How to identify and deal with Whitefly

Whitefly

Trialeurodes vaporariorum & Aleyrodes proletella

also called Glasshouse whitefly, Cabbage whitefly

Tiny white moth-like insects that rise in clouds from leaf undersides, weakening plants and leaving honeydew. Hang yellow sticky traps and encourage or introduce parasitic wasps.

Insect Sap-sucker Serious SpringSummerAutumnWinter

How to know it

Moth-like insects 1–2 mm long with powdery-white wings, sitting under the leaves and flying up in a cloud when the plant is disturbed. The immature scales are flat, translucent-green oval discs fixed to the leaf underside and are what actually does the damage.

What you'll see

A cloud of small white insects lifts off the moment you brush the foliage.
Flat oval scales and white rings of eggs stud the undersides of the upper leaves.
Leaves are sticky with honeydew, then blacken with sooty mold that spoils fruit and blocks light.
Growth slows and leaves yellow and thin, worst on plants under glass.
On brassicas the flies rise from the outer leaves in winter, but the hearts stay fit to eat.

Where it comes from

Eggs are laid in rings on the newest leaf undersides; the crawlers settle within a day and spend the rest of their lives as fixed scales, so damage stays on the upper leaves while the empty cases work down the plant. Under glass it breeds all year with no dormant stage — the reason a greenhouse population never quite goes away — while the cabbage whitefly overwinters happily outdoors on brassica leaves.

A greenhouse or conservatory pest above all: warm, still, frost-free air lets generations overlap until the numbers are absurd. Outdoors it builds through late summer and collapses in the first hard frosts, except on brassicas.

Dealing with it

Hang yellow sticky traps at plant height — they catch the adults and, more usefully, tell you the numbers early.
Introduce Encarsia formosa, the parasitic wasp, as soon as the first flies appear under glass and above 18 °C; it turns the scales black and does the job for the season.
Vacuum the adults off in the cool of early morning, when they are sluggish and clustered.
Wipe or hose the honeydew off before sooty mold takes hold on it.
Spray only with insecticidal soap or plant oils, undersides included — nothing systemic reaches a fixed scale better than that, and hard chemicals kill the Encarsia.

Keeping it out

Quarantine every bought plant for a fortnight before it enters the greenhouse.
Clear the greenhouse of green material and old crops in winter to break the year-round cycle.
Grow a few French marigolds among tomatoes under glass — the scent genuinely confuses incoming flies.
Ventilate and space plants so the air moves; still humid corners are where colonies build.
Strip and compost the outer leaves of overwintering brassicas rather than leaving a nursery standing.

What it goes for

LeavesShoots & stemsFruit & pods

Tomatoes, cucumbers, aubergines, peppers, fuchsias and most soft greenhouse plants; outdoors, the cabbage whitefly lives on kale, cabbage, sprouts and other brassicas.

and 14 more in the catalog

Trouble it brings with it

Who hunts it for you

Big-eyed bugGeocoris spp. A stubby 3 mm bug with enormous wrap-around eyes, working the soil surface and the low foliage for aphids, mites, thrips and insect eggs. It is the main natural check on chinch bugs in a lawn and it is killed outright by insecticide. Grow to recruit it: Calendula,Clover,Cosmos,Goldenrod,Sweet alyssum
Dragonflies & damselfliesOdonata Aerial hunters that take mosquitoes, midges and small flies on the wing all day, and whose larvae spend a year or more clearing the same insects from the water. A pond with planted margins and no fish is the way to have them. Grow to recruit it: Bulrush,Iris,Nymphaea,Pickerel Weed
Green lacewingChrysoperla carnea Its bristly larvae — "aphid lions" — impale aphids, mealybugs, thrips, and mite colonies, eating hundreds a week. Adults feed on nectar and pollen from small open flowers and overwinter in hollow stems left standing. Grow to recruit it: Cilantro,Cosmos,Dill,Sunflower,Yarrow
Ladybird (lady beetle)Coccinellidae The gardener’s favorite ally: adults and their alligator-shaped larvae each eat thousands of aphids, plus scale, mealybugs, and other soft-bodied sap-suckers. Grow flat, open flowers for the adults and never spray a colony that ladybird larvae are already working. Grow to recruit it: Calendula,Cilantro,Cosmos,Dill,Fennel,Yarrow
Minute pirate bugOrius spp. A pinhead-sized true bug that is the best natural check on thrips, and happily spears aphids, mites, and insect eggs too. It needs pollen when prey is scarce, so keep something flowering nearby. Grow to recruit it: Cosmos,Fennel,French marigold,Sweet alyssum
Parasitic waspsBraconidae & Trichogramma Tiny, stingless wasps that lay eggs in or on caterpillars, aphids, and moth eggs — the white cocoons riding a hornworm are their calling card, so leave that caterpillar be. Umbel flowers with shallow nectar keep the adults fuelled. Grow to recruit it: Cilantro,Dill,Fennel,Lovage,Parsley,Sweet alyssum
SpidersAraneae Webs, ambushes, and night patrols make spiders the garden’s steadiest generalist predators, taking whatever flies or crawls past. Mulch, ground cover, and undisturbed corners keep the population dense. Grow to recruit it: Lavender,Thyme