Thrips feeding on plants leaves causes white patches or streaks, but the more serious problem is that thrips can carry and transmit viruses such as Tomato Spotted Wilt Virus.
Use yellow or blue sticky traps to monitor thrips populations, especially in greenhouses. Outdoors, thrips have numerous natural enemies that are attracted by planting plenty of flowers. Avoid over-fertilizing your plants, because heavy feeding increases the risk of damage from thrips. In warm climates where outbreaks of Tomato Spotted Wilt Virus are common, use reflective mulches to help hide your plants from passing thrips, and grow resistant varieties.
Place a light-colored cloth beneath infested plants and shake individual branches to dislodge many thrips. Insecticidal soaps or oils can be used to control thrips, but spraying is seldom necessary in the open garden, where thrips face numerous natural enemies.