The hornet mimic hoverfly
Val spots a large hoverfly that scavenges on wasps’ nests
SOME of you may remember me writing about a wasps’ nest positioned underneath the seat at the front of our house (AG 4 July). I’m afraid it met an untimely end when this year’s exceptional spring turned into an unseasonably wet and cool summer. The wasps became weak and unable to fly, and by early August the sodden nest had been abandoned.
Once we made sure that nothing flew out when the garden seat was rattled, the Best Beloved and eco-warrior granddaughter Ellie removed the nest in the last week of August, without fear of being stung. The nest had been successful at one time, though, because it contained at least a dozen large queen cells.
One thing led to another, as it does in the natural world, and a little later we spotted a huge hoverfly roughly the length of an adult’s thumbnail.
Its Latin name is Volucella zonaria and it’s a hornet mimic hoverfly with a yellow-and-black banded body topped by raised chestnut-brown humps on its upper abdomen. It’s the largest hoverfly seen in British gardens, although it’s not one of our 285 native hoverfly species.
This hoverfly was first noted in the 1930s on the coast of southern England. It spread northwards, and since 1995 it has expanded its range into North Wales. Numbers peak in August, although this large hoverfly can still be seen on the wing in November.
We’ve never seen this giant in the garden before, but the interesting thing is the maggot-like larva of Volucella zonaria survive by scavenging in the nests of social wasps and hornets. They feed on the debris at the bottom of the nests. Perhaps this one came from our nest under the seat.
Hoverflies are true flies and they follow the same lifecycle of adult, egg, larva and pupa. Like all true flies, the adults have just one pair of true wings. They have small club-like structures, known as halters (or halteres), which act like a gyroscope and give these insects incredible control over their flight, allowing them to hover.
Most hoverflies are great for a natural garden, because many have maggotlike larvae that predate aphids. The white larvae are said to be more effective at finding aphids than ladybirds, even though they are blind. They eat aphids more quickly and can reach into tighter spaces than ladybirds. They will also feed on other soft-bodied insects.
Some larvae feed in the day, others at night. Day feeders tend to look rather like small milky slugs, but nocturnal feeders are often more transparent and resemble bird droppings. They all pupate, often close to pollen sources, and then a new hoverfly emerges.
Despite its hornet-like size, Volucella zonaria has tiny mouth parts and needs to find small or simply shaped flowers – a trait shared by all hoverflies. I saw my giant hoverfly on the grey-blue flowers of a scabious-like plant named Succisella inflexa ‘Frosted Pearls’, a perennial with pollen-rich anthers held above the petals.
Adults need energy-giving nectar for flight and protein-rich pollen for viable eggs, and can live for a month or two months, depending on the species. The females lay single eggs, rather like slim, off-white rugby balls, close to aphid colonies. After five days these hatch into larvae. A larva passes through three stages, or instars, and at the end of each stage it sheds its skin so it can increase in size. The third-stage instars do most of the feeding and can live for a long time.
One of our most common hoverflies is the marmalade hoverfly (Episyrphus balteatus). Each of its nocturnal larvae could consume 250-600 cabbage aphids in its lifetime. Although common in late July, it can also be seen on a sunny day in midwinter because adults overwinter. It’s very common and often mistaken for a social wasp.