Merodon equestris
Jane McGary (Mon, 19 Nov 2012 16:28:59 PST)

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