Pest
Glasshouse whitefly
| Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Watch for | Watch for in March | Watch for in April | Watch for in May | Watch for in June | Watch for in July | Watch for in August | Watch for in September | Watch for in October |
What it is
Glasshouse whitefly is a sap-sucking bug about 1.5 mm long, with powdery white wings held flat over its body. It cannot survive British winters outdoors but lives year-round in greenhouses, conservatories and on houseplants, and moves outside onto nearby plants in summer.
Symptoms
Adults fly up in a small cloud when you disturb the plant and quickly settle again on the undersides of leaves. Leaves become sticky with honeydew and then blackened by sooty mould, which reduces vigour and spoils fruit. Flat, oval, pale green scales on the undersides of lower leaves are the immature stages. Heavy infestations cause yellowing and leaf drop. Cabbage whitefly looks very similar but lives outdoors on brassicas.
Cause
Females lay eggs in rings on young leaves near the tops of shoots. The eggs hatch into tiny crawlers that settle and feed as scales before emerging as winged adults. The cycle speeds up in warmth, so generations overlap and all stages are present at once by midsummer.
Control
Hang yellow sticky traps just above plants to catch adults and monitor numbers; shake plants gently to stir them up. The parasitic wasp Encarsia formosa is a reliable biological control once temperatures stay warm; parasitised scales turn black. The predatory bug Macrolophus is another option for tomatoes. Wipe or rinse sticky leaves to remove sooty mould. As a last resort, soap or plant-oil sprays reduce numbers; follow the label and avoid using them alongside biological controls.
Prevention
Check new plants before buying, looking under the leaves. Clear the greenhouse of plants and weeds for a few weeks in winter if you can, and clean it thoroughly. Grow French marigolds among tomatoes if you like, though evidence that they repel whitefly is limited, so do not rely on them.
Plants often affected
Could it be something else?
These problems share some of the same symptoms. Compare them, or run the symptom checker with these signs ticked.
Months to keep an eye out: March, April, May, June, July, August, September and October.
Sources
Identification and life-cycle details were checked against the references below. Text and advice are our own. See our editorial policy.