How to Deal With Whitefly on Geraniums and Pelargoniums

Whitefly are tiny, moth-like insects that congregate on the undersides of geranium and pelargonium leaves, feeding on sap and leaving behind sticky honeydew. When you brush or move the plant, they scatter in a white cloud. A light infestation causes little obvious damage, but a heavy one weakens the plant, covers the foliage in honeydew, and often leads to sooty mould growth on the sticky residue. They are also persistent — the immobile scale-like nymphs resist many sprays — so controlling them requires a multi-stage approach.

Identifying Whitefly

The adults are 1–2mm long, white, and fly when disturbed. More diagnostic are the nymphs — flat, oval, pale-green or white scales on the underside of leaves that look like tiny flat discs. You may also notice yellow stippling on the leaf surface caused by feeding damage, and a shiny, sticky coating of honeydew. Black sooty mould growing on the honeydew can obscure the leaf surface and reduce photosynthesis. The glasshouse whitefly (Trialeurodes vaporariorum) is most common on pelargoniums grown under cover.

Yellow Sticky Traps

Yellow sticky traps hung near affected plants capture flying adults continuously and reduce the reproducing adult population. They will not eliminate an infestation alone but are a valuable monitoring tool — a suddenly full trap tells you a new generation has emerged and treatment is needed. Position traps at plant height, just above the canopy, and replace them when full or dirty. In a greenhouse, two to three traps per five square metres give good coverage.

Insecticidal Sprays

Insecticidal soap, fatty acid, pyrethrum, and plant oil sprays kill adults and young nymphs on contact but do not penetrate the hard outer casing of older nymphs or pupae. This is why a single spray rarely works — surviving immature stages mature into new adults within days. Spray every five to seven days for at least three to four weeks to treat successive generations. Always spray the undersides of leaves where the insects are concentrated, in the early morning or evening to reduce evaporation and drift.

Biological Control

In a greenhouse or conservatory, the parasitic wasp Encarsia formosa is an excellent biological control for glasshouse whitefly. Introduced as parasitised scale cards hung near affected plants, the wasps search for and lay eggs inside whitefly nymphs, which then turn black as the parasite develops. Effective from spring through early autumn when temperatures are above 18°C. Do not use insecticides once Encarsia is established. The predatory bug Macrolophus is an alternative for higher-temperature environments.

Managing Honeydew and Sooty Mould

Once the whitefly population is under control, the sooty mould will gradually weather off or can be wiped away with a damp cloth. Improving airflow and reducing humidity helps too. A mild detergent solution (a few drops in a litre of water) wiped onto affected leaves loosens the mould and honeydew. Do not scrub — work gently. Once the plant is back to full health and producing vigorous new growth, the cosmetic damage will be quickly diluted by clean new leaves.