White flies are drawn to a variety of plants, and herbs are no exception. These pests may infest herb plants regardless of where they are grown, whether indoors in pots or outside in beds. The small, white-colored insects pierce plants leaves and stems, sucking out the sap. They lay their eggs on the underside of the leaves, which is also where they prefer to feed. They spread disease and may also kill the plant or ruin the leaves, the part of the plant that many herb plants are grown for.
Inspect the underside of the herb plant's leaves at least once a week, particularly from mid- to late-summer when the white fly population is largest. Remove leaves if the underside feels like sandpaper and is covered in sap. The sandpaper texture is caused by the white fly eggs and nymphs, so dispose of them before the flies develop wings and infest other plants.
Combine 1 tsp. of a mild liquid dish soap with 1 gallon of water. Place the mixture in a spray bottle that has a mist setting.
Spray the underside of the leaves with the detergent solution. Allow the solution to dry on the plant for 12 hours. The soap penetrates the waxy coating on the white flies and causes them to die.
Spray the herbs every two to three days with the solution if no damage is caused by the first treatments. If the infestation persists and the plants are responding favorably to the detergent, gradually increase the mixture to 2 tbsp. per gallon of water.
Rinse the foliage with clear water 24 hours after each treatment. Rinsing prevents the detergent from accumulating on the leaves, which can cause damage over time.