What Causes White Flies on Herbs?

Identification

• Herbs with tiny, powder covered flies scattered on the undersides of leaves are infested with white flies. These flies are often referred to as "plant dandruff" because they fly up when plants are disturbed, scatter about and then settle back onto host plants. Warm, sunny climates are favorable for whitefly reproduction and development. White flies have a wide host range of plants, but prefer herbs, vegetable plants and shrubs.

Life Cycle

• Adult white flies are gnat-like flies that lay their eggs on the leaves of host plants. Within three to seven days, these eggs hatch and nymphs emerge which are responsible for the majority of damage to host plants. White fly nymphs are translucent and do not move around the plant a great deal. Nymphs gather underneath the leaves of host plants, where they feed for two to four weeks before pupating into adult flies. When winter weather is mild, white flies typically lay eggs for eight weeks.

Signs And Symptoms

• Herbs infested with white flies may have specks of white or yellow on the foliage, which is from the removal of plant sap. Infested herbs may drop leaves prematurely and experience reduced vigor. When white flies remove plant sap from host plants, they produce an excrement known as honeydew. This sticky liquid lands on plant leaves, where it attracts sooty mold fungi. Herbs with sooty mold have an unsightly black fungal crust all over the plant. Sooty mold fungi does not kill plants, but it can cause reduced vigor when fungal crusts are thick and block out sunlight.

Management

• Hanging yellow sticky traps near your infested herbs can help reduce infestations. White flies are drawn to the yellow color, where they become trapped. Discard traps when they are full and replace them. Light infestations of white flies may be removed by hand and crushed. Many natural predators live in your garden and control whiteflies. Green lacewigs and lady beetles are beneficial insects that reduce white fly populations. If beneficial insects are present in your garden, avoid applying insecticides because these chemicals kill beneficial insects as well as white flies. Insecticidal soaps and horticultural oils are soft-insecticides that can be used when white flies are causing significant damage to your herbs. These products must thoroughly coat plant leaves to be effective, so be sure to spray the undersides of plant leaves where white fly nymphs gather to feed.