Glasshouse whitefly is a common sap-feeding pest, mainly of houseplants and greenhouse plants. Whiteflies excrete a sticky substance (honeydew) on foliage, which allows the growth of sooty moulds.
What is glasshouse whitefly?
Glasshouse whitefly is a sap-sucking insect pest that 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 houseplants. 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, evergreen azalea and rhododendron are other species of whitefly specific to those plants.
It thrives in warm conditions, which is why it is not usually a problem on outdoor plants. Glasshouse whitefly is active all year round on houseplants and in greenhouses.
Due to this pest's 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, urea/mineral lattice, plant extracts or plant oils (see below) prior to its introduction.
Other non-chemical controls
Hang sticky yellow sheets (Agralan Yellow Sticky Traps, Bayer Greenhouse Fly Catcher, Growing Success Greenhouse Whitefly Traps, Vitax Whitefly Traps) above or among the plants to trap adult whitefly.
Frequent sprays with deltamethrin (Bayer Sprayday Greenfly Killer), lambda cyhalothrin (Westland Plant Rescue Fruit & Vegetable Bug Killer), plant extracts (Growing Success Fruit & Veg Bug Killer, Growing Success Greenhouse 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 urea/mineral lattice (SB Plant Invigorator) are required to control established infestations. Resistance to deltamethrin and lambda cyhalothrin can occur.
There are several systemic insecticides that can be applied as foliar sprays or compost drenches. Systemic insecticides are absorbed into the plant tissues and are taken up by sap-sucking insects when they feed.
- 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
- Thiacloprid (Provado Ultimate Bug Killer Ready To Use or Provado Ultimate Bug Killer Concentrate 2) can be sprayed on the foliage of greenhouse-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 ornamental plants, or as a foliar spray (Scotts Bug Clear Ultra or Bug Clear Ultra Gun) on ornamental plants only
- Thiamethoxam* (Westland Plant Rescue Bug Killer Ornamental Plants) is a foliar spray for use on ornamental plants only
* Please note this product is being withdrawn. The withdrawal (in effect a ban) comes into force on 30 September 2013, but there is to be a period of grace to use up these materials by 30 November 2013. After this time it will be illegal to use them, any remaining products should be taken to a manned local authority household waste site where they should be handed over to the staff. For information on waste disposal sites see www.pesticidedisposal.org. It remains legal to use other neonicotinoid-based products that are not affected by the withdrawal. This withdrawal follows concern over their effects on bees.
Pesticides for gardeners (Adobe Acrobat pdf document outlining pesticides available to gardeners)
This sap-sucking insect pest 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. Glasshouse whitefly cannot survive winter out of doors but continues breeding throughout the year on indoor plants.
Image: © GWI/John Swithinbank. Available in high resolution at www.gardenworldimages.com