How To Deal With Whitefly in Your Irish Garden
What is Whitefly?
Whitefly are tiny white-winged insects that infest greenhouse and conservatory plants throughout Ireland. The glasshouse whitefly (Trialeurodes vaporariorum) is the most common species in Irish gardens, though cabbage whitefly (Aleyrodes proletella) attacks outdoor brassicas. Adults are about 2mm long with powdery white wings and rise in clouds when disturbed plants are touched. They cluster on the undersides of leaves where they lay eggs and feed on plant sap. Whitefly have a complex life cycle with eggs, scale-like nymphs and adults all present on infested plants simultaneously. They reproduce rapidly in warm greenhouse conditions and populations can become enormous if unchecked. Like aphids, whitefly excrete sticky honeydew that becomes coated with unsightly black sooty mould.
Damage Caused by Whitefly
Whitefly weaken plants by sucking sap from the undersides of leaves, causing yellowing, wilting and poor growth. Heavy infestations can significantly reduce the vigour and yield of greenhouse crops including tomatoes, cucumbers, peppers and aubergines. The honeydew they excrete attracts sooty mould, which blocks light reaching leaf surfaces and further reduces plant health. Ornamental plants become disfigured and sticky. In severe cases, whitefly-damaged plants drop their leaves prematurely. Brassica whitefly infests Brussels sprouts, cabbage and kale outdoors, contaminating harvested crops.
How to Prevent Whitefly
Maintain good greenhouse hygiene — clear away all old plant material at the end of the season and wash down the greenhouse interior with disinfectant. Use fine mesh screens on greenhouse vents and doors to prevent whitefly entering from outside. Hang yellow sticky traps above plant canopy level from early in the season to monitor and catch adult whitefly. Avoid overcrowding plants, which creates the warm, sheltered conditions whitefly prefer. Companion planting with French marigolds and basil may help deter whitefly from greenhouse tomatoes.
How to Treat Whitefly
Introduce the parasitic wasp Encarsia formosa as a biological control in greenhouses from April onwards — it is highly effective against glasshouse whitefly. Spray with organic insecticidal soap, ensuring thorough coverage of leaf undersides where eggs and nymphs are found. Repeat sprays every 5-7 days for three to four applications to catch successive generations. Yellow sticky traps reduce adult populations. For outdoor brassica whitefly, a strong jet of water dislodges adults and nymphs. Organic neem oil sprays also provide some control.
Whitefly in Ireland
Whitefly is primarily a greenhouse pest in Ireland, as the outdoor climate is generally too cool and wet for glasshouse whitefly to establish outside. However, cabbage whitefly is well-adapted to Irish conditions and is common on outdoor brassicas throughout the country, particularly from July onwards. The increasing popularity of polytunnel growing in Ireland has expanded the habitats available to glasshouse whitefly. Irish gardeners growing tomatoes, peppers and cucumbers under cover should be vigilant from May onwards when rising temperatures allow rapid whitefly reproduction. Conservatory plants brought indoors for winter often harbour whitefly populations that persist year-round.
Plants Affected by Whitefly
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