There are two granular “grub killer” products marketed for use on lawns which are effective on large narcissus fly grubs. One of these contains Dylox and the other contains Imidacloprid. The Dylox product is said to kill grubs in 24 hours. Imidacloprid is said to be effective for (I think) around two months. I assume this means that the Dylox breaks down in the soil and is not effective for long periods of time (although I am not sure of this). The Imidacloprid persists in the soil and is effective for some months, basically, a season. I believe that Dylox is registered for narcissus fly control, but imidacloprid is not. However, it is pretty clear that both are effective. My experience with these granular formulas of the two chemicals began 3 or 4 years ago. The first season, I put a tablespoon or so of the dylox-containing product around the base of each plant or clump before bloom. Later that season, thinking that the Dylox would not last, I used the imidacloprid. Each year since then, I have used only the imidacloprid product mid to late in the blooming season. What this has done, I think, is to interrupt the life cycle of the flies by killing the grubs, either as they travel up from having spent the winter eating bulbs, or having hatched from the egg stage, travelling down to enter the base plate. Whatever, my garden has been virtually free of bulb fly for the past few years. (I think I found one hole in the hundreds of bulbs I dug last year.) Occasionally, I see an imposter fly or two buzzing around, and if I feel angry enough I might zap them with a contact poison like malathion, which is cheap enough and is not persistent. If only basal rot were as easy to fix! The granular formulations of both chemicals are sold as lawn grub killers and are widely available under various brands. Just check the bag to see what the active ingredient is. The foregoing is just my experience; it would be good if there were some controlled trials to see what the most effective treatment is, and what the environmental side-effects may be.