Narcissus fly attacks other bulbs as well as narcissus - daffodils.
They are also responsible to damage of the bulbs of Hyacinths, Irises and Snowdrops.
The Narcissus fly swoops just as the foliage is dying down on the bulbs - after flowering - and lays its eggs in the crown of the bulb. The eggs hatch and become the maggot - grub - of the fly and bore down into the bulb where they feed on the inside fleshy scales and emerging flower bud.
The bulbs which have been affected, become hollowed out inside and have a soft spongy feeling. The planted bulb just produces a few leaves and no flower, as this has been eaten away by the Narcissus Fly larvae.
You should never plant or select bulbs that have a soft spongy feel - always go for good firm bulbs when buying.
With bulbs that are to be left in the garden after flowering, you should rake up a little soil over the neck of the withering foliage just after the flowers have finished. This prevents the Narcissus fly from laying its eggs in the crown of the bulb to a certain extent.
the damage being caused by the maggot of the fly will rarely be seen for the foliage will normally be dying down after flowering anyway. The grub continues to feed - hollowing out the core of the bulb. The grub is happy to stay in the bulb overwinter and then emerge the following spring as it warms up, pupating and ready - as an adult narcissus Fly - to re-start the life cycle of laying eggs in the crown of withering bulb foliage.
The effects of this are sometimes known as Bulb Blindness, and indeed the bulb will have no flower. However, if the bulb is left, then it will renew itself over two to three years and start flowering again.