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.
This involves introducing tiny parasitic wasps, Encarsia formosa, which attack the whitefly nymphs. The parasite 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.
A small (<2 mm) black ladybird, Delphastus pusillus is sometimes available for glasshouse whitefly control. This can give good control of heavy whitefly infestations.
Other non-chemical controls
Hang sticky yellow sheets (widely available from garden suppliers) above or among the plants to trap adult whitefly. 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 pest 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.
- Glasshouse whitefly often requires repeat spraying to control infestations
- Products with short persistence such as plant extracts (organic e.g. Vitax Organic Pest & Disease Control Concentrate, BugClear for Fruit & Veg Gun!), fatty acids (organic e.g. Bayer Bug Free, Doff Universal Bug Killer) or products which contain a blend of surfactants and nutrients (e.g. SB Plant Invigorator) can be used with care prior to introduction of biological control.
- Contact pyrethroid insecticides such as deltamethrin (e.g. Bayer Sprayday Greenfly Killer or Bayer Provado Ultimate Fruit & Vegetable Bug Killer), lambda-cyhalothrin (e.g. Westland Resolva Bug Killer), cypermethrin (e.g. Py Bug Killer Concentrate) pyrethrins (organic e.g., Bug Clear Gun for Fruit & Veg, Defenders Bug Killer, Pyrol Bug & Larvae Killer, ecofective Bug Killer (also contains fatty acids) may control established infestations. However resistance to pyrethrins, deltamethrin, cypermethrin and lambda-cyhalothrin can occur.
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. Organic products can also be used on edible plants provided label instructions are followed
The systemic neonicotinoid insecticide acetamiprid may also be used. 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
Pesticides for gardeners (Adobe Acrobat pdf document outlining pesticides available to gardeners)
Biological control suppliers (Adobe Acrobat pdf document)