If desperate put 4 or 5 cloves of garlic, lightly squashed and a good half-bucketful of nettles into a bucket, 3/4 fill with water, stir every time you pass it for a day or two and, using an old emulsion or wallpaper paste brush, swoosh the plants with it (it won't go through a sprayer!)
I don't know whether this will harm bees or hoverflies so have only ever used it in the late evenings. It is very effective!!