You reach in to deadhead a rose and a tiny white insect flies out from under a leaf. On the underside of that leaf is a strange white spiral. You have discovered whiteflies.
Whiteflies are not related to flies at all. Instead, they are close cousins to aphids, mealy bugs, soft scale and armored scale.
Whiteflies got their name because most whitefly species look like tiny white flies. Adult bodies are actually yellow, but it’s hard to see. They have 4 white wings and are often no bigger than 1 mm. Some whitefly species are black or yellow. You can see a chart of the most common whiteflies found in California at the UC IPM Pest Note on whiteflies.
Whiteflies are quick and very difficult to see. It is much easier to tell if a garden is infested with whiteflies by noticing white spirals on the underside of leaves. These spirals are whitefly eggs. The adult walks a spiral path, laying eggs in a waxy trail that holds the eggs to the leaf. In warmer parts of California, whiteflies can breed year-round. Whiteflies go through four instars. Eggs hatch out nymphs (crawlers). The next three stages are nearly immobile, much like scale insects. The final instar is a pupal stage. At each stage, these pests are sucking plant juices, spreading disease, and leaving a behind drops of honeydew. Whitefly larval feeding can distort leaves and cause significant losses in some vegetable crops.
Whiteflies can reproduce at mind-boggling rates. They use piercing mouthparts to suck the sap from the phloem of a wide variety of garden edibles and ornamentals. Heavy infestations can cause leaf drop. The honeydew (sugary poop) left behind is host to sooty mold. Honeydew also attracts ants, which then protect whiteflies from their natural enemies. Some whitefly larva may transmit viral diseases and adults can also carry pathogens. Low whitefly populations are not a significant problem, but they can make citrus, pomegranate and avocado trees look shabby.
How to control whiteflies
Natural predators are the best defense against whitefly. Very often, population explosions are caused when gardeners apply broad-spectrum pesticides that kill off predators, or when ants are allowed free access to host trees. Ants can be prevented from protecting whiteflies by applying sticky barriers around the trunks of trees and shrubs. Dusty conditions can also put the odds in favor of whiteflies. Hosing off dusty plants can help reduce whitefly populations.
Heavy whitefly populations are very difficult to control. Any leaves with whitefly eggs should be removed and discarded. If the leaf looks particularly healthy, you can simply rub your thumb over the eggs to destroy them. Reflective mulches repel adult whiteflies, aphids, and leaf hoppers. Also, yellow sticky traps can be used to trap and monitor whitefly populations.
According to companion planting lore, whiteflies can be discouraged from an area by planting basil, mint, thyme, and nasturtium. Whitefly predators, such as parasitic wasps and flies, and hummingbirds, can be attracted to an area by planting zinnias, bee balm, hummingbird bush, and pineapple sage.
I hope this information inspires you to grow more of your own food. You can ask your garden questions on my Home page.