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Tiny sap-sucking scale insects. They are 1-6mm long, and their whitish, grey-brown shells are covered by protective, waxy scales. The insects don't cause too much damage unless they are present in huge numbers, but some excrete an unsightly, sticky, sugary substance (called honeydew) which encourages the growth of a fungus called sooty mould. Early summer is the best time to tackle the problem, when the newly hatched insects are crawling around looking for a place to feed and settle.
Solution
Organic
First try to remove them with your thumbnail. If there are too many, use an insecticidal soap spray. Alternatively, leave them for the birds to pick off. In a greenhouse, use the parasitic wasp, Metaphycus helvolus, bought by mail order as a biological control.
Chemical
Use an insecticide containing imidacloprid or thiacloprid in early summer when the insects are more vulnerable.
