Tiny White Flies When You Touch the Plant — Whitefly and How to Control It

Photo: 乌拉跨氪, CC BY-SA 3.0, via Wikimedia Commons

If brushing a plant sends up a small cloud of white insects that settle again within seconds, that is whitefly. They are related to aphids and scale rather than to true flies, and like both they feed on sap and excrete honeydew.

Confirming it

Turn a leaf over. Adults sit on the undersides and scatter when disturbed; the immature stages look like tiny flat translucent scales stuck to the leaf and do not move at all. Those static stages are the reason a plant that "looked clear yesterday" is covered again today.

Why they are hard to shift

Only the adults fly, and only the adults are hit by most sprays. The eggs and the fixed nymph stages sit protected on leaf undersides, so a treatment that looks like a total success is followed by a new generation four to five days later.

Control

1. Yellow sticky traps, immediately. They intercept adults before egg-laying and give you a population count — falling catches mean you are winning.
2. Vacuum the adults in the morning while they are sluggish. It sounds absurd and it is remarkably effective on a bad infestation.
3. Insecticidal soap or oil on the leaf undersides, every four to five days for three weeks, to match the hatch interval.
4. Remove the worst-encrusted lower leaves rather than treating them repeatedly.
5. In a greenhouse, consider Encarsia formosa, a parasitic wasp that handles whitefly better than any spray and is the standard commercial answer.

Prevention

Whitefly arrives on new plants and thrives in warm, sheltered, still conditions. Inspect undersides at purchase, avoid over-feeding nitrogen (soft growth suits them), and keep traps up permanently in a greenhouse so you find the first arrivals rather than the hundredth.

Not sure that's what you have?

Symptoms overlap, and the wrong treatment wastes weeks. Photograph the affected leaf with Sherleaf and you get the diagnosis, a severity rating and a treatment plan in about ten seconds.