Make the furrow in which the onion sets will be planted. Heavily dust the bottom and sides of the furrow with granulated sugar and fresh, uncooked coffee grounds. Do not stir to incorporate into soil. Plant onions. This method works for onions, radishes plus other underground root vegetables prone to root maggots.

Thanks nandina. Any logic to the method that you've described?

Coffeehaus, I cannot give you a logical answer. Back in the dark ages when my Dad was editor of a well known gardening magazine the sugar/coffee idea was submitted to him by a local farmer. Dad mentioned it to some of his horticultural associates, all of whom gave it a try in their home gardens. Reports back indicated that this off-beat method is effective. I have no idea why. But, from this I developed my experiments using just granulated sugar (or dried molasses) to control root-knot nematodes when I moved south and discovered they were a serious problem here. I have not had to deal with onion maggots in years so cannot say if just sugar/dried molasses would control them without adding the coffee. You might want to trial it both ways.

Thanks so much for your response nandina. I'll try it the next time I plant.