How to recognise it
Affected bulbs contain a single, cream/brown larva, which is up to 20mm long, usually surrounded by a muddy brown excrement. If foliage is produced it is narrow, yellow and distorted. It is unlikely that the bulb will flower. These symptoms are similar to those caused by the stem eelworm.
The flies, which emerge in late spring/summer, are about 15mm long and are similar to to small bumblebees, also making a humming noise when in flight. You are most likely to see them on warm, sunny days when there is little wind.
The female usually gets to the neck of the bulb through the hole left by the foliage in the summer. She lays up to 100 eggs near the neck of the bulb which hatch about a week later. The larvae crawl to the base of the bulb and tunnel into the interior to start feeding on the central part of the bulb, which includes the following year’s flower buds. Generally there is only one larve per bulb, which feeds for around 6 months until March/April when it leaves the bulb to pupate in the soil. The adults then emerge in mid-May to June.
Why it’s a problem
The damage to the bulb can affect both flowering and foliage and, if severe, can kill the plant.
Where you are likely to find it
Can affect Narcissus, Hyacinthus and other spring flowering bulbs, particularly those growing in open, sunny situations.
How to deter it
Ensure that you purchase bulbs from reliable sources – commercial producers will treat bulbs with hot water and chemicals to kill any larvae. If you receive bulbs from friends or other sources, check them carefully before planting and destroy (preferably by burning) any which have holes or rotting areas.
How to get rid of it
You are unlikely to spot the problem until the damage has been done, ie when the plant fails to grow or flower in the spring. Lift and burn any infected bulbs.
In areas where the large nacissus fly is rife, or you have already had an infection, the risk of infestation can be reduced by planting in the shade or providing shading/a barrier (eg covering with insect-proof mesh netting or horticultural fleece) from May to June, as the leaves die down, to deter adults from laying eggs. Piling soil or a mulch over base of the dying foliage can also ‘bung up’ any holes which would allow the females to get to the bulb. Firming down the soil can have a similar effect. An insecticidal dust can also be partially effective if applied to plants during the egg laying period.
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Other useful information
This should not be confused with the small narcissus fly (Eumerus tuberculatus and Eumerus strigatus), which is much smaller, more numerous and only tends to infest plants which have already been damaged by other problems, rather than being the primary cause of damage.