Bulb flies are hoverflies, the larvae feed on Narcissus (daffodils) and other bulbs in the Amaryllidaceae including Crinum, Eucharis, Galanthus (snowdrop), Haemanthus, Hippeastrum, Ismene, Leucojum, Nerine, Sprekelia and Vallota. Occasionally the underground storage organs of other families, such as Lachenalia, Iris and Scilla, can be affected.

Large narcissus bulb fly (Merodon equestris) can be the more serious problem, as this has larvae that can feed within and destroy healthy bulbs. The large bulb fly became established in the UK in the late 1800s and has since become widespread.  

Maggots of small bulb flies (Eumerus strugatus and E. funeralis) are usually a secondary problem; they feed in bulbs that have already been damaged by another animal, disease or lave been physically damaged.

Hoverflies are true flies and belong to the order Diptera, family Syrphidae. There are more than 6000 species of fly in Britain of which over 280 are hoverflies. Hoverflies get their name from their ability to hover in mid-air. Many hoverflies mimic wasps, honey-bees or bumble-bees with stripes, bands and markings of black and yellow. Large bulb fly is a bumblebee mimic. Adult hoverflies are pollinators and the larvae of most species are predators or help recycle organic matter (detritivores), very few feed on living plant material. Read more about hoverflies.

Bulb flies can be difficult to diagnose and they are only one of several reasons bulbs can fail. Often the bulb has completely rotted by the time any issue is noticed, at which point it is not possible to determine the initial cause of damage. There are several causes of lack of flowering (blindness) in daffodils.

Signs of bulb fly include:

• Affected bulbs are often killed or produce a few grass-like leaves but no flowers. This should not to be confused with daffodil blindness
• The centre of the bulb is eaten out by a plump, creamy white maggot that is up to 18 mm long. This is the large narcissus bulb fly (Merodon equestris) which is a primary problem that can feed on healthy bulbs
• The centre of the bulb is filled with the larva’s muddy excrement
• If there are several creamy white maggots, up to 6 mm long, in a decaying bulb, these are likely to be larvae of small bulb flies (Eumerus strigatus or E. funeralis). These are usually secondary and  only found in bulbs that have already been damaged by other creatures, diseases or have been injured

• Plant bulbs in shaded and exposed places; adult bulb flies like warm sheltered places, so daffodils growing in shaded and exposed places are less likley to be affected
• After flowering, firm down the soil around the plants to make it more difficult for female flies to deposit eggs
• Avoid introducing large narcissus bulb fly and other bulb problems into your garden by buying firm, good quality bulbs from reputable suppliers
• Newly acquired high value bulbs can be protected by covering the area where they have been planted with insect-proof mesh, during mid-May to June when the female flies are laying eggs.
• Any plants which show signs of bulb fly attack in the spring should be dug up and disposed of before the larvae leave the bulbs to pupate in the soil

Adult large narcissus bulb flies are about 15 mm long and resemble small bumblebees, but like all flies they only have one pair of wings. They have hairy bodies that vary in colour – some are yellowish brown, while others have bands of black, white and/or brown hairs. They emerge on sunny days in mid-May to June, when the females seek out suitable host plants.

Eggs are usually laid on the neck of the bulbs. After hatching, the maggot crawls down to the bottom of the bulb and enters through the basal plate. Once inside, the maggot eats the central portion of the bulb, including next year’s flower buds. The maggot is fully grown by late winter – early spring of the following year.

The larva usually leaves the bulb and pupates in the soil nearby.