Whiteflies can kill otherwise healthy plants. They dwell primarily on plant leaves, but the eggs can also infect the soil. Once the eggs hatch, the larvae feed on plant material sitting on the soil's surface until they grow wings and move into the foliage of the plant. Control of these pests is difficult, and a small population can quickly grow to deadly proportions.
Whiteflies resemble small white grains when feeding. When flying, they resemble pure white moths. The insects are small and difficult to see, except when they are feeding in clusters. The flies typically feed on the underside of the leaves or along the stems, so you may not notice them unless you turn the plant leaves over. Whiteflies spend most of their lives on the plant, but larvae may end up in potting soil, so a plant may look healthy but harbour a future infestation. Reusing infected soil spreads the whiteflies to other plants.
The insects feed by sucking sap from plant leaves. Their feeding weakens the plant, so leaves eventually yellow and die. The flies exude a sticky honeydew as they feed. Mildews and moulds grow on the honeydew, further weakening the plant. Severe infestations can kill a plant quickly. They can quickly infest every plant in your home or in a greenhouse. Natural predators typically prevent whiteflies from posing a problem in outdoor beds. Whiteflies can also carry plant diseases, which they then transfer to the plant as they feed.
Insecticides cannot adequately control severe whitefly infestations. Sticky traps, available from garden centres, attract and kill the mature whiteflies. Replace the traps every two days until no new whiteflies are found on the plants. This may take two weeks or longer, depending on the extent of the infestation, since new whiteflies continue to hatch from any successful eggs laid on the plants or in the soil. Predator insects, such as lady beetles and bigeyed bugs, feed on the whiteflies and help control pest populations, but they aren't suitable for indoor use. Removing badly infested foliage also helps control whiteflies.
Preventing an infestation from occurring is the best practice for whitefly management. Place a reflective mulch over the soil in plants to deter whiteflies from accessing the soil and from settling on the plant leaves. Quarantine new plants from existing plants for two to three weeks so they do not introduce whitefly populations to your existing healthy plants. If whiteflies are suspected, repot the plant into new, sterile potting soil, and monitor it for two weeks or more for any whitefly activity before bringing it into your home or greenhouse.