Scale insects
Immobile, shell-covered insects that cling to stems and leaves, draining sap and weakening trees and houseplants.
Also known as: Soft scale, Armoured scale
Symptoms and identification
Small brown, grey or white bumps along stems, branches and leaf veins that can be scraped off with a fingernail. Leaves turn yellow and drop, branches may die back, and soft scale species produce sticky honeydew followed by black sooty mould. Ants often travel along affected branches.
Causes
Young scale insects (crawlers) move short distances before settling and forming a protective shell. They spread on new plants, wind and clothing. Stressed plants and those growing in sheltered, dusty spots are more likely to be attacked, and ants protect them from predators.
Organic treatment
Scrape small numbers off with a soft brush or a cotton bud dipped in rubbing alcohol. Spray horticultural oil or neem oil to smother the insects, covering stems thoroughly, and repeat every 10–14 days. Sprays work best against young crawlers, so treat as soon as you notice new activity.
Prevention
Inspect stems and the undersides of leaves of trees and houseplants regularly. Keep plants healthy with good watering and feeding, prune out lightly infested twigs, and control ants on trunks with sticky bands.
Chemical treatment (use with care)
Severe infestations on ornamental plants and fruit trees can be treated with a systemic insecticide or a mineral oil spray labelled for scale, following the label. Do not use systemic products on herbs or edible leaves.