Whitefly

What you are looking at

Brush past a tomato or a kale plant and a cloud of small white insects lifts off, circles briefly and settles again. That behaviour is diagnostic on its own. No aphid colony does it, and no scale insect does anything at all.

The adults are about two millimetres long, with four wings held roof-like over the body and dusted with a white waxy powder. They sit on the undersides of the youngest leaves near the growing point, so a plant can look perfectly clean from above.

Despite the name they are not flies. They belong with the aphids, scale insects and mealybugs, and like all of those they feed by pushing a stylet into the phloem and drinking sap.

The stage nobody looks at

Turn a leaf over and look past the adults. The eggs are laid in a neat arc or a complete circle, because the female pivots around her inserted mouthparts as she lays.

What hatches crawls for a short time, then settles, flattens, and becomes an oval translucent scale stuck to the leaf. It stays there through three more stages until it emerges as an adult. Those scales are most of the population, they are almost invisible, and they do not move.

That is why swatting, vacuuming and spraying the adults achieves so little. The plant carries a fortnight’s worth of replacements on the leaves you have just cleared.

Two species, two different problems

Glasshouse whitefly, Trialeurodes vaporariorum, is a tropical insect that cannot survive a British winter outdoors. It lives permanently under glass and indoors, breeds all year in a heated greenhouse, and is the one that ruins tomatoes, cucumbers, fuchsias and poinsettias. It is also notoriously resistant to insecticides.

Cabbage whitefly, Aleyrodes proletella, is a native outdoor insect that overwinters on living brassicas and does relatively little harm. It makes kale, sprouts and cabbage unpleasant to prepare rather than reducing the crop. Anyone told to spray a kale bed for this is being sold a solution to an inconvenience.

Treat the greenhouse one as a pest and the brassica one as a nuisance, and the whole subject becomes much more manageable.

What the damage amounts to

Sap loss slows a plant down, and heavy populations yellow the leaves and reduce cropping. That is the smaller half.

The larger half is honeydew. It falls onto the leaves and fruit below, black sooty mould grows on it within a couple of weeks, and the mould blocks light without infecting anything. On a greenhouse tomato it coats the trusses; on kale it turns the crop grey.

Glasshouse whitefly also transmits several plant viruses, which is a further argument for keeping numbers low rather than tolerating them.

What to do about them

Keep it out. Glasshouse whitefly arrives on bought plants, so quarantine new arrivals for three weeks and look under the leaves each week. Clear the greenhouse right out between crops, and overwinter pelargoniums and fuchsias somewhere other than beside next year’s young tomatoes.

Then use Encarsia formosa. A parasitoid wasp under a millimetre long, it has been used commercially against this insect since the 1920s and remains the best answer available to a gardener. It lays into the settled scales, and the parasitised ones turn black, so you can watch it work. Release it when you find the first few adults, repeat every fortnight for three releases, and keep the greenhouse above about 18 degrees C with reasonable light.

Hang yellow sticky traps to monitor rather than to control. A trap that fills faster this week than last is telling you the wasps need topping up.

Strip the lower leaves off tomatoes and brassicas as the season goes on, since they carry most of the settled young and do little for the plant. Bag them.

For cabbage whitefly, wash the undersides with a hose, clear old brassica stumps in spring, and rinse the crop before cooking.

Why no spray is listed here

Glasshouse whitefly has developed resistance to several classes of insecticide, and the settled scale stages are protected by a waxy covering that contact products struggle to wet. A spray therefore kills the adults, which are the easiest and least important part of the population, and leaves the scales to emerge next week.

It also kills Encarsia. Anyone who releases the wasps and then reaches for a bottle when the numbers do not fall in three days has spent money twice and undone the more effective of the two.

Fatty acid and plant oil products have the same coverage problem and the same effect on beneficial insects.

What it gets mistaken for

Fungus gnats are dark rather than white, they come up out of the compost rather than off the leaves, and they drift about the pot instead of settling back on the foliage.

Aphids sit still in dense clusters on shoot tips and do not fly up when disturbed, and the white flecks among them are shed skins rather than insects.

Scale insects are immobile domes on stems and leaf veins, and while the settled young of a whitefly do look similar, the adults flying about settle the question in a second.

White fluff in the leaf joints is mealybug, an animal under a coat of wax rather than an insect with wings.

Small white flakes that fall off and do not fly are usually the waxy scurf whitefly leave behind, or in a greenhouse, the fine debris from a shed leaf.