Whiteflies are members of the order Hemiptera, a group of insects classed together because of the similarity of their sucking mouthparts, with which they feed on the juices of plants. They are closely related to aphids. There are over 1,500 species of whitefly, but two are a particular nuisance in North American greenhouses and gardens, the glasshouse whitefly (Trialeurodes vaporariorum), and the tobacco whitefly (Bemisia tabaci).
Adult whiteflies look like extremely tiny, bright white moths, and are usually seen after they have been disturbed and take to the air. They infest the tops of plants, their growing tips, and the undersides of leaves, where they suck nutrients from the plants’ tissue.
Whiteflies lay their eggs on the undersides of leaves, and this is where the larvae develop and begin to feed on the plants. The pupae (cocoons) are usually found on the undersides of older leaves or lower on the plant.
Both the adults and larvae suck nutrients that the plants need to develop properly. This results in stunting and poor productivity. The larvae excrete honeydew as they feed, which becomes home to sooty moulds, which then infect the rest of the plant. Whiteflies also transmit viruses from plant to plant.
Control
Ladybugs and green lacewings are voracious predators of whitefly eggs and larvae. A number of predatory wasp species feed on whiteflies in all their life stages. Calendula and marigolds both produce chemicals that repel whitefly, and can be planted densely as companion plants. Nasturtiums and basil are thought to have a similar effect, and mint does well as a trap crop. Citronella can be sprayed on the tops of unaffected plants to repel adults from landing.
Whiteflies rapidly develop resistance to pesticides, so Agriculture Canada and the USDA both recommend prevention and biological controls. Destroy infested plants in the garbage, not the compost.