Narcissus Bulb fly (Merodon equestris)

Discover the Darker side of Hoverfly Ecology

Narcissus Bulb fly (Merodon equestris) 

Wing Length 8.5-10.25mm Body Length 10-14mm Flight late Apr-early Jul.

Species to spot in May

Have you ever planted bulbs that never quite made it to flowering, and you’re left wondering why? If you’re fairly sure they haven’t been dug up by a local grey squirrel, this hoverfly might be the hidden culprit.

It’s a remarkable species with around 34 different disguises, often easily mistaken for several types of bumblebee. The adults begin emerging from the soil from late April through to the end of June, so it’s well worth checking your bulb pots. You might spot one warming up in the sunshine or drying its wings on the leaves of your plants.

This hoverfly was first recorded in the UK in 1869, after being found in a London garden. It’s believed to have arrived as an unintended consequence of the bulb trade with the Netherlands. During the Victorian era, newly imported bulbs from Amsterdam were highly fashionable among horticulturists, and vast numbers were shipped from Dutch fields to English estates. This species followed those same commercial routes, travelling inside consignments of Narcissus (daffodil) bulbs. As the larvae live hidden within the bulbs for many months, they remained undetected — quietly hitching a ride without the bulb connoisseur ever knowing.

Today, this hoverfly is found across much of the world, not just in the UK. It was only recently recorded in South Korea in 2018, where it had previously been misidentified as a species of Mallota*. Merodon is now recognised as the most globally widespread genus within its group. While the UK has just a single species, there are over 50 species in Europe — most occurring in Turkey — with only five found in north west Europe. Worldwide, there may be more than 200 species.

*A quick look at the rot hole dronefly (Mallota cimbiciformis) makes it easy to see why the confusion arose.

It is a robust hoverfly typically 10-15mm in length, with a stout, heavy-bodied appearance. As with other hoverflies, the sexes can be distinguished by the eyes: males are holoptic, with the eyes touching at the top, while females are dichoptic, with the eyes clearly separated. One of the key identification features is the distinctive loop in vein R4+5 towards the tip of the wing. Beneath this loop is a closed cell shaped rather like a sock — a feature unusual among hoverflies. The legs are entirely black, and the hind femur carries a prominent triangular, tooth like projection on the underside near the tip.

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Many years ago, I had a wonderful time in a churchyard in Hornsea where that morning there had been an eruption of Merodon equestris, each exhibiting a different form. I was very new to Hoverfly Spotting in those days, but I spent several hours noting several different forms of what I would later find to be just the one species. If my memory is correct, I counted 9 variations within that one visit.

Polymorphism: - refers to the occurrence of two or more distinct forms or traits within a population of a species. It is a discontinuous genetic variation where individuals of a species exhibit sharply distinct form.

These forms of polymorphism in Merodon equestris exhibit themselves in the colouration of the hairs on the thorax and abdomen. Below are 6 different main forms, but there are 34 different colour variants to be found.

Common – bulborum, equestris and flavicans
Less common – Validus
Rare – nobilis and transversalis

How many variants have you seen?

    Source: Hoverflies of Britain and North-West Europe: A Photographic Guide

They are now quite a common hoverfly in the British countryside, reflecting our ongoing love affair with bulbs as a nation of gardeners. You’ll often find them in gardens, parks, and urban spaces wherever daffodils, tulips, and other bulbs are growing.

They mimic several species of bumblebee — red tailed, buff tailed and carder bees, as any bumblebee spotter will recognise. The mimicry isn’t just in their colouring but also in their behaviour, as they zig zag through vegetation with a loud buzz and a clear preference for visiting blue flowers. One reliable way to tell whether you’re looking at a hoverfly rather than a bee is to check the antennae and legs: hoverflies have short antennae, large eyes and lack pollen sacs on their hind legs. If you notice those features, you’re looking at a hoverfly.

The larvae of this species don’t feed on aphids. Instead, the female lays her eggs low down among bulbous plants, often close to small gaps where the newly emerged larvae can crawl into the soil. From there, they make their way to the basal plate of the bulb and burrow inside, where they feed and then hibernate until the following spring.

They are often considered a pest in the horticultural trade. However, the larvae don’t kill the bulb, although their presence can stunt its growth, meaning it may fail to flower that season. Despite this, the bulb usually recovers, rebuilding its resources so that normal service resumes the following year.

One tell‑tale sign when buying bulbs is softness — this can indicate that a hoverfly larva may already be inside. But please don’t throw it away. Plant it instead and allow the hoverfly to complete its lifecycle. Keep your camera ready too, as the insect will pupate nearby and often stays around the parent plant for some time, offering a great opportunity to observe and photograph it.

This happened a couple of years ago, when not one but two of them crawled out of a pot of bulbs. They hung around for quite a while, patiently drying their wings before eventually flying off. In the video below you can hear me offering a bit of running commentary — it’s going, going… and then it’s gone. I won’t be expecting a call from the BBC any time soon, and it’s probably why I’m not on YouTube.

Sources:

Ball.S and Morris R 2023: Wildguides Hoverflies of Britain and Ireland 3rd Edition: Princetion University Press

Graham E.Rotheray and Francis.S.Gilbert 2011: The Natural History of Hoverflies Forrest Text

Bot.S & Meutter Frank Van de 2023: Hoverflies of Britain and NorthWest Europe A Photgraphic Guide: Bloomsbury Naturalist