Brush against a tomato plant in a greenhouse and a cloud of tiny white insects lifts off the leaves. That is the usual introduction to glasshouse whitefly, a sap-feeding insect that breeds fast under glass and on houseplants. The Royal Horticultural Society (RHS) sets out how to recognise it, how it arrives and how it can be managed without pesticides. This article summarises the RHS page on glasshouse whitefly and its page on sap-feeding insects on indoor plants. It covers greenhouses and houseplants in the UK, reports RHS gardening advice rather than a legal standard, and is general information.
What glasshouse whitefly is
The RHS says whiteflies are sap-sucking true bugs in the family Aleyrodidae. Adults are typically white and fly up from host plants. About eight species are found in Britain, some limited to a narrow host range and others found indoors on many plants. Glasshouse whitefly (Trialeurodes vaporariorum) feeds on many greenhouse vegetables and ornamentals as well as houseplants, and the RHS names cucumber, melon, tomato, peppers, chrysanthemum, gerbera, pelargonium, fuchsia, lantana, poinsettia and verbena.
It thrives in warm conditions, which the RHS gives as the reason it is not usually a problem on outdoor plants, and it is active all year round on houseplants and in greenhouses. The RHS adds that outdoor plants can host it, especially in warm summers, but whiteflies on brassicas, Viburnum tinus, honeysuckle, evergreen azalea and rhododendron are other species specific to those plants.
How to recognise it
The RHS says whiteflies are relatively easy to see. When a plant is disturbed, clouds of small white-winged insects about 1.5 mm long fly up, which distinguishes whitefly from insects such as aphids. White objects on the top surface of leaves are more likely to be aphid skins. On the underside of leaves there may be flat, oval, creamy-white scale-like nymphs. If those nymphs are black, the RHS says they have probably been parasitised by an Encarsia wasp and are dead.
Adults and nymphs excrete sticky honeydew, which falls on foliage, stems and fruit and lets black sooty moulds grow. The RHS says it can be recognised as a black or brown powder on upper leaf surfaces and that sooty mould is usually a tell-tale sign of an insect infestation.
Why it builds up so quickly
The life cycle explains the speed. According to the RHS, adults and nymphs live on the undersides of leaves and feed on sap, weakening the plant. Females lay grey-white cylindrical eggs singly or in circles on leaf undersides, and each can lay more than 200 eggs. Males are rare and reproduction takes place without fertilisation. The eggs hatch into crawling nymphs that move around briefly before becoming fixed and feeding, and the final nymphal stage is the pupa, from which the adult emerges.
Temperature sets the pace: at 10 degrees C the life cycle takes several months, but it can be completed in about three weeks at 21 degrees C. In an unheated greenhouse the insect can stay active through winter if suitable host plants are present, whereas the RHS says it does not usually survive winter outdoors. Warm greenhouses can produce many generations in a season.
What the RHS advises
The RHS’s management steps are practical and non-chemical:
- Watch new purchases. The insect is often first brought into a glasshouse on new plants, so if possible new plants should be quarantined to give eggs and nymphs a chance to develop and be recognised.
- Remove unwanted plants from the glasshouse to cut the number of host plants.
- Provide good ventilation, which helps check the growth of sooty mould.
- Clean glasshouses in winter to reduce overwintering populations.
On indoor plants it refers readers to its indoor sap-feeder advice, which adds that manual removal is difficult for whitefly adults because of their mobility, and that the RHS does not support pesticide use.
Encarsia and other biological control
The RHS calls the introduction of tiny parasitoid wasps, Encarsia formosa, an established method of control. The wasps prey on the whitefly’s scale-like nymphs and can be bought by mail order from biological control suppliers. Two points matter for timing. First, the parasitoid must be introduced before plants are heavily affected, because it cannot give instant control. Second, parasitised nymphs turn black, so progress is easy to monitor. The RHS says other predators, such as the mite Amblyseius andersonii, which preys on eggs, are sometimes available in addition. The indoor page adds that all biological controls are susceptible to pesticides and so cannot be used with most pesticidal controls.
Yellow sticky traps
The RHS suggests hanging sticky yellow sheets above or among plants to trap adult whitefly, but it frames them as a way of monitoring whitefly activity rather than controlling it. It warns that sticky traps often catch non-target organisms, including the parasitoid wasps released as biological control, so their use should be considered carefully. On the indoor-plant page it says sticky traps are less likely to catch other insects inside the home, though they may not look attractive.
Whitefly versus other pests
The RHS page says white objects on the top of leaves are more likely to be aphid skins, and its aphid page notes that woolly aphids can be confused with scale insects, mealybug or whitefly. The site’s related RHS-based articles cover ants and aphids in the garden, glasshouse red spider mite and mealybugs and scale insects on houseplants.
The bottom line
According to the RHS, glasshouse whitefly is a warm-conditions pest that usually enters on new plants and multiplies fast in a heated greenhouse. Quarantining purchases, thinning surplus plants, ventilating and cleaning in winter are the first steps, and Encarsia formosa wasps are the established biological control if introduced early.
Sources
- Royal Horticultural Society, “Glasshouse Whitefly: Identification and Control” (rhs.org.uk)
- Royal Horticultural Society, “Indoor plants sap feeders: Identify” (rhs.org.uk)