Bumblebee males and new
The hatching of males as adults usually signals the beginning of
the end of the co-operation and organization of the nest. The males drink the
stores of honey, but do not forage to replace it. Males are usually produced
once the stores reach a sufficient quantity, or if the queen dies or loses her
When the adult males emerge they spend a few days in the nest, but
do no work, and then they leave the nest for good and forage for themselves.
They can often be seen sheltering under the heads of flowers when it rains or
when it gets dark. Well, that is what most of the books say, but recently it has been found that some North American bumblebee males do help in the nest by incubating the young, so their adult life is not just drinking, chasing queens and staying out all night long.
New bumblebee queens
New queens emerge about a week or so after the males. The new queens leave the nest to
forage for themselves, returning to the nest for shelter, but they do not add
to the existing nest provisions. When the new queen is ready to mate she flies
to where the attractant chemical has been deposited by the male and waits for a
suitable mate. Then the two mate. Most queens mate just once though, but some species, e.g. B. hypnorum and B. huntii have been found to mate 3 times.
Bumblebee males patrol circuits
The adult male bumblebee (in common with most adult male
insects) has only one function in life - that is to mate. Unfortunately it had been estimated that only 1 in 7 males actually achieve a mating. He will fly in a
circuit depositing a queen-attracting scent pheromone produced from a gland in his head in suitable places, usually in the
morning, and replacing the scent if it rains. Different species have different
preferred flying heights for this, and different queen attracting scents. Sometimes the same route is used by the same species year after year although all males die and so the new males have no idea where the males from the previous year made their circuit.
The pheromone is used to scent-mark prominent objects (tree trunks, rocks, posts, etc) on the circuit which is usually a few hundred metres long. The scent of some species can be detected by some humans. Usually they patrol at species specific heights. Bombus lapidarius, terrestris and lucorum males patrol at tree-top height. Bombus sylvestris and hortorum within 1 m of the ground. However this depends on the habitat. Bombus hypnorum does not bother with the hassle of patrolling a circuit and laying down scent. The males just find a suitable nest where the new queens have not yet emerged and they hang around the nest entrance and make a nuisance of themselves until the virgin queens do emerge. Then it is every man for himself.
Mating usually takes place while resting on the ground or on vegetation, but
there have been sightings of large queens flying with small males attached to
them. The photograph of a Bombus lapidarius queen and male mating above right was sent in by AsB, and the Bombus hypnorum photograph below was sent in by Julia Hedges. Both these photographs and that below of Bombus impatiens show that the mating is successful. At the start of mating the male grabs on to the queen's thorax to get himself into the correct position, this can be seen below in the Bombus lucorum photograph, only when everything is in position does he move back.
In order to mate the female must extend her sting so that the male can insert his genital capsule. The photograph below shows the underside of a Bombus sylvestris genital capsule.
|Note the size variation. Lapidarius is a species where there is a great range of sizes with the queen being much bigger than the workers and males. The mites on the queen will survive with her during hibernation and take up residence in her new nest the following spring. They use her as a means of transport.
The time taken for matings varies widely from 10 minutes to 80 minutes.
The sperm is transferred within the first 2 minutes of mating, and the bees are
in a rather vulnerable position, so why do they continue for so long? Well
after the male passes his sperm into the queen he pumps a sticky mixture into
her genital opening. This genital plug takes time to harden, and once hardened
can completely or partially block the entry of sperm from other males for up to
three days. So even though the two are in a vulnerable position, it is in the
interest of the male to hang on to ensure that his genes have a good chance of
being passed on to the next generation, and as most males don't even manage to mate, and those that do usually just mate once, he wants to be sure his genes will be passed on.
The photographs on the right and below Bombus impatiens mating. B. impatiens is a bumblebee native to the Eastern U. S., but is also found in the western states. The photograph below shows clearly the size difference between the male and the queen. Also the creamy yellow hair on the male's "nose". In this species males often have hair that is a brighter colour than the queen and workers. The image below was sent in by a visitor to the bumblebee.org site. New queens usually search for a hibernation site soon after mating.
New queens and cuckoo females drink lots
of nectar to build up their fat body and fill their honey stomach. This will enable them
to survive the winter hibernation, then they find a
suitable place to hibernate. They hibernate individually, but a good hibernation site can be dug into by many bumblebees. In the UK this is often under a tree root or at
the base of a wall, but it is never in a place that could be warmed up early in
the year by the sun. This is to prevent premature emergence. So in the UK the
hibernation site and nest site are usually located in very different
situations. As with many other animals that hibernate, it appears that
bumblebees must reach a certain weight in order to survive the winter. For the
largest bumblebee in the UK, Bombus terrestris, the queens must weigh at least 0.6 g to successfully hibernate and emerge
next spring. It is during hibernation that queens can become parasitized by the nematode Sphaerularia bombi.
During hibernation if the temperature falls below a certain point glycerol is automatically produced in the queen's body. This is a form of anti-freeze and prevents ice crystals forming which would cause the fluids inside her cells to expand and her body to burst.
Hibernation actually lasts as long or longer than any other part of a queen bumblebee's life as it can last for 6 - 9 months depending on the species and the spring temperature.
Link to video of Bombus impatiens mating
Bumblebee colony success and failure
The failure rate of bumblebee colonies is high. Not all nests go
on to produce males and queens, many fail in the early stages, some are
damaged, and some never build up enough reserves to produce reproductives. Some
nests produce only queens, others only males, and some nests produce both males
and queens. Studies have shown that as many as 75% of colonies can fail to produce new queens.
By the time males and new queens emerge the old queen is almost bald and may have lost some of
her influence over the persistent or larger workers, then gradually the stores
dwindle and the remaining workers and old queen die.
The timing of
events in a bumblebee nest depends largely on three things
- The species. B. terrestris queens are usually the first to emerge, B. pratorum usually produces the first workers,
males and new queens, and B. pascuorum is usually
the slowest to produce males and new queens.
- Climate. In colder areas
and in colder years events start later and end earlier.
- Food supply. This means suitable flowers throughout the colony life. If
there is an abundant supply of flowers with plenty of nectar for the colony and
not a lot of competition from other bumblebees and nectar/pollen gatherers then
the colony will be larger than if the food supply was limited.
The photograph on the right was sent in by George at
Some species can produce more than one
colony per year
There is some evidence that Bombus pratorum and B.
hortorum may, under certain circumstances, be able to go through two
colony cycles in a year. This would mean that instead of hibernating the new
queens would immediately start a colony and the queens that emerge from this
colony would then hibernate through the winter. B. pratorum and B.
hortorum usually have quite small colonies, and B. pratorum colonies
can reach their maximum size earlier in the year than any other species. This
has been reported in the south of England where the winters have been milder
recently, and there is plentiful pollen and nectar available from non-native
In New Zealand, where the winters are milder, Bombus terrestris, introduced just over 100 years ago to pollinate clover, can forage throughout the year, and in such
colonies the number of workers can reach thousands.