Whitefly look just like they sound - small white flies that 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 scales 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(much magnified).
Whitefly adults(much magnified).
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 (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, ans 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 chemical pesticides to kill them (generally it is only the adult that is affected) you have to spray very regularly to control a severe outbreak. 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!. But brassicas 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.
There are a number of chemical pesticides that you can use on whitefly. Those containing bifenthrin and thiacloprid provide the best control.
Bayer Provado Ultimate Bug Killer is systemic and is taken up by the plant where it will help control whitefly - and other sap-sucking insects. There are only certain edible crops that the ready-to-use formulation can be used on; check the label before using.
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. The second is delphastus, which is a member of the ladybird family.
This predator eats all stages of the whitefly.-->
Encarsia is available from various mail order suppliers, including Defenders, who also has a very useful Pest Problem Solver booklet.
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.