I used regular Dawn Dish Soap and water to spray down my orange tree when it was infested with aphids. Worked like a charm. Just be sure not to spray down your plants on a hot sunny day! Always do it either in the early morning or the late evening when the sun isn't shining so brightly. I've killed a couple plants that way...
Thanks for the warning. My bonsai trees are in an area that gets shade in the afternoon, so I presume that once the sun has moved away from that area, it's safe to spray them.