Hornet robberfly
A ferocious looking mimic, which is absolutely harmless to humans
I have walked hundreds of miles this field season, undertaking a great number of surveys across farmland and estates, and by looking at my Fitbit stats, it says I have walked just over 500 miles, which is the equivalent of me walking from my house in a sleepy village in the Test Valley in Hampshire, to Glencoe in Scotland, which is quite impressive and also quite exhausting to think about!
While doing so, I have encountered so many wonderful things that not everyone gets to encounter, like breathtaking sunrises and sunsets, recording rarities, watching the seasons change, witnessing the rut – I was nearly bowled over by a roe buck and doe crashing through a hedge – having oblivious hares run right up to me when I have been hidden in a hedge, watching migrant species come and go, listen to swifts screeching above, stare back at little owls peeping out of their holes, but also spotting flora and fauna that I haven’t seen before, like the hornet robberfly (Asilus crabroniformis), which I saw for the first time last month and is my species of the month for September.
While doing butterfly surveys on the Martin Down Farmer Cluster, one whizzed over my shoulder and landed on the field edge in front of me, clasping onto its latest prey. It is quite a menacing-looking insect, which would make anyone stop in their tracks due to its size and appearance. Arguably Britain’s largest fly (Diptera), the hornet robberly is one inch long (2.5cm) with black and yellow markings and a gingery ‘beard’, resembling its namesake – the hornet. Its scientific name, crabroniformis, means ‘hornet-form’. The hornet robberfly watches for its prey from a perch out in the open, so this imitation of a hornet helps protect them from predation themselves.
They are brilliant aerial hunters and feed on several species as adults, including other flies, wasps, grasshoppers and beetles. They catch their prey by launching from its perch, grabbing insects mid-air and then piercing its prey with its sharp mouthparts to drink its body fluids, before discarding the dry, unwanted body away. For those who might be faint-hearted, this may sound awful, but that’s nature for you!
These perching posts are also great vantage points for males to seek out a passing mate. When the time comes, the females lay their eggs in cow, horse or rabbit dung, where the larvae feed on dung beetle grubs in the ground beneath. The robberfly spend three years as larvae before they pupate and emerge as winged adults the following summer – with just five weeks to start the cycle again!
Due to this lifecycle, they are very dependent on grazing animals and insect-rich habitat found on heathland and appropriately managed species rich grassland, which has confined them to about 40 isolated breeding sites in southern England and west Wales.
Between 1980 and 2000 the hornet robberfly declined by 21% in the UK. This has been due to the conversion of heaths; intensively managing meadows (or ‘improving’ grassland); the increase of arable land; replacing cattle with sheep and even under-grazing on nature reserves; and the reduction or complete removal of livestock in the landscape. Taking land away due to development has significantly reduced this robberfly’s range.
As I said, this species is very vulnerable to change of land use and land management, especially the use of general insecticides such as avermectins for worming cattle, which also kill off the dung beetles the hornet robberfly prey on. The numbers have stabilised, but the species still face local threats, which makes them very vulnerable, as they cannot fly beyond a few hundred metres from suitable habitat.
I know they might not seem the most endearing species, but the hornet robberfly is an important indicator of the state of our countryside for invertebrates. So, spotting this species on your land is a real accolade to you and what you are doing for biodiversity – like the members of the Martin Down Farmer Cluster, it is a result of the great work they are doing, individually and together, to increase biodiversity at a landscape scale.
I hope you get a chance to see this scary-looking but harmless predator, as wouldn’t it be great if we could increase their range by decreasing insecticide use and managing grassland in a way to increase diversity?
P.S. How on earth is it September already? My mother was right: the years go faster the older you get!
Megan Lock
Advisory
Image credit: Megan Lock