Valuable bulbs that are susceptible to bulb fly can be protected by planting them in a fairly small area and covering the area with a floating row cover such as Reemay before the flies emerge. You can also plant them in a dense perennial cover such as mat-forming thyme. Another strategy is simply to put a scoop of grit, sand, or bark over the vulnerable bulb when it begins to senesce. All these prevent the insect from laying its eggs where the larva can easily get into the bulb via the hollow left by the leaves and scape. Galanthus and Sternbergia are even more attractive to these pests than Narcissus, I find. Jane McGary Portland, Oregon, USA