a bird, as was the one on the right where the thorax has been emptied. They rub the sting off before eating the abdominal contents which would include the honeystomach.
In the U. S.
there are wasps called beewolves in the genus Philanthus, these wasps specialize in hunting bumblebees. The
bumblebee is caught while feeding and is paralysed with a sting, it is taken
back to the nest and enclosed with a wasp egg in a cell; there are usually
about five bumblebees in each cell.
All of the above are predators of foraging bumblebees.