Hornet Robberfly

• Its larvae develop in the soil beneath cow and horse dung, where they prey on beetle grubs, so the species disappears from pastures without extensive grazing.
• The beard of stiff bristles on its face protects it from the kicks of the prey it catches in flight.
• It is considered a good indicator of well-preserved grassland under traditional livestock management.
• It is one of the largest flies in Europe and a fearsome aerial predator: it hunts beetles, grasshoppers, bees and other flies on the wing, sometimes larger than itself.
• It kills by injection: it grips the prey with its spiny legs, stabs it with its rigid rostrum and injects neurotoxic saliva and digestive enzymes that paralyse it and liquefy it in seconds.
• Its appearance mimics that of a hornet, with a yellow-and-black abdomen, but it does not sting: it is Batesian mimicry.
• It depends on livestock. Its larva lives in the soil beneath dung pats and is believed to hunt the dung beetle larvae that develop there, so the species only thrives where traditional extensive grazing survives.
• That is why it is in decline across western Europe and is a priority conservation species in the United Kingdom: it disappears with the intensification of livestock farming and with the use of antiparasitic treatments that kill the fauna of the dung.
• It has a dense beard of bristles between its eyes, called the mystax, which protects its face from prey that fight back.

Skeleton type

Invertebrate - chitin exoskeleton (insect)

Habitat and environment

Dry grassland and wood pasture with cattle and horse dung

Sexual maturity

The adults begin to emerge in mid-June and the bulk appear in July; they hunt from a perch and catch their prey in mid-air

Gestation / incubation

The larva develops in and under droppings, mainly old cowpats and horse dung, and takes two to three years; the exact food of the larva remains unknown

Litter / clutch size

The female lays the eggs on or around herbivore droppings, preferring those whose surface has already dried

Sources consulted: Buglife; Natural England, planes de acción de especies; Pinheiro et al., trabajos sobre Asilidae; Worcestershire Biological Records Centre; Enciclopedia de la vida; Cumbria Wildlife Trust; Worcestershire Biological Records Centre, estudios de campo sobre Asilus crabroniformis; Ficha de especie de Asilus crabroniformis; Enciclopedia de la vida, ficha Hornet robberfly

Best time: July to September, in grazed grassland