Meet the hornet robberfly on Motttistone Down

As well as watching where you put your feet when walking across cow meadows on the Isle of Wight’s Mottistone Estate in summer, keep a watchful eye open for a large fly that’s also looking out for cow pats.

Grazing at Mottistone Down

Mottistone Down is an area grazed by rabbits and – in summer – by cattle. This helps to control the spread of scrub and brambles onto the grassland. But these cattle are also good news for a fly which is large, bristly and downright unpleasant to its enemies.
 

Enter the hornet robberfly

The hornet robberfly is one of Britain’s biggest flies at just over an inch long (28mm).  It has eye-catching bright yellow and black patterning, looking rather like a big hornet.  Sadly it is now a scarce species in Britain so we’re actually pleased that it has chosen to live at Mottistone.  
 

A cowpat love affair

The female hornet robberfly chooses to lay her eggs in cowpats, because when they hatch we believe her larvae feed on the larvae of dung beetles, also living in the cow pat. Doubly bad if you are the young of a dung beetle.
 
After spending about two years as a larva, the hornet robberfly lives for another two to three years as an adult. The flies are active from July to October and can often be seen sunning themselves on dry cowpats. As you would.
 

What's for lunch?

The adult robberfly feeds on a variety of insects, including grasshoppers, bees and wasps. It will often dart out to catch its prey mid-air and then suck it dry – a process which may take up to 30 minutes. We did warn you that they were unpleasant.
 
Since we want to let insects like this carry out their natural lifestyle, we encourage our farm tenants only to treat their cattle with worming drugs that do not kill creatures that live in or on their dung.
 
More gruesome details about this unpleasant but fascinating fly can be found on the Buglife and Natural History Museum websites.