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Soft scale

Also known as Soft scales, Coccidae, Brown soft scale (Coccus hesperidum), Black scale (Saissetia oleae), Hemispherical scale (Saissetia coffeae)

PestHarmful
Brown soft scale (Coccus hesperidum) on a leaf Photo: AfroBrazilian · CC BY-SA 3.0

In short

Soft scales are immobile sap-feeding insects whose adult females look like small smooth or waxy bumps stuck to stems and leaf veins. They excrete plenty of honeydew, so sticky leaves, black sooty mould and busy ants are often the first clue.

How it looks

Adult females are up to about 6 mm (1/4 in) long, domed, with a smooth, waxy or cottony surface that is the insect's own body wall and cannot be lifted off separately. They sit along the midrib and main veins, often on leaf undersides, and on stems. Some species lay their eggs under a covering of white waxy fibres. The only mobile stage is the crawler, a tiny yellow to orange first-stage nymph; once it settles its legs are lost and it stays put. Scales killed by parasitoid wasps may look puffy or darkened, or show a round exit hole.

Signs

Sticky honeydew on leaves and surfaces below, black sooty mould, ants on the plant, and yellowing leaves or dieback when infestations are heavy.

Life cycle

Crawlers hatch under the mother, wander briefly, then settle to feed. Most soft scales in temperate climates have one generation a year and overwinter as second-stage nymphs, but brown soft scale has several generations a year. A single female can produce hundreds to thousands of eggs.

What favours it

Adult females hardly move, so infestations usually arrive on new plants. Ants that collect honeydew protect scales from their natural enemies.

Prevention

  • Inspect stems and the undersides of leaves along the veins before bringing a plant home, and quarantine new plants.
  • Control ants, for example with sticky barriers.
  • Monitor for crawlers by wrapping double-sided sticky tape around a stem and checking it weekly for yellow or orange specks.

Treatment

Start with the first step and move down only if the problem persists.

  1. Scrape or wipe scales off, or dab them with a cotton bud dipped in alcohol; dead scales stay attached, so check whether new ones appear rather than whether old shells remain.
  2. Prune off heavily infested shoots on small plants.
  3. Protect natural enemies such as parasitoid wasps and ladybirds; the RHS lists the nematode Steinernema feltiae for soft scale.
  4. Time insecticidal soap or horticultural oil sprays for when crawlers and young nymphs are present; follow the product label.
  5. As a last resort, systemic neonicotinoid-class products control most soft scales; they can harm pollinators, so avoid flowering plants and follow the product label.

Pros and cons

Cons

  • Honeydew and sooty mould make leaves sticky and black.
  • Heavy infestations weaken plants and cause dieback.

Look-alikes

Armored scales
Flat, often smaller covers that lift off to reveal a separate body, and no honeydew.
Mealybugs
Mobile, powdery white, with waxy filaments around the body.
Whitefly nymphs
Flat, pale, translucent ovals on leaf undersides, with white-winged adults nearby.

Affected plants

Many indoor, greenhouse and orchid plants, as well as outdoor shrubs and trees.

Sources

  1. Scales — UC IPM Pest Notes
  2. Indoor plants: sap-feeding insects — RHS
  3. Orchid Insect and Mite Pests in South Florida (ENY2114) — UF/IFAS
  4. Insect pests of cacti and succulents grown as house plants — Missouri Botanical Garden

Written in our own words from these sources. Check a product label before using any treatment.