Updated: 25th February 2021
Earlier this year I was asked during one of my talks “Why do bumble bees dig in the ground?”
The lady was quite sure it was a bumble bee she had observed, rather than a mining bee, which may be seen burrowing into soft soil to create a nest.
In asking the question, she wanted to know why the bumble bee was digging – whether it was to create a new nest, or whether the bumble bee was digging a hole for the purpose of hibernating.
In studying bumble bees, scientists have uncovered several reasons why bumble bees may dig in the ground.
When bumble bees are observed to dig holes in the ground, generally, it will be either because the bumble bee is
But how do we know which?
Certain species of conopid flies affect the behaviour of some species of bumble bee, and generally workers rather than queen bumble bees. Female conopid flies rest on flowers and attack bumble bees in the air, or whilst they are foraging on flowers. Eggs are deposited into the body of the bee where they grow and develop.
Before dying, the host bumble bee may dig into the soil
about 5-10 cms, where the conopid flies are now able to hibernate to remerge
the following year.
These parasites live and mate in the soil, and it is the females which attack queen bumble bees, usually during their winter hibernation, but more rarely they are also believed to attack early emerging queens. The female nematode is fertilized, and releases its eggs into the body of the bumble bee queen.
The bumble bee queen is sterilised as a result of
the infestation, and her behaviour alters.
It is suggested by some that instead of searching for nest sites, the
queen searches for hibernation sites, although scientists do not necessarily agree
on this point.
Others suggest the queen searches for both hibernation and nest sites, yet digs shallow holes in situations unsuitable for either. However, the juvenile nematodes are released into the soil via the bumble bee queen’s faeces (poop).
It is suggested that in some cases, infestation may spread from one queen to another, when an uninfected queen inspects a hole dug by an infected one, whilst searching for a nest site.
Toward the end of the life of a bumble bee colony, new queens
will emerge. They will mate and feed to
prepare themselves for winter hibernation.
Ted Benton writes in his book ‘Bumblebees’:
In the scenario described by the lady, I suspected the bumble bee was actually parasitized, for the simple reason that I believed it was far too early in the season for hibernation to be occurring.
Take a look at my page Which bees dig in the ground?
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