The life of a pollinator can be quite hazardous. There are plenty of creatures willing to devour one of them, from insects and spiders to birds. In addition to that, there are many parasites and other pathogens. Perhaps the highest mortality comes from the so called parasitoids and cuckoos. Parasitoids behave somewhere between a parasite and a predator; they parasitize the host, feeding on it, and eventually killing it, while predators kill and eat the prey all at once. Cuckoo bees and wasps, just like cuckoo birds take over the nest, lay their eggs and let the workers of the host species take care of the brood. Many cuckoos are related species, mainly other bumble bees; in some cases they mimic the appearance of the host species.
In general, most birds seem to avoid bumble bees; although there are a few species that specialize on bees. Skunks, voles and moles can attack their nests and eat their reserves and grubs. These creatures destroy a good number of nests. There is a story about Darwin saying that the British armies owed a lot to old spinsters. This was his explanation: Old spinsters owned cats which ate field mice. Fewer mice meant that more bumble bee nests survived. In turn, the bumble bees pollinated the alfalfa that fed the cattle that fed the soldiers and kept them strong and healthy. It was just an unproven entertaining hypothesis, but still a nice illustration of the intricate web of life.
Crab spiders are so called because they look and move like crabs. They are among the most common predators. They build no webs; instead they hide inside blossoms, waiting patiently for prey and ready to pounce on the first flower visitor that comes within reach. They are well camouflaged and are capable of changing their colors if they have to move to another flower.
Ambush bugs deserve the name. They sit for long hours, well hidden inside a flower or cluster of small flowers, waiting for flower visitors. Their contour is jagged making it hard to detect them. They are very good at holding still, tucked in between petals. Their front claws look like those of a preying mantis and they have a sharp beak used for stabbing their prey and sucking vital juices. Their venom must be very powerful because they can paralyze a bumble bee twice their size in the blink of an eye.
Some flies of the Conopidae family parasitize bumble bees. They hang around flowers waiting for bumble bees and lay just one egg in the narrow space between body segments. The egg hatches, the larva gets inside the unfortunate bee and proceeds to feed on its blood, (its real name is hemolymph because it is more similar to lymph than to blood). As it grows it starts eating the tissues, eventually killing the host.
Bee wolf wasps attack many types of bees, bumble bees included, and also other insects. They overpower them and take them to their nests to feed their young. They have a very thick, pitted body cover that serves them as armor.
Other enemies of bumble bees are bumble bees themselves, the so called cuckoo bumble bees. There are several species of them. They build no colonies, nor have any workers. A queen attacks and subjugates a colony of honest, hard working bumble bees. She usually kills the resident queen and destroys any eggs. Then proceeds to lay eggs of her own which are nurtured by the workers of the host species.
Bumble bees also suffer from many of the pathogens that afflict honey bees, such as mites, fungi, nematode worms, bacteria and viruses. The life of a pollinator is not an easy one.