Cuckoo Bumble Bees — Social Parasites

  What is a Cuckoo Bumble Bee?

Ashton's Cuckoo Bumble Bee, Bombus ashtoni
Ashton's Cuckoo Bumble Bee
  A Cuckoo Bumble Bee, like the bird it is named after, lays its eggs in another bumble bees nest and leaves the workers of that nest to rear the young. The eggs she lays are either females or males — there are no queens.

The cuckoo females emerge from hibernation in late spring or early summer, much later than ordinary bumblebee queens. So by the time the cuckoo females have emerged the bumble bee queens will have already established their nests.

One key to their success as bumble bee parasites is that they usually have the same patterns of color as the bumble bee whose nest they lay their eggs in.

In Wisconsin, there are 2 native cuckoo bumble bee species, both of which are very rare, the Ashton Cuckoo Bumble Bee and the Yellow Cuckoo Bumble Bee.

  Cuckoo Bumble Bee Life Cycle

Entomologists refer to cuckoo bees as kleptoparasites due to the nature of procreation. Unlike other bee species, cuckoo bumble bees do not take care of their own young. Instead, they trick their host species to do it for them through deception and sneaky tactics.

  How Are Cuckoo Bumble Bees Different From Social Bumble Bees?

The cuckoo has evolved a number of characteristics, which mean it is totally reliant upon its host for its future survival.

Cuckoo bees are notoriously hard to detect. Since they are usually related to their host, the similarities in appearance make it really difficult to tell them apart. They instinctively lurk around the host’s colony to take on their odor. Masking their presence with both colors and chemicals makes infiltrating the nest a lot easier.

Yellow Cuckoo Bumble Bee, Bombuc Citrinus
Yellow Cuckoo Bumble

Because they need to move behind enemy lines, cuckoo bees are a bit bigger and a lot tougher than their host. Some species infiltrate and kill the host queen, which means they need to be bigger, have stronger mandibles, and harder exoskeletons.

First, cuckoo species are unable to establish their own nests. They do NOT have the ability to excrete wax from their abdomens for making egg cells in which to lay their eggs, nor can they make honey pots from which newly emerged brood may feed, and from which they may feed themselves whilst they incubate the brood. The cuckoo female must use the larval cells and cups made by the host queen.

The cuckoo female also cannot collect pollen for returning to her nest, since she has no pollen baskets (corbicula) on the hind legs, although they do of course, eat pollen, just as social bumble bees do. Pollen is important as it helps the ovaries to mature in the fertilized females, and it is used for feeding the brood.

Also, cuckoos are unable to rear workers. The cuckoo merely lays females and males like herself, and these offspring are likewise limited, in that they cannot perform the tasks that worker bees of the social species can undertake, in order to sustain the colony such as building wax cells and collecting pollen.

  How Do Cuckoo Bumble Bees Take Over A Nest?

Cuckoo females typically emerge from hibernation a few weeks after the target host species. Some cuckoos target only one specific species of bumble bee, whilst others may select from 2 or 4 target hosts.

The queen cuckoo bumble bee must first locate a ready prepared nest at an appropriate stage of development. Female cuckoo must choose her target host carefully. There must be a colony already established of at least a few workers to help the cuckoo rear her offspring. On the other hand, if there are too many workers in the nest, she may be attacked and easily defeated.

A nest with 2 broods already reared, may have enough workers to overpower the cuckoo, and prevent her establishing herself in the nest.

Once the cuckoo has found a suitable nest, she will typically lurk around it for some time, in order to ‘pick up the scent’ of the host. This will help her to usurp the resident queen if necessary, and gain acceptance of the workers through physical attacks and the use of her pheromones.

Once the cuckoo enters the nest, there are a variety of scenarios that may occur, but a common one is that the original host queen is killed. The cuckoo will then set about laying her own eggs in the nest for the workers of the original queen to tend to and feed.

Timing is critical. If the cuckoo has entered a nest that is well developed, and there are many workers, they may attack the parasite, and kill her. On the other hand, if there are too few workers to support her, then she will not be able to rear many offspring.

  Is the Queen Always Killed?

Very often, she will be. Although cuckoos closely resemble social species, the female parasite is often a little larger than her host. She has a more powerful, longer sting, and a thicker, though less hairy coat. She can often overpower the founder queen unless there are sufficient workers to attack her.

However, there have been reports of both host and cuckoo cohabiting in a nest for quite some time, with the cuckoo even incubating the host queen eggs, and offspring from both the cuckoo and host queen emerging!

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