Glasshouse whitefly

Glasshouse whitefly is a common sap-feeding insect, mainly of house-plants and greenhouse plants. They excrete a sticky substance (honeydew) on foliage, which allows the growth of sooty moulds.

Glasshouse whitefly. Image: ©www.gardenworldimages.com

Quick facts

Common name: Glasshouse whitefly
Scientific name: Trialeurodes vaporariorum
Plants affected: Many house-plants and greenhouse plants
Main symptoms: Sticky honeydew on foliage, black sooty moulds, small white-winged insects
Most active: All year round

What is glasshouse whitefly?

Glasshouse whitefly is a sap-sucking true bug that can reduces the vigour of plants and excretes a sticky, sugary substance, called honeydew, on the leaves, stems and fruits of its host plants.

It can feed on many vegetables and ornamental plants grown in greenhouses as well as house-plants. These include: cucumber, melon, tomato, peppers, Chrysanthemum, Gerbera, Pelargonium, Fuchsia, Lantana, poinsettia and Verbena. Outdoor plants can also be attacked but not to such a damaging degree. Note that whiteflies seen on brassicas, Viburnum tinus, honeysuckle, evergreen azalea and rhododendron are other species of whitefly specific to those plants.

Glasshouse whitefly thrives in warm conditions, which is why it is not usually a problem on outdoor plants. This insect is active all year round on houseplants and in greenhouses.

Symptoms

  • It is relatively easy to see whiteflies on infested plants. When a plant is disturbed clouds of small white-winged insects, 1.5mm (about 1/16in) long, will fly up this distinguished infestation from other insect such as aphids, white objects on the top surface of leaves are more likely to be aphid skins than whitefly. 
  • You may also see flat, oval, creamy white scale-like nymphs on the underside of leaves
  • Adult whitefly and the nymphs excrete sticky honeydew on the foliage, stems and fruits, which allows the growth of black sooty moulds

Control

Check susceptible plants frequently from spring onwards so action can be taken before a damaging infestation has developed. When choosing control options you can minimise harm to non-target animals by starting with the methods in the non-pesticide control section. If this is not sufficient to reduce the damage to acceptable levels then you may choose to use pesticides. Within this group the shorter persistence pesticides (that are usually certified for organic growing) are likely to be less damaging to non-target wildlife than those with longer persistence and/or systemic action.

Due to this insects rapid reproductive rate and the widespread occurrence of pesticide-resistant strains, biological control often gives better results than insecticides on greenhouse plants.

Non-pesticide control

Biological control

This involves introducing tiny parasitoid wasps, Encarsia formosa, which prey on the whitefly scale nymphs. The parasitoid is available by mail order from the Biological control suppliers . It is important to introduce the parasitoid before plants are heavily infested as it cannot give instant control. Parasitised whitefly 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 invigorators, plant extracts or plant oils (see below) prior to its introduction.

Other non-pesticide controls

  • Hang sticky yellow sheets (widely available from garden suppliers) above or among the plants to trap adult whitefly, this can help monitor whitefly activity rather than give control
  • Glasshouse whitefly can feed and breed on weeds so good weed control inside and around the glasshouse will remove alternate host plants
  • Watch for signs of whitefly on new purchases as the insect is often first brought into a glasshouse on new plants. If possible quarantine new plants in order to give eggs and nymphs a chance to develop and be recognised
  • Good ventilation will help to check the growth of sooty moulds
  • Cleaning glasshouses in winter can help reduce overwintering populations

Pesticide control

Glasshouse whitefly often requires repeated treatment to control infestations
  • Organic sprays, such as natural pyrethrum (e.g. Bug Clear Gun for Fruit & Veg, Neudorff Bug Free Bug and Larvae Killer), fatty acids (e.g. Solabiol Bug Free, Doff Greenfly & Blackfly Killer) or plant oils (e.g. Vitax Plant Guard Pest & Disease Control, Bug Clear for Fruit and Veg) can give good control of whitefly. These pesticides have a very short persistence and so may require reapplication to keep whitefly numbers in check. Plant oil and fatty acid products are less likely to affect larger insects such as ladybird adults. 
  • Plant invigorators combine nutrients to stimulate plant growth with surfactants or fatty acids that have a physical mode of action against aphids (e.g. Ecofective Bug Control, RHS Bug and Mildew Control, SB Plant Invigorator and Westland Resolva Natural Power Bug & Mildew). These are not considered organic
  • More persistent contact-action insecticides include the synthetic pyrethroids lambda-cyhalothrin (e.g. Westland Resolva Bug Killer), deltamethrin (e.g. Provanto Ultimate Fruit & Vegetable Bug Killer, Provanto Sprayday Greenfly Killer) and cypermethrin (e.g. Py Bug Killer)
  • In glasshouses it is possible to use glasshouse fumigants. Glasshouse should be sealed and instructions on the product label must be followed. An organic fumigant based on garlic is available as Pelsis Pest-Stop Biofume Greenhouse Fumigator and can be used when crop plants are present. Products based on the synthetic pyrethroid permethrin are available as DeadFast Greenhouse Smoke Generator, DeadFast Greenhouse Smoke Fumigator
  • The systemic neonicotinoid insecticide acetamiprid (e.g. Bug Clear Ultra) is also available

Follow label instructions when using pesticides. On edible plants make sure the food plant is listed on the label and follow instructions on maximum number of applications, spray interval and harvest interval. Plants in flower should not be sprayed due to the danger to bees and other pollinating insects.
Inclusion of a pesticide product does not indicate a recommendation or endorsement by RHS Gardening Advice. It is a list of products currently available to the home gardener.

 

Downloads

Pesticides for gardeners (pdf document)
Biological control suppliers (pdf document)

Biology

Glasshouse whitefly is a sap-sucking insect that breeds rapidly and produces many generations in warm greenhouse conditions. The adults and their whitish scale-like nymphs live on the underside of the leaves where they feed on sap, weakening the plants. They lay greyish white cylindrical eggs either singly or in circles on the underside of the leaves.

Each female can lay more than 200 eggs. Males are rare and reproduction takes place without the need for fertilization. The eggs hatch into small crawler scale-like nymphs which crawl around for a while before they begin feeding and become immobile. The nymphs are a flat, oval shape, whitish-green in colour, and just over 1mm in length when fully developed. The final nymphal stage is called a pupa and the adult whitefly eventually emerges through a slit in the dorsal surface. The length of the life cycle varies according to the temperature. At 10ºC (50ºF) the life cycle takes several months, but can be completed in about three weeks at 21ºC (70ºF). The insect can remain active during the winter in an unheated greenhouse, provided suitable host plants are present. Glasshouse whitefly does not usually survive winter out of doors.

Image: © GWI/John Swithinbank. Available in high resolution at www.gardenworldimages.com


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