I have a large infestation of white fly on my very old lemon tree, what is the best way to get rid of them. 

• There are several ways to control whitefly, the best way is to keep your tree healthy and actively growing by regular feeding in spring and again in late summer, early autumn with a balanced fertiliser. For garden plants feed with Tui Citrus Food, for citrus in pots and containers fertilise with Tui Enrich Fruit, Citrus, Tree & Shrub controlled release fertiliser.
• Regular watering throughout the growing season and mulch around the tree to conserve soil moisture.
• Remove weeds that can be host to whitefly such as milk weed, and any grass growing right up around the tree as it competes with the lemon tree for nutrients and water.
• Try regularly hosing off the whitefly but it sounds like a large tree, so this may not be practical.
• Hang yellow sticky pheromone traps in the tree which help break the lifecycle of the whitefly, these are available from garden centres and hardware stores.
• Talk to your local garden centre about a suitable control for white fly, try a pyrethrum based insecticide or plant spraying oil.
• Continue to spray every 7-10 days to break the lifecycle of the insects.
• Regular applications of Tui Organic Seaweed Plant Tonic will also help create natural resistance to pests and diseases in plants by strengthening the cell wall of the leaves, essentially it toughens the plant and makes it difficult for insects to pierce the leaf surface.