Whitefly on My Greenhouse Melons — What Can I Do?

Glasshouse whitefly (Trialeurodes vaporariorum) is one of the most persistent pests of cucurbits grown under glass, and cantaloupe melons are commonly affected. The tiny white-winged adults rise in clouds when you disturb the plants, while the flat, scale-like nymphs feed unseen on the undersides of leaves, secreting sticky honeydew that turns black with sooty mould. Heavy infestations weaken the plant and make harvested fruit unpleasant to handle. In warm, enclosed environments, populations can build rapidly without effective intervention.

Identifying glasshouse whitefly

Adults are about 1.5 mm long, white-winged, and rise in small clouds when the plant is touched. The immature stages are flat, oval, and transparent to pale yellow — found on the undersides of leaves. Yellowing leaves, a sticky surface, and black sooty mould growth on the upper leaf surfaces are the signs of a developing infestation. Unlike aphids, whitefly colonies build up on the undersides of leaves and are not immediately obvious unless you check regularly.

Biological control

Encarsia formosa — a tiny parasitic wasp — lays its eggs inside whitefly nymphs, which then turn black and die rather than completing development. The wasps are harmless to humans, other wildlife, and the plants. Order them from a biological control supplier and introduce them as soon as whitefly is first seen — the earlier the introduction, the more effective. Encarsia requires temperatures above 18°C to be effective, which matches the conditions needed for melon growing.

Chemical and physical control options

Yellow sticky traps catch adult whitefly and provide an early warning of developing infestations. Insecticidal soap spray applied to the undersides of leaves kills nymphs on contact and needs to be repeated every five to seven days. Avoid using any insecticides that are harmful to Encarsia if you are using biological control — the two approaches are incompatible. Good greenhouse hygiene at the end of the season — removing all plant debris and washing down surfaces — reduces the overwintering population significantly.