Hornet Robberfly

Arguably Britain’s largest fly, the Hornet Robberfly (Asilus crabroniformis) has an impressive appearance. At over 2.5cm long with black and yellow markings, the Hornet Robberfly resembles a hornet (indeed the Latin name “crabroniformis” means ‘hornet-form’). This mimicry helps protect them from predators whilst basking or perching in the open, whilst their brown undersides camouflage them in their night-time roosts of tall vegetation. Harmless to humans, they are ferocious predators of other insects.

Like other robberflies they are hunters, catching beetles, grasshoppers, wasps, and other flies, including each other!  The robberfly waits on a perch, watching the air for movement. Once prey is spotted, it launches and grabs the insect in mid-air with spiny legs, then lands and uses its piercing mouthparts to drink the prey’s body fluids. A thick, gingery “beard” protects its mouth from struggling prey.

A particular threat to Hornet Robberflies is the use of general insecticides, such as avermectins for worming cattle, horses and other livestock which sterilise their dung. Robberflies often lay their eggs in dung and often use them as hunting perches. Between 1980 and 2000 the Hornet Robberfly declined by 21% in the UK. Though numbers may have now stabilised they remain at a much lower level, the species still faces the same threats and is confined to only around 40 isolated breeding sites. They are particularly vulnerable to local extinction since they cannot fly beyond a few hundred meters from suitable habitat. Due to this, they are a Priority species in England and Wales.

Being reliant on grazing animals and insect-rich habitats makes the Hornet Robberfly vulnerable to changes in farming practices. The conversion of heaths and wildflower-rich grassland to species-poor ‘improved’ grasslands or arable land, replacing cattle with sheep and even under-grazing on nature reserves have reduced the robberfly’s range.

Though it may not be the most endearing species, the Hornet Robberfly has a dramatic and fascinating lifestyle and is an important indicator of the state of our countryside for invertebrates.

• Size: >25mm in body length.
• Life span: Annual.
• Diet: Both adults and larvae are predatory. The adults will feed on a variety of insects, including grasshoppers, beetles, bees, wasps and other flies. The robberfly larvae prey on dung beetle larvae, especially the larvae found in the soil beneath animal dung.
• Reproduction: Hornet Robberflies lay their eggs on edges of dry cow pats, horse dung, under rabbit droppings or on nearby vegetation. The adults may be seen sitting on other types of dung, such as sheep or dog, but they are more likely using these as hunting perches rather than for egg-laying.
• When to see: Late July to early October.
• Population Trend: Declined but now more stable (albeit at a diminished level of population and range).
• Threats: Loss of habitat, changes in livestock and grazing, over-use of veterinary medicines.
• Interesting Fact: Hornet Robberflies are fussy about their dung! Sites in Hampshire that replaced their cattle with sheep found that local populations of the Hornet Robberfly disappeared. They prefer cattle, horse and even rabbit dung as hunting and breeding sites.

Buglife is working to restore populations of the Hornet Robberfly through  our projects, including partnership project Life on the Edge, but we need your help!

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