Advice
RHS Help & Advice
Glasshouse whitefly (Trialeurodes vaporariorum)
Plants affected
Many vegetables and ornamental plants in glasshouses and houseplants. Outdoor plants can also be attacked but not to such a damaging degree. Note that whiteflies seen on brassicas, Viburnum tinus, evergreen azalea and rhododendron are other species of whitefly specific to those plants.
Symptoms
Clouds of small white-winged insects, 1.5mm long, fly up when a plant is brushed against in a glasshouse. The foliage becomes sticky and coated with a black sooty mould. Scale-like nymphs are also likely to be present on the underside of leaves.
Cause
This is a sap-sucking insect that breeds rapidly in warm greenhouse conditions. The adults and their whitish-green, scale-like nymphs feed on sap from the undersides of the leaves, weakening the plants. Their sugary excrement (honeydew) makes the foliage sticky and allows the growth of black sooty moulds.
Non-chemical control
Hang sticky yellow sheets (Agralan Yellow Sticky Traps, Bayer Greenhouse Fly Catcher, Growing Success Greenhouse Whitefly Traps) above or among the plants to trap adult whitefly.
Biological control
Because of this pest's rapid reproductive rate and the widespread occurrence of pesticide-resistant strains, biological control often gives better results than insecticides on glasshouse plants during April to October.
This involves introducing tiny parasitic wasps, Encarsia formosa, which attack the nymphs. The parasite is available by mail order from the suppliers of biological controls. It is important to introduce the parasites before plants are heavily infested as they cannot give instant control. Parasitised nymphs turn black so it is easy to monitor the progress of the control. As Encarsia is killed by most insecticides, avoid spraying with products other than fatty acids, plant extracts or plant oils (see below) prior to its introduction.
Chemical control
Frequent sprays with bifenthrin (Scotts Bug Clear Gun, Bayer Sprayday Greenfly Killer Plus, Doff All-In-One Garden Pest Killer), plant extracts (Growing Success Fruit & Veg Bug Killer, Growing Success Whitefly Killer or Agralan Whitefly Killer), plant oils (Vitax 2 in 1 Organic Pest and Disease Control, Scotts Bug Clear for Fruit & Veg), fatty acids (Doff Greenfly and Blackfly Killer, Bayer Organic Pest Control or Greenfingers Organic Pest Spray) or mineral lattice (SB Plant Invigorator) are required to control established infestations. Resistance to bifenthrin can occur.
There are several systemic insecticides that be applied as foliar sprays or compost drenches. Imidacloprid is available as a concentrate foliage spray (Provado Ultimate Bug Killer); this can be used on ornamental plants and glasshouse tomatoes, peppers and aubergines. Bayer Provado Vine Weevil Killer 2 is a compost-applied systemic insecticide containing thiacloprid for application to the roots of ornamental plants growing in pots or containers only. A ready-diluted spray containing thiacloprid (Provado Ultimate Bug Killer Ready To Use) can be used on glasshouse-grown tomatoes, peppers, aubergine and cucumber, as well as ornamental plants. Acetamiprid can be applied as a compost drench (Scotts Bug Clear Ultra Vine Weevil Killer) on container-grown plants or as a foliar spray (Scotts Bug Clear Ultra or Bug Clear Ultra Gun) on ornamental plants only. Thiamethoxam (Westland Bug Attack) is a foliar spray for use on container-grown ornamental plants. Systemic insecticides are absorbed into the plants and are taken up by sap-sucking insects when they feed.