The best insecticides to use for whiteflies are insecticidal soap or insecticidal oil, such as neem or narrow-range oil. However, these methods only have a limited effect on eliminating whiteflies, states University of California Agriculture and Natural Resources. Horticultural oils also work well to smother whiteflies.
To make sure that the insecticide use is effective, it is important for gardeners to apply it to the tops and undersides of all leaves, especially ones at the lower parts of plants. The best time to use an insecticidal soap is when the plants are watered well and the temperature is 80 F or below. Yellow sticky traps are also effective at catching adult whiteflies.