You can buy traps to kill whiteflies through catalogs or from garden centers, but the sticky substance on commercial traps dries out quickly, and when they get covered with bugs, you have to throw them out.
I prefer to make reusable traps out of window glass (I use old panes of greenhouse glass, but any clear glass will do). I spray one side of the glass with bright yellow paint (whiteflies are attracted to yellow). After the paint dries, I spread a thin coat of motor oil on the other side. Then I set the piece of glass—oil side up—near infested plants. The whiteflies go for the yellow color and get stuck in the oil. Every month or so, I wipe the glass clean and apply a fresh coat of oil.