Conops quadrifasciatus and Gasteruption jaculator

Conops quadrifasciatus is a fly in the family Conopidae; known as the Thick-headed Flies. As the name suggests, heads of these flies are typically the widest part of the insect and this can be seen clearly in the photograph. Many species have wasp-like colouration and they can be seen regularly feeding (and mating) on flowers. The example in the photograph is feeding on an Eryngium flower.

Although the adult flies enjoy nectar, the larvae are parasitioids (they parasitise and eventually kill their hosts); of various bees and wasps. The females of Conops quadrifasciatus specialise in attacking bumblebees in flight. They wait on flowers for a bumblebee to arrive and then hijack the bee and whilst holding on; inject an egg into the bee's abdomen. The egg hatches inside the bee and the larva feeds on the bee's abdominal contents. The bumblebee eventually dies and the larva then pupates and emerges the following summer.

Interestingly, parasitised bumblebees change their behaviour and tend to stay outside and visit the nest less frequently than other workers, even remaining outside during spells of cold weather.

This extraordinary-looking insect is the parasitic wasp Gasteruption jaculator. I've had quite a few in the garden this summer; nectaring on Lobelia and Campanula flowers. More importantly, I've watched them investigating my west-facing house wall on sunny evenings.

Gasteruption jaculator is a cleptoparasite (it "steals" the nest), of various other species of solitary bees and wasps. When "investigating" my house wall, they are looking for the nest burrows of potential host bees/wasps. If they find one, they use their long ovipositor to deposit an egg into the brood chamber. When the egg hatches; the larva eats the host egg/larva and the pollen/nectar food store collected by the host bee. This behaviour is not uncommon amongst our bees and wasps. Indeed; of the 200+ species of solitary bee in the UK, well over a quarter are parasites of other bees!

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