The large Narcissus fly (Lampetia equestris) flies in October and November. This fly is the same size as a blow fly with the same distinctive buzz. The female lays eggs on the top of a bulb and the resulting caterpillar feeds on the inside of the bulb right through summer. It is known to attack daffodils nerine, snow flakes and snow drops. Prevention is best by ensuring the soil covers the top of the bulb or, if you wish, diazanon is a useful deterrent.