Usually my pest control efforts consist of taking the seed heads off invasive perennials, swatting flies, killing slugs and waiting for insect pests to disappear naturally. Plants very rarely die from insect infestation and a few holey leaves can be picked off or ignored. But I will take action if pest damage just beside my front door is particularly ugly.
The problem is a black coating which has appeared on the leaves of evergreen shrubs such as camellia, holly and pieris. This is a fungus called sooty mould which lives off the honeydew excreted by various sap-sucking insects living higher up. It can be softened by wiping with soapy tepid water, then rinsing off. The soot builds up on older leaves and stems, especially if there is no rain reaching the leaves, and requires annual cleaning unless the insect problem is dealt with.
The pestilential insects on these shrubs are often scales, looking like small, light brown bumps on the underside of the leaves. Most of the time the insect is shielded from insecticides by its shell-like scale, and is only vulnerable when it moves to new leaves in May or June. A good way to reduce insect populations is spraying or wiping both surfaces of the leaf with an insecticidal soap solution at this time. Horticultural oil can also be sprayed on, following the directions on the container, and as this acts by smothering the insect, the timing is not so vital.
A more direct solution is to prune out and dispose of infested branches, and scrape off remaining scales if there are not too many of them. Sometimes one side of the shrub is infested first, so cut this away and pick off the remainder. Often a massive infestation occurs in a shrub which is planted in inhospitable conditions, such as under the eaves. Consider moving or removing it.
Check tall shrubs or those beneath overhanging aphid-infested trees regularly, and clean off the ugly sooty mould. A spray program to kill off the offending insects involves massive collateral damage to other insects, many of which help us by eating the bad guys and pollinating our fruit. If we kill the helpful insects, we have to take over the work they do.
– Sheila Watkins, Master Gardener