Why Do Bumble Bees Dig In The Ground?

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. 

Reasons Why Bumble Bees 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

  • parasitized, or indeed,
  • the bumble bee is creating a small area in which to hibernate. 

But how do we know which?

Parasitized Bumble Bees

1. Bumble Bees Parasitized By Conopid Flies 

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.

Conopid fly on pink hemp agrimony.  Side view.  The fly has a thin waspish waist and is dark in color.  Conopid flies can cause bumble bees to dig in the ground.Conopid fly on hemp agrimony.

2. Nematodes (Sphaerularia bombi)

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.

Bumble Bee Queens May Dig In The Ground To Hibernate

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’:

“Fertilised queens continue to feed and to return to the parental nest, building up their fat store – large, whitish clumps of cells, which eventually fill much of the abdominal cavity.  They also fill their honey-stomachs prior to settling into their chosen place for hibernation.  Little is known about where or when the different species hibernate. 

Favoured places are said to be north-facing banks, where they dig into deep leaf litter, or into loose soil, forming a small chamber.  In some species at least, the fertilised young queens enter their winter quarters as early as July or August, but individuals of other species (such as B. pascuorum and B. terrestris) may be seen on the wing well into October.”

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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