There is an interesting article on Narcissus bulb fly here:
http://www.pacificbulbsociety.org/pbswiki/index.php/NarcissusBulbFlySome quotes:
There are two flies: the large Narcissus Fly, Merodon equestris, and the small Narcissus Fly, Eumerus strigatus.
Don't remove the detritus from the bulb. Plant the bulb as is since the detritus contains anti-fungicidal properties that prevent the bulb from rotting and enable it to survive long enough to aid sprouting of the surviving secondary meristem.
Destruction of the central meristem of the bulb by the larva will often trigger the secondary meristem in the outer scales into growth (if the bulb is large enough; smaller bulbs are usually totally consumed). Thus, if the bulb is left in the soil, a number of small leaves will emerge the following spring. Three growing seasons later, the bulb will be large enough to bloom again and provide the food source for another generation.
I have never found an attacked Galanthus in my garden, but I don't grow any special variety so I wouldn't miss one or two. And maybe there is some truth in what Thomas Seiler says. In Western Europe you can find snowdrops everywhere in the wild or half-wild.
http://www.walkinginsunshine.nl/images/amw_2006_0306_121007AA.JPG (Utrecht NL, Amelisweerd park). They thrive without any fungicide or insecticide or whatever threatment. A field like that must be heaven for the Narcissus Fly but still it grows bigger and bigger each year.
