Whiteflies are

Sap is a fluid that circulates through a plant's vascular system, in a similar way to blood moving through our veins. Phloem sap carries the sugars produced in the leaves by photosynthesis down to roots and other storage organs, as well as carrying minerals and hormones. Xylem sap is watery and transports nutrients absorbed by the roots upwards to the rest of the plant. Some plants leak sap from wounds or pruning cuts, and this is known as bleeding.

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.

• It is relatively easy to see whiteflies on affected plants. When a plant is disturbed clouds of small white-winged insects, 1.5mm (about 1/16in) long, will fly up. This distinguishes whitefly from other insect such as aphids. White objects on the top surface of leaves are more likely to be aphid skins
• You may also see flat, oval, creamy white scale-like nymphs (sometimes called larva and pupa) on the underside of leaves. If these nymphs are black they have probably  been parasitised by an Encarsia wasp and are dead
• Adult whitefly and nymphs excrete sticky honeydew which falls onto foliage, stems and fruits and allows the growth of black sooty moulds

Check susceptible plants frequently from spring onwards so action can be taken before a damaging population has developed. When choosing control options you can minimise harm to non-target animals by using the methods in the non-pesticide section below. Pesticide treatments are likely to kill natural enemies and are only likely to be successful if the entire plant can be reached.

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

• Removing unwanted plants from the glasshouse can reduce the number of host plants for glasshouse whitefly
• 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
• 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. Sticky traps often catch non-target organisms including parasitoid wasp predators so there use should be considered carefully

Biological control

An established method of control for glasshouse whitefly is to introduce 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 affected 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  insecticides, avoid spraying with products other than fatty acids, plant invigorators, plant extracts or plant oils (see below) prior to its introduction.

Other predators such as the mite Amblyseius andersonii (egg predator) are sometimes available in addition to Encarsia. See Biological control suppliers.

Pesticides

• Organic sprays, such as natural pyrethrum (e.g. Bug Clear Ultra 2, Neudorff Bug Free Bug and Larvae Killer) or plant oils (e.g. Vitax Plant Guard Pest & Disease Control, Bug Clear Fruit & Veg, Vitax Rose Guard) have a largely physical mode of action. These are broad spectrum so will kill a wide range of insects. 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 like ladybirds.
• 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, Growing Success Bug Stop, RoseClear 3 in 1 Action and SB Plant Invigorator). These products contain some synthetic ingredients and so are not considered organic
• 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
• Further information about the use of pesticides available for management of glasshouse whitefly is available on the pesticides for gardeners leaflet

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.

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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 (also known as larva and pupa) live on the underside of 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