Imidacloprid is in Merit insecticide, and this may be as good a product as any. It is a systemic that gets inside the leaves, and when applied to the soil around the trees it will move up into the foliage and last  for possibly 5 or 6 months, killing the insects as they suck the plant fluids. The difficulty with spraying contact insecticides on the foliage is the wax that covers the larval stages of the whitefly. This wax tends to repel water based products so that the insect is not properly contacted. 

This fact sheet from UFL also highlights the predators and parasites that currently are known to feed on the Ficus Whitefly, and encourages us NOT to spray if good numbers of these beneficial insects are present. In California we had a serious problem years ago with the Ash Whitefly, and the introduction of a tiny parasitic wasp was highly successful in beating the whitefly to tolerable levels. It was working so well that pest control people and homeowners were asked to please not spray the trees so that the wasps could do their work instead. One advantage of the soil application of the systemic insecticides is that it does not contact the beneficial insects, and in theory is only available to the pest insects that are feeding on the plant.