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Whitefly are a difficult plant pest that can be a problem to control.
The flies can be found on the leaves of plants - both on the upper and lower surfaces, but mainly on the latter.
These are the adults, which feed on growing shoots and lay eggs that hatch into tiny white scales that remain attached to the underside of leaves. Like aphids (greenfly, blackfly), whitefly are sap-sucking insect pests. This sap sucking weakens the plant and may introduce plant viruses that further weaken the plant, and may even kill it.
Whitefly scales (larvae) on the back of a leaf
Unfortunately, whitefly don't go round in ones or two - they go round in hordes of hundreds and a severe attack can severely weaken a plant. You will often see a 'cloud' of whitefly flying away from a plant whose leaves have been disturbed.
But this has another drawback, too. The large amounts of sugar-rich exudate they excrete plays host to a fungus called sooty mould. When the exudate lands on leaves below where the whitefly are feeding, these leaves then become covered in sooty mould. Although this does no direct damage, it does stop the leaves from receiving sunlight, so they are unable to manufacture food for the plant (via photosynthesis). Some gardeners believe they have a disease problem when, in fact, they have a pest problem!
Whitefly tend to be more of a problem in greenhouses and indoors, but one species is very troublesome on brassicas, and another on tomatoes and related crops.
Sadly, whitefly can be difficult to control - some people say impossible - because they have a very fast lifecycle; they can develop from egg to adult in as little as three weeks in the summer. This means that if you are using contact or organic chemical pesticides to kill them, you have to spray very regularly to control a severe outbreak; in the height of a hot summer this could be every two or three days. And then you have to be very vigilant to catch other outbreaks early. If you've only a few plants it is simpler to check over them regularly and squash adults and scales (you can just about see them) as soon as they appear.
Prevention is better than cure and covering plants with a layer of horticultural fleece will stop the pest getting at them - although it might not look very pretty in the ornamental garden! But brassicas and other vegetables can easily be protected as long as uninfected plants only are covered, the edges are buried in the ground, and the fleece is kept on until harvest.
Covering greenhouse vents with fleece and making a door screen from the same material will help reduce attacks under cover.
Systemic insecticides, containing thiacloprid, thiamethoxam or acetamiprid, provide the best control; they also kill on contact, but are also taken inside the plant where they go on to protect against further attack for several weeks.
Bayer Provado Ultimate Bug Killer and Scotts BugClear Ultra are systemic and are taken up by the plant where they will help control whitefly - and other sap-sucking insects. There are only certain edible crops that these systemic insecticides can be used on; check the label before using. Westland Plant Rescue Bug Killer for Ornamental Plants is similar, but can only be used on ornamental plants.
I've also tried Plant Invigorator SB with good results. This is not a pesticide per se, but a plant growth stimulant that also controls pests by physical methods.
In the greenhouse you can catch them by hanging yellow sticky traps just above the leaves of plants - they are attracted to the colour yellow, which is why I've never been able to work out why growing-bags are that colour! Some people even vacuum their plants regularly to remove the little flying pests!
Whitefly are best controlled in the greenhouse by biological control - using another insect that kills them and so do the work for you.
Encarsia is a small parasitic wasp which lays its eggs in the scales where the larvae develop and so kill the scale The larvae hatch into adults, which find more scales to parasitise. However, once it has done its job there is no food source for the larvae and so the lifecycle stops. If a further outbreak occurs then you have to re-introduce the encarsia. Luckily, encarsia rarely kills all its hosts, so there are usually some to carry on the lifecycle.
Encarsia is available from various mail order suppliers.
In order to keep your whitefly killers safely within the greenhouse where they will sort out your whitefly problem, it's a good idea to again use fleece over the vents and doors. And, you can't use insecticides (although soft soap can be used before the controls are introduced) while the controls are busy at work - so you'll have to trust them.
Buy a range of pest control products, inluding biological controls, from my affiliate companies, Harrod Horticultural and Greenfingers.com
- Agralan, The Old Brickyard, Ashton Keynes, Swindon, Wilts SN6 6QR; firstname.lastname@example.org (biological controls, sticky traps and fleece)
- Bayer Garden, 230 Cambridge Science Park, Milton Road, Cambridge CB4 0WB; email@example.com (Provado)
- Biowise, Hoyle Depot, Graffham, Petworth, West Sussex GU28 0LR (biological controls)
- Defenders, PO Box 131, Ashford, Kent TN25 5EN; firstname.lastname@example.org (biological controls, sticky traps and fleece)
- Green Gardener, Chandlers End, Mill Road, Stokesby, Great Yarmouth NR29 3EY (biological controls, sticky traps)
- Just Green, Unit 14, Springfield Road Industrial Estate, Burnham-on-Crouch, Essex CM0 8UA; email@example.com (biological controls)
- Organic Gardening Catalogue, Riverdene Business Park, Molesey Road, Hersham, Surrey KT12 4RG (various products including biological controls)
- PlantsByPost.com, Moor Lane, Gotham, Nottingham NG11 0LH; firstname.lastname@example.org (SB Plant Invigorator biological controls, other control methods)
- Scarletts Plant Care, The Glasshouses, Fletching Common, Newick, Lewes, East Sussex BN8 4JJ (biological controls)
- The Natural Gardener, The Steppes, Hope under Dinmore, nr Leominster, Herefordshire HR6 0PP (biological controls)
- The Scotts Miracle-Gro Company, Salisbury House, Weyside Park, Catteshall Lane, Godalming, Surrey GU7 1XE; email@example.com (BugClear)
- Westland Horticulture, 14 Granville Industrial Estate, Granville Road, Dungannon, Co Tyrone BT70 1NJ (Plant Rescue)
If you want to know more, or if you've got a gardening problem you need help with, then send an e-mail to: firstname.lastname@example.org