During winter it's easy to spot shrubs that have their leaves turning silver, especially rhododendrons and viburnum. What causes this to happen? Small insects called thrips are to blame. They thrive on the sheltered microclimate on the underside of the leaves. These tiny insects have strong suction pipes which they insert into the leaf and proceed to suck out all the goodness including the chlorophyll (green colored matter) turning the leaf silver - hence the name silver leaf. Unfortunately the leaf does not recover even if you kill all the insects. Controlling thrips is quite easy. Regular autumn sprays of all seasons spraying oil in late summer and autumn. The oil smothers the eggs and insects effectively killing them. Spraying needs to be done regularly and every year to make sure that infestations are kept under control. For additional control add some systemic insecticide to the oil mixture. Where infestations are known pick up all the dead leaves, which may have thrips and eggs on them, and dispose of them but not in the compost heap.