June 2016

Great fleas have little fleas upon their backs to bite ‘em.  And little fleas have lesser fleas, and so ad infinitum.  As doggerel, at least it scans and roughly rhymes, but as far as scientific accuracy goes, it leaves a lot to be desired.  Firstly fleas don’t bite other fleas, they only bite mammals, and secondly the chain of parasites stops somewhere at the level of single celled organisms, which is way short of infinity.  But having just returned from Sweden working for the bumblebee reintroduction project (www.bumblebeereintroduction.org ), I am more concerned about what is biting bumblebees, rather than fleas.

One of the first short-haired bumblebee queens caught in Sweden in 2015

When the bumblebee queens are returned to the UK, they are put in quarantine for two weeks.  The quarantine is not an import requirement, but a decision by the project to avoid as far as possible bringing new strains of bumblebee disease into the UK.  (There are no such restrictions on the import of commercial bumblebees for pollination purposes to the UK, which is crazy.)  We expect only 50% of the queens to make it through quarantine.  Mites are the largest creatures that infect bumblebees, and if you get a close look at one in your garden, you may see small pale yellow or white spots, especially at the joint between the middle bit and the end bit. (Okay, thorax and abdomen if you want to get technical.)  If you use a magnifying glass, you may see that the pale spots have eight legs.  Mites don’t do the bumblebees any harm, but a serious infestation may affect its ability to fly because of their weight.  If any mites are found on our reintroduced bumblebees, they can be removed during quarantine after a visual inspection.  (Note that though bumblebees may carry the varroa mite, it does no harm. It is only a problem for honey bees because their colony stays warm throughout the winter.) 

A short-haired bumblebee queen released at Dungeness in
 2015 feeding on periwinkle

Following the flea rhyme, the next parasite in descending order of size would be a parasitic wasp (Syntretus species) which lays her eggs in the thorax of the queen.  These eggs then develop into larvae (maggots) which collect in her abdomen and will eventually kill her.  There are a large number of parasitic wasps that prey on a variety of insects and go about their dastardly business in much the same way. 

The next smaller in size would be a nematode worm – Sphaerularia bombi – that infects the gut of many bumblebee species.  Nematode worms infect just about every animal species on earth and it has been estimated that 80% of all animals now alive are nematode worms.  They are roundworms (to distinguish them from flatworms) and yes, the roundworm that infects human guts is a nematode worm.  The horrible Loa loa worm that Sir David Attenborough uses as an argument against a benevolent creator is a nematode as well.  There are around 28,000 species of nematode worm known and some even infect plants.  When nematodes develop in the bumblebee, they alter her behaviour in such a way that instead of founding a colony after emerging, she feeds herself and then goes and finds somewhere to hibernate instead.  She then dies and the worms leave her body, mate, and then wait until another queen hibernates there in the autumn.

Last, and probably least, as far as we know, is a single-celled organism called a trypanosome.  This is Crithidia bombi which infects the gut of the bumblebee.  Though this may weaken the bee, it doesn’t kill it and the queen may still go on to found a colony.  So if we find a bumblebee with Crithidia while we are collecting them, she will be released again near to where she was found.  In order to find out if the bees have Crithidia, the project has a bee poo specialist (@BombusJones) who looks at a poo sample under a microscope to see if there is any there.  She spends much of her day waiting for the bees to poo, and the rest of it peering at bee poo through a microscope, and curiously, she seems to thoroughly enjoy doing it.

Yet another threat to bumblebees - a cuckoo bumblebee Bombus rupestris that will kill or eject a red-tailed bumblebee from her colony, and then, after killing or eating any eggs or larvae in the colony, will lay her own eggs and induce the workers from the original colony to raise her brood.