Some interesting bee/fly relationships ...

22nd March 2014
It's getting well into the "bee season" now, with quite a few social and solitary species active locally. We hear quite a lot in the media about the pressures on honeybees (varroa mite, insecticides, colony-collapse disorder etc), and these are real issues. Because our solitary bee species are less well understood and less easily identified, their pressures don't get the same degree of attention but they are just as real and just as interesting!

Along with our early spring bee species, there are some flies that are also evident at this time of year. Amongst them is the Bee-fly (Bombylius major). This is also known as the Large or Greater Bee-fly or the Dark-edged Bee-fly. This species is the UK's most common Bee-fly. The fly is tawny-coloured with long hair; somewhat resembling a bumblebee. The long pointed proboscis is used for gathering nectar. The fly is harmless to humans (it does not bite or sting), but the larvae are parasitoids of the nests of mining bees.

The nests of Andrena mining bees are regularly parasitised by this fly. The female fly deposits her eggs in areas used by mining bees to construct their nest burrows. The fly larvae then enter the nests and consume the bee larvae and remaining food stores.

The lifecycle of the Bee-fly is interesting enough, but this pales into insignificance when compared with that of the Stylops fly! This Andrena mining bee has been parasitised by the Stylops fly. The Stylopidae is a family within the Strepsiptera; the Twisted-wing Flies. They are a very strange group of invertebrates.

When insects are infested with this parasite they are said to be stylopised. The adult forms can be seen protruding through the abdominal tergites (segments). In this Andrena bee there is a flattened female Stylops (nearest) and a male Stylops. A compound eye of the male is just visible, together with what looks like part of the complex antennae. The females remain in the host insect and do not develop external features like legs, wings, eyes or antennae; they remain flattened and grub-like. The males pupate in this position and then emerge in more typical insect form. The males fly off to find an infected bee and then mate with the projecting part of the female fly. You can sometimes see the empty pupal cases in the bee's abdomen.

This Andrena bee has two female Stylops in-situ. After they have been inseminated by the male, the females produce thousands of six-legged larvae (triangulins) which emerge from the female and seek out new hosts, often waiting on flowers. When a suitable bee arrives, they climb on and are carried back to the nest site. At the nest site the triangulins burrow inside the developing bee larvae. They remain in the developing bee continuing to moult and develop; utilising the bee's body fluids and internal organs as nutrients. Often the bee's internal sexual organs are affected and the bee fails to develop typical secondary sexual characters, resulting in inter-sex appearance and characteristics.

Stylopsed bees typically emerge earlier from the nest sites than non-infected individuals. They are also less active and reasonably easy to approach and photograph. I easily encouraged this one onto my hand. It's worth checking "early" bees exhibiting this behaviour to see if they are stylopised. It's often difficult to determine the species of stylopised bees but this one looks a little like Andrena chrysosceles. It was the right size. This species is very regularly stylopised; some Andrena species don't seem to get affected.

One of my hopes is to one day find a male Stylops fly that has left a bee and is on the wing seeking a female. Even better, encounter one mating on a parasitised bee. Some photographers have been fortunate enough to witness this. I'm always on the lookout!

[Click on any image for a larger version]


Photo comment By George Pilkington: Fantastic pictures and a truly amazing story. Thanks for sharing. I have re tweeted. Is n't science fantastic! Cheers George
Photo comment By Lucy Corrander: Disturbing - but fascinating. (And an unusual ambition!)

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