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

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 attacks 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
  • 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

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 suppliers of biological controls. It is important to introduce the parasite before plants are heavily infested as it 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 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, Ecofective Bug Killer), fatty acids (e.g. Solabiol Bug Free, Doff Greenfly & Blackfly Killer) or plant oils (e.g. Vitax Organic Pest & Disease Control, Bug Clear for Fruit and Veg) can give good control of whitefly nymphs. These pesticides have a very short persistence and so may require reapplication to keep whitefly in check. Plant oil and fatty acid products are less likely to affect larger insects such as ladybird adults and can be used with care, prior to introduction of biological control. Organic products can also be used on edible plants provided label instructions are followed
  • More persistent insecticides include the synthetic pyrethroids lambda-cyhalothrin (e.g. Westland Resolva Pest Killer), deltamethrin (e.g. Provanto Ultimate Fruit & Vegetable Bug Killer) and cypermethrin (e.g. Py Bug Killer). There are resistant strains of whitefly to these products. Products containing deltamethrin, cypermethrin and lambda-cyhalothrin can be used on edible plants listed on the label provided instructions on maximum dose and harvest interval are followed
  • The systemic neonicotinoid insecticide acetamiprid (e.g. Bug Clear Ultra) is also available. Systemic insecticides are absorbed into the plant tissues and are taken up by sap-sucking insects when they feed. They also usually have some contact action. There are several formulations of one active ingredient available. Acetamiprid can be applied as a compost drench (e.g. Bug Clear Ultra Vine Weevil Killer) on container-grown ornamental plants only, or as a foliar spray (e.g. Bug Clear Ultra or Bug Clear Ultra Gun) on ornamental and some listed edible plants, provided the label instructions on maximum dose and harvest interval are followed
  • Plants in flower should not be sprayed due to the danger to pollinating insects
  • Inclusion of a pesticide product does not indicate a recommendation or endorsement by the RHS. It is a list of products currently available to the home gardener

Downloads

Pesticides for gardeners (Adobe Acrobat pdf document outlining pesticides available to gardeners)

Biological control suppliers (Adobe Acrobat pdf document)

Biology

This sap-sucking insect 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 pest 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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