Whitefly: tiny white insects that lift off the leaves when a plant is touched

Check your plant with the questions

Brush against an infested plant and a small cloud of white specks rises from it. Those specks are adult whiteflies, and the flying adults are the least of it. Their young sit flat and motionless beneath the leaves, sucking sap; they do most of the harm, and they are what any treatment has to reach. The species met most often indoors and in greenhouses is the greenhouse whitefly, Trialeurodes vaporariorum. In British guides it goes by glasshouse whitefly.

Adults and nymphs

Whiteflies are not flies. They are sap-sucking bugs, relatives of mealybugs, aphids and scale insects, and their name comes from the white, mealy wax dusting the body and all four wings. An adult is about a sixteenth of an inch long, with a yellowish body under the wax.

The nymphs are harder to see and more telling. Tip a leaf toward the light and look along the underside for pale, flattened ovals, whitish-green and barely more than a millimeter long when full grown. They have no visible legs and do not move, so they are easy to take for a light crop of young scale. Scattered among them you may find the eggs, grayish white and cylindrical, laid one by one or arranged in small circles.

How numbers climb so fast

Warmth drives everything. A female can lay upward of two hundred eggs, and males are scarce enough that she needs none to reproduce. What hatches is a mobile crawler that wanders for a few hours, then settles, sinks its mouthparts into the leaf and stays put through the remaining nymphal stages. The last of these is sometimes called a pupa, and the adult splits it open along the back to climb out, leaving an empty case with a T-shaped opening.

In a cool spot a full generation can take several months; in a warm room or greenhouse it may take about three weeks. Indoors there is no winter break, so eggs, nymphs and adults overlap all year, and a problem that looked minor in one month can be dense by the next. Outdoors, in colder climates, this species does not usually survive the winter.

Honeydew, sooty mold and other damage

Every stage feeds on the sugary sap in the leaf veins and passes the surplus as honeydew. It falls on leaves, stems and fruit lower down, which turn sticky and then dark as sooty mold spreads over them, coating the upper faces with what looks like dark brown or black dust. The mold is a fungus living on that sugar, not a disease of the plant itself. Fresh air moving around the plant slows the mold down.

Heavy feeding leaves foliage yellow and dry, and those leaves may drop. Adults alone seldom cause serious harm unless they are carrying a plant virus, which is mostly a concern in vegetable crops; a light population on a houseplant is usually tolerable. Real losses come when nymph numbers are high. Honeydew also draws ants, which fend off the predators and parasites that would otherwise keep whiteflies in check.

Look-alikes

• Fungus gnats are dark, slender flies that hang around the surface of damp potting mix. Whiteflies are white and come off the foliage.
• Aphid skins. White flecks resting on the top of a leaf tend to be the cast skins of aphids feeding higher up than whitefly, which gathers underneath.
• Scale. Whitefly nymphs and young scale look very alike. Adults taking flight from the same leaves settle the question.

Bringing numbers down

Heavy infestations are hard to clear, so act on the first few adults you see.

• Check before you buy, and wait before you mix. Whitefly often arrives on a new plant. Keeping a newcomer apart for a spell gives any eggs and nymphs time to hatch and show themselves.
• Pull off the worst leaves. Removing leaves thick with nymphs early on can hold a population back. When a whole plant is heavily infested, discarding it may be the sensible choice.
• Rinse the plant. A strong spray of water knocks adults off the foliage.
• Hang yellow sticky cards over the pots or between them. They are better at showing whether whitefly is present, and whether numbers are going up, than at controlling it. In a home they catch few other insects; in a greenhouse they also trap the beneficial wasps.
• Spray soap or oil thoroughly. Insecticidal soap, neem oil and other plant oils kill only the insects the spray actually touches, so soak the undersides of every affected leaf, including the low ones that are hardest to reach, and repeat. Avoid spraying a plant that is short of water or sitting in direct sun.
• Go easy on stronger insecticides. Whiteflies quickly become resistant to many of them, and most do not work well at home. Imidacloprid applied to the soil can control nymphs, but it harms pollinators and can set off a spider mite outbreak.
• In a greenhouse, use the parasitic wasp Encarsia formosa. Release it before the plants are badly affected; it cannot act quickly. Other options include a predatory mite that eats whitefly eggs plus a predatory bug, Macrolophus pygmaeus.

Signs the tide has turned

Nymphs that have turned black under a leaf have been parasitized by Encarsia and are dead. Empty cases with a round or oval hole also mean a parasite came out, where a T-shaped slit means a whitefly did. A falling count of adults on the sticky cards from one week to the next shows the population is shrinking.

Plants whose sources name whitefly

This page draws its plant list from the data behind the site, taking in every plant whose NC State Extension or RHS profile names whitefly as a pest.

• African Hemp RHS
• Allamanda RHS
• Angel's Trumpet RHS
• Areca palm NC State Extension
• Avocado RHS
• Banana Vine RHS
• Barberton daisy RHS
• Black-eyed Susan Vine RHS
• Blood Leaf RHS
• Blue passion flower RHS
• Bougainvillea RHS
• Brazil Raintree RHS
• Bristly Copperleaf RHS
• Calamondin NC State Extension
• Cape leadwort RHS
• Chinese hibiscus RHS
• Coleus RHS
• Common Hoya RHS
• Cooperleaf RHS
• Crape myrtle RHS
• Dumb cane NC State Extension
• Elephant ear RHS
• Emerald Ripple Peperomia NC State Extension
• Eternal Flame Plant RHS
• Furry Feather Calathea RHS
• Gardenia RHS
• Goeppertia concinna RHS
• Goeppertia orbifolia RHS
• Goeppertia picturata RHS
• Grape ivy RHS
• Hardy fuchsia NC State Extension
• Hoya gracilis RHS
• Indian azalea RHS
• Ivy Geranium RHS
• Jungle Rose RHS
• Kangaroo Vine RHS
• Lantana camara RHS
• Madagascar periwinkle RHS
• Majestic Prayer Plant RHS
• Mosaic Calathea RHS
• Nasturtium RHS
• Peacock Plant RHS
• Pink Silver RHS
• Pinstripe Calathea RHS
• Poinsettia RHS
• Polka dot plant NC State Extension
• Prayer Plant RHS
• Purple Waffle Plant NC State Extension
• Shrimpplant RHS
• String of pearls RHS

If the flies are dark and rise from the potting mix, read the fungus gnats page instead. For anything else, start a diagnosis.

Answer a few questions about your plant