Identification
Immobile, brown or yellowish shields (1-4 mm) attached to stems and the underside of leaves, especially along the veins. Easily mistaken for growths on the plant. They suck sap, causing yellowing and weakening, and excrete sticky honeydew. Common on figs, orchids and plants with tough leaves. The collective name covers two groups with different biology. Armoured scales (Diaspididae) have a hard waxy shield separate from the body, which can be prised off - beneath it emerges a tiny, legless and wingless female; they do not excrete honeydew. Soft scales (Coccidae) have a convex, cup-shaped cover fused with the body, and it is they that excrete it abundantly. Sticky honeydew - the domain of soft scales and other honeydew-producing scale insects, not of armoured scales - settles on the leaves and attracts sooty mould fungi (sooty mould, sooty blotch) as well as ants, which defend the colony against predators. The only mobile feeding stage is the crawler - a flat, tiny larva which, after hatching from beneath the mother's shield, disperses over the plant, then settles, inserts its stylet into the conducting tissues and loses the ability to move; adult females remain immobile for the rest of their lives. Males pupate and, as tiny, winged, short-lived insects, do not feed. In heated interiors the pest breeds all year round, producing several overlapping generations; outdoors usually 1-3 generations, and fertilised females or eggs hidden beneath the shield overwinter. Feeding symptoms are yellowing and premature leaf drop, dieback of shoots, stunting and weakening of the plant, and secondarily a black coating of sooty mould on leaves covered with honeydew. On fruit trees infested by San Jose scale (Comstockaspis perniciosa), reddish discolouration appears around the feeding sites, and heavily colonised branches die back. Because the shields are immobile and their colour blends into the bark and veins, an infestation is easily overlooked until honeydew appears or the plant is noticeably weakened - which is why regular inspection of the leaf undersides and shoot axils is key.
Treatment
Scraping off the shields, wiping with a cotton bud soaked in alcohol, potassium soap and oil-based preparations (which block the scales' respiration). Systemic products for heavy infestations. Regular inspection of the leaf undersides allows the pest to be detected early. The main difficulty in control stems from the hard shield sheltering the adult insect from contact products. This is why contact sprays (potassium soap, oils, pyrethroids) are most effective against crawlers - not yet covered by wax - whose hatch is worth tracking so the treatment can be timed to it and repeated every 1-2 weeks. Adults, being protected, are instead reached by systemic insecticides, taken up by the plant and distributed in the sap. On potted plants with a slight infestation, mechanical removal of the shields (scraping them off, wiping with a cotton bud soaked in alcohol) and washing the plants with a potassium soap solution, repeated several times, is enough; heavily infested shoots are best cut out. A new plant is worth isolating and inspecting before adding it to the collection. On fruit trees and shrubs, oil sprays (paraffin oil) in early spring destroy the overwintering forms before the buds break. Biological control offers support, especially under cover and in potted cultivation: parasitic wasps (chalcids of the families Aphelinidae and Encyrtidae) and predatory ladybirds (including Chilocorus, Rhyzobius, and Cryptolaemus against mealybugs) and lacewings can curb the population, though they rarely eliminate it completely. It is also worth controlling the ants that protect the colonies and avoiding excess nitrogen fertilisation, which favours the development of scale insects on lush, soft growth.
How to tell it apart from look-alikes
| Feature | Armoured and soft scale insects | Mealybugs |
|---|---|---|
| Appearance | Hard or waxy, brown/yellowish shield stuck fast to the plant | White, cottony, mealy coating; the insect resembles a tuft of cotton wool |
| Mobility | Adult females completely immobile, permanently attached | Move slowly throughout their life, have legs |
| Location | Along the veins, on stems and hard parts, leaf undersides | Leaf axils, bases of leaf stalks, crevices, potted-plant roots |
| Egg protection | Eggs laid beneath the mother's shield, invisible from outside | Eggs in a white, cottony sac (ovisac) |
Control — organic and chemical methods
Organic and biological methods
| Method | How it works | When to use |
|---|---|---|
| Mechanical removal and washing | Scraping off the shields and wiping with a cotton bud dipped in alcohol or a soap solution; for light infestations of potted plants, needs repeating. | Year-round, at the first outbreaks |
| Horticultural oils (paraffin, rapeseed) | Form a film that blocks the spiracles and suffocates the insect; most effective on crawlers, and on fruit trees on the overwintering forms. | At crawler hatch; on trees spray in early spring |
| Potassium soap | Dissolves the waxy coatings on contact and dehydrates crawlers and uncovered females; does not reach beneath the hard shield of adults. | Repeat every 7-14 days during crawler hatch |
| Natural enemies (biological control) | Parasitic chalcid wasps, predatory ladybirds (Chilocorus, Rhyzobius) and lacewings curb the population; a supporting method, rarely eliminates an outbreak. | Under cover and in potted cultivation |
Chemical methods
| Method | How it works | When to use |
|---|---|---|
| Systemic insecticide | Taken up in the sap, it reaches beneath the shield that protects adults from contact products; used for heavy, extensive infestation. | When contact methods fail |
| Contact product for crawlers (pyrethroid, oil-based preparation) | Acts only on uncovered crawlers, so it must be timed to the hatch; pyrethroids also harm beneficial insects. | At the peak of crawler hatch |
We give active-ingredient groups and modes of action, not brand names — specific products and their approval vary by country and are sometimes withdrawn. Always check and follow the current label.
Sources
- Atlas of ornamental-plant pests
