Arguably Britain’s largest fly, the Hornet robberfly (Asilus crabroniformis) has an alarming appearance. Harmless to humans, they are ferocious predators of other insects.
Latin name: Asilus crabroniformis
Notable feature: Large hornet-like fly with a bristly, ginger body and long, spiny legs
Where in the UK: Heaths and Grasslands in Southern England and West Wales
At over an inch long with black and yellow markings, the hornet robberfly resembles a stinging 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.
Like other robberflies they are hunters, catching beetles, grasshoppers, wasps, and other flies, including each other! The robberfly watches the air for movement from a perch. Once prey is spotted, it launches and grabs the insect in midair 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.
Males are also watching for passing mates. When the time comes, the female lays her eggs on cow, horse or rabbit manure. The larvae are thought to dine on dung beetle grubs in the ground beneath, where they spend three years of their lives. They then pupate and emerge as adults the following summer with just five frantic weeks to start the cycle again.
Being reliant on grazing animals and insect-rich habitats makes the Hornet robberfly vulnerable to changes in farming practices. The conversion of heaths and meadows 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.
A particular threat to Hornet robberflies is the use of general insecticides, such as avermectins for worming cattle, which also kill off the dung beetles the robberflies prey on.
Between 1980 and 2000 the Hornet robberfly declined by 21% in the UK. Though numbers have now stabilised, the species still faces the same threats and is confined to only around 40 isolated breeding sites. They are particularly vulnerable to local effects since they cannot fly beyond a few hundred meters from suitable habitat.
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. Buglife is working to conserve the Hornet robberfly by increasing awareness of the species and educating farmers and landowners to its needs, including the publication of a Species Management Sheet.