Today 
Turn over a leaf and you'll see 2mm long white flies. With a life cycle of just three weeks, whitefly breed very fast. Both adults and offspring are sap feeders and a severe attack will weaken a plant. They excrete the sticky excess, called honeydew, onto lower leaves where it encourages black sooty mould to grow. If left, this weakens the growth of the plant.
Solution
Organic
Hang up yellow sticky traps to catch flying adults. Grow strong-smelling plants, such as French marigolds, alongside your tomatoes as a deterent. Every two to three days spray the underside of leaves with a soap-based spray containing fatty acids to kill emerging adults. When the temperature remains above 10 celsius in the greenhouse you can use a biological control, such as the parasitic wasp, Encarsia formosa, which attacks the juvenile forms of whitefly.
Chemical
Repeated sprays with products containing bifenthrin, though some strains of whitefly can become resistant. Also try those containing imidacloprid and thiacloprid, which are absorbed by the plants (causing no damage) to be taken up by the sap-sucking whiteflies.
