A bumblebee's worst nightmare?

The arrival of these mating thick-headed flies, which I think are probably Conops quadrifasciatus, is very bad news for bumblebees nesting in our garden. They're parasites, whose larval stages feed on the body fluids (haemolymph) and ultimately on the internal organs of their bumblebee hosts.

Female thick-headed flies spend much of their time on flowers, waiting for bumblebees to approach, then they grab them - often while the bee is still in flight. They have specially-adapted reproductive structures on their tails which prise apart the hard segmental plates of the bee's exoskeleton, so that they can insert an egg through the softer tissue in between. As many as a third of worker bumblebees in a colony can be parasitised and there is some evidence that even in the early stages of infection the behaviour of the bees is affected, so that they spend more time collecting nectar for their own consumption and return to the nest less frequently.

There are plenty of published accounts of the way in which Conops lays its eggs in its host but in a quick search of Google images I couldn't find any of the parasite attached to, or laying its eggs in the host either when they were at rest or in flight - it seems to happen quickly and would be something that anyone would be lucky to see, let alone photograph.

It's a tough life being a worker bumblebee: if you're lucky you work from dawn until dusk all your short life; if you are unlucky you meet Conops.............

Thick-headed Flies

Conops quadrifasciata pairs remain together like this for some time after mating has taken place, and take to the air in unison

Females have a more slender abdomen that's sharply curled under at the tip

The exceptional width of the male's head is visible here. Maybe it's because the female's vision is obscured by the male above, so he has to act as look-out for both of them

These flies, mimicking wasps and linked together during mating in characteristic biplane arrangement, as thick-headed flies Conops quadrifasciatus that I found in a field of ragwort in Durham. The bottom photo shows the broad head which gives these insects their common name. They’re parasites of bumblebees, laying eggs on the adult bees which are then literally eaten alive by the parasitic larvae. One study carried out in Switzerland and published in 1990 found that almost 35 per cent of workers of early bumblebee Bombus pratorum and common carder bee B. pascuorum sampled in August contained this parasite’s pupae. At this time of year there are always a lot of sleepy-looking bumblebees apparently resting on flowers and showing little sign of feeding, and many of these are likely to be suffering from parasites, including conopid fly infestations. There’s no doubt that habitat destruction has been a major factor in bumblebee decline, but it’s also true that parasite infestations can have a very significant impact on local populations too.