Earlier this year I was asked during one of my talks “Why do bumblebees dig in the ground?”
The lady was quite sure it was a bumblebee 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 bumblebee was digging – whether it was to create a new nest, or whether the bumblebee was digging a hole for the purpose of hibernating.
In studying bumblebees, scientists have uncovered several reasons why bumblebees may dig in the ground.
Why Do Bumblebees Dig In The Ground?
Some species of bumblebee may be observed to dig holes in the ground. Generally, it will be either because the bumblebee is parasitized, or indeed, the bumblebee was creating a small area in which to hibernate. But how do we know which?
Bumblebees Parasitized By Conopid Flies Or Nematodes
Some species of conopid flies affect the behaviour of some species of bumblebee, and generally workers rather than queen bumblebees. Female conopid flies rest on flowers and attack bumblebees in the air, or foraging on flowers. Eggs are deposited into the body of the bee where they grow and develop.
Before dying, the host bumblebee may dig into the soil about 5-10 cms, where the conopid flies are now able to hibernate to remerge the following year.
Nematodes (Sphaerularia bombi)
These parasites live and mate in the soil, and it is the females which attack queen bumblebees 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 bumblebee queen.
The bumblebee is sterilised as a result of the infestation, and her behaviour alters. It is suggested by some that instead of searching for nesting sites, the queen searches for hibernation sites, although scientists do not necessarily agree on this point. Some suggest the queens search for both hibernation and nest sites, yet dig shallow holes in situations unsuitable for either. However, the juvenile nematodes are released into the soil via the bumblebee queen’s faeces.
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.
Bumblebee Queens May Dig In The Ground To Hibernate
Toward the end of a bumblebee colony life, 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 bumblebee was actually parasitized, for the simple reason that I believed it was too early for hibernation to be occurring.
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