Whitefly in Vegetables: Identify, Keep Them Out, and Control by IRAC Group

Shake a tomato, chilli, aubergine, cucumber or cotton plant in a warm place, and a cloud of tiny white insects may fly up. Those are whiteflies.

A few do little harm. But they breed fast, they cover leaves with sticky honeydew and black sooty mould, and one species in particular spreads some of the most damaging plant viruses in the world, including tomato yellow leaf curl.

And whiteflies have become resistant to many insecticides.

In this video: how to recognise whiteflies and their life stages, the damage they do, how to scout, how to keep them out with clean seedlings, nets and crop breaks, the natural enemies that control them, and how to choose insecticides by IRAC group so that they keep working. The advice is general and global.

Your local extension service and product labels always come first.

Whiteflies are not flies.

They are small sap-sucking bugs, related to aphids, about one to two millimetres long, with four white, waxy wings. Two species matter most in vegetables.

The silverleaf or sweet potato whitefly, Bemisia tabaci, is found in warm regions and greenhouses worldwide.

It is small and yellowish, and holds its wings like a slightly open tent. The greenhouse whitefly, Trialeurodes vaporariorum, is a little larger and whiter, and holds its wings flat over its back. It prefers cooler conditions and greenhouses.

Their life stages are on the underside of the leaves.

The female lays tiny eggs there.

The first-stage nymph, called a crawler, walks a short distance, then settles and stays put. The later nymphs look like flat, oval, pale scales stuck to the leaf. The last stage, often called the pupa, develops red eyes before the adult emerges.

In warm weather the whole cycle takes about three to four weeks, so generations overlap quickly.

Whiteflies harm crops in three ways.

Adults and nymphs suck sap, weakening the plant.

Their honeydew makes leaves and fruit sticky,

and black sooty mould grows on it, reducing photosynthesis and spoiling produce.

And Bemisia tabaci transmits many viruses.

The most feared are the begomoviruses, such as tomato yellow leaf curl,

cotton leaf curl and cassava mosaic. Infected tomato plants are stunted, with small, upward-curling, yellow-edged leaves, and they set little fruit.

Both species also spread some criniviruses,

which cause yellowing of older leaves in tomato and cucurbits.

Feeding by Bemisia tabaci can also cause disorders of its own, such as silvering of squash leaves and uneven ripening of tomato fruit. Where viruses are present, a few whiteflies can matter more than thousands would without them.

Scout at least once a week, and more often in warm weather.

Yellow sticky cards, hung just above the crop, catch adults and show when they arrive and whether numbers are rising. Then turn over leaves: count adults on upper leaves, and look for eggs and nymphs on the middle and lower leaves. A hand lens helps a lot. Also look for parasitised nymphs, which turn black in the greenhouse whitefly and yellowish-brown in Bemisia, a sign that natural enemies are at work.

Action thresholds depend on the crop and especially on viruses.

Where tomato yellow leaf curl or a similar virus is common, the threshold is very low, and prevention matters more than any spray.

The best whitefly is the one that never reaches the crop.

Start with clean seedlings, raised under fine insect-proof netting, and inspect them before planting.

In greenhouses and net houses, insect-proof screens on vents and doors, with fine enough mesh to exclude Bemisia, and double doors, keep most whiteflies out.

UV-absorbing films can reduce their activity in some greenhouses.

In open fields, do not plant new crops next to old, infested ones, and remove and destroy crop residues as soon as harvest ends, because old plants are whitefly nurseries.

Control weeds that host whiteflies and viruses.

In regions where leaf curl viruses are a problem, a crop-free period of several weeks between seasons, agreed across a whole area, breaks the cycle of insects and virus.

And where available, plant varieties that tolerate or resist the local leaf curl virus.

Whiteflies have many natural enemies, and in greenhouses biological control is often the main method.

Tiny parasitic wasps lay their eggs in whitefly nymphs: Encarsia formosa is widely used against the greenhouse whitefly,

and Eretmocerus species against Bemisia.

Predatory bugs such as Macrolophus and Nesidiocoris eat eggs and nymphs, predatory mites such as Amblyseius swirskii feed on eggs and young nymphs, and lacewing larvae and some lady beetles help too.

In humid conditions, insect-killing fungi such as Beauveria bassiana can be applied as biological sprays.

In the open field, the same enemies are already present in many places.

Broad-spectrum insecticides kill them faster than they kill whiteflies, so avoid them, and choose selective products when you must spray.

If you need to spray, first match the product to the stage you are targeting, then choose by IRAC group.

Sprays must reach the underside of the leaves, where the nymphs are, so use enough water and good coverage.

Several groups are used against whiteflies.

Insect growth regulators act on eggs and nymphs: buprofezin in group sixteen, and pyriproxyfen in group seven C. Group twenty-three, the tetronic and tetramic acids, is especially active on nymphs. Group nine and group twenty-nine stop feeding.

Group four includes the neonicotinoids in four A, the sulfoximines in four C and the butenolides in four D, which act on the same receptor and should be treated as one family. Some diamides in group twenty-eight also have whitefly activity. Insecticidal soaps and horticultural oils act by contact.

Bemisia tabaci, especially some populations known as the Q or MED type, is resistant to many insecticides, including neonicotinoids.

So treat by generation window: use one group for about one whitefly generation, three to four weeks, then switch to a different group. Never use the same group on consecutive generations,

respect the label limits and the pre-harvest interval,

and check which products are registered in your country.

To recap.

Whiteflies live and breed on the underside of leaves, with crawlers, scale-like nymphs and pupae.

Bemisia tabaci holds its wings like a tent and spreads leaf curl viruses;

the greenhouse whitefly holds its wings flat.

Scout with yellow sticky cards and by turning leaves.

Keep them out with clean seedlings, insect-proof netting, prompt removal of old crops and weeds, and crop-free periods. Use parasitic wasps, predatory bugs and mites, and avoid broad-spectrum sprays.

When you spray, cover the undersides, choose by IRAC group, and switch groups every generation.

For more on vegetable pests, and on every other crop pest, visit laisha.ai. Thank you for watching.