Brown soft scale
Brown soft scale is the flat, oval, yellowish-brown scale insect found along the stems and leaf midribs of houseplants, citrus and glasshouse plants. Adults are 1/16–1/5 inch (1.5–5 mm) long, do not move, and coat the leaves below with sticky honeydew and black sooty mould. Unlike most soft scales it breeds all year, so every stage is present at once. Scrape off small numbers, keep ants away, and time any oil or soap spray for the mobile crawlers. A heavily infested houseplant can be simpler to replace.
Brown soft scale (Coccus hesperidum) females along a leaf midrib, with honeydew. Photo: pending.
Identification
Adult females are flat to slightly raised ovals, 1/16–1/5 inch (1.5–5 mm) long, yellowish green to pale or dark brown. They sit close to the midrib and main veins, usually on the underside of leaves, and on green stems. The cover is the insect's own body wall, so a soft scale lifts off whole; an armoured scale's cover comes away separately and leaves the insect on the plant.
Honeydew gives it away. Brown soft scale produces large amounts, so sticky leaves, a sticky windowsill or black sooty mould on the leaves below usually come before anyone notices the insects. Armoured scales produce no honeydew. Mealybugs leave honeydew too, but their segmented bodies are usually covered in wax, and they have legs and can move, if slowly.
Easily confused with
- Parasitised brown soft scaleBeneficial — leave itA scale with a round wasp exit hole in its back, or one darkened or turned black, has been killed by a parasite such as Metaphycus. Leave it be.
- Citricola scaleAlso a pestA similar soft scale on the same citrus trees, but it has one generation a year; a brown soft scale colony shows every life stage at once.
- Hemispherical and black scalesAlso a pestHemispherical scale is a hard, smooth, rounded dark-brown dome, and black scale is dark brown to blackish, with an H-shaped ridge on its nymphs; brown soft scale is only slightly raised and ranges from yellowish green to pale or dark brown, often mottled.
- MealybugsAlso a pestMealybugs have distinctly segmented, wax-covered bodies and older ones can still walk; brown soft scale is a mottled, yellowish, rounded shell.
Damage symptoms
Brown soft scale sucks phloem sap and excretes copious honeydew, on which black sooty mould grows. A few scales do the plant no harm; heavy infestations slow growth, yellow the leaves and can kill twigs. Ants farm the honeydew and in return protect the scale from its parasites and predators, so ants on the stems are a sign as well as a problem.
- Sticky, shiny leaves, and sticky surfaces beneath the plant
- Black sooty mould on the leaves below the colony
- Flat yellow-brown ovals along the midrib and on green stems
- Ants travelling up and down the stems
- Yellowing leaves, slow growth and twig dieback when numbers are high
Lifecycle & degree days
Most soft scales have one generation a year. Brown soft scale is the exception: it has several overlapping generations, and adults and nymphs can be found in any month. Females lay a few eggs at a time beneath their bodies and the eggs hatch almost immediately, so crawlers keep appearing rather than arriving in one flush. That is why a single treatment rarely finishes it.
- Generations
- 3–5 a year outdoors in California citrus; continuous indoors
- Eggs
- A few at a time; hatch almost immediately
- Crawlers settle
- Within a day or two of emerging
- Adult female
- 1/16–1/5 in (1.5–5 mm)
- Outdoor peak
- Mid-summer to early autumn (California citrus)
Host crops
The RHS describes brown soft scale as very widespread and common on a wide variety of garden and glasshouse plants. Outdoors in California, UC lists it as annoyingly abundant when ants tend it, on avocado, citrus, holly, manzanita and palm. The sources name its hosts broadly, and none of them names orchids, hibiscus or aroids for this species, so treat those entries below as likely indoor hosts rather than a list from the literature.
Monitoring & action thresholds
UC IPM says there are no quantitative action guidelines for scales. Record the density that caused damage (dieback or unacceptable honeydew) and use it as your own threshold. Before treating, check that the scales are alive: a live one exudes fluid when squashed; parasitised ones may be puffy or darker than the rest, or have a round exit hole.
Check the undersides of leaves along the midrib and the green stems, and look for honeydew and sooty mould on the leaves below. To time a spray, wrap transparent double-sided sticky tape tightly around a few infested stems and change it weekly: crawlers show up on it as yellow or orange specks. UC advises spraying after crawler numbers on the tape have peaked and begun to fall. Check for the round exit holes of parasitic wasps before deciding anything.
Biological control
Outdoors, biological control of this scale is mostly conservation. UC finds a complex of Metaphycus parasitoid wasps attacking it, M. angustifrons the commonest in southern California, and lady beetles including Rhyzobius lophanthae. Ants protect the scale from both, so controlling ants comes first. Indoors, a bought predator is an option on a plant too large to clean by hand. The evidence for lacewing larvae against this scale is thin. The RHS notes that the nematode Steinernema feltiae is also sold for soft scale in the UK, but can infect non-target animals, so use it with care.
Cultural controls
On a lightly infested houseplant, taking the scale off by hand is the simplest control, though it may not keep up with a large population. Outdoors, the job is keeping the ants off so the natural enemies can work.
- Scrape off adult scales when you see them, checking the leaf undersides along the midrib
- Prune out heavily infested stems, if only a few are affected
- Keep ants off: a sticky band on a wrapped trunk, or bait stations near the nest
- Rinse dusty foliage, because dust interferes with natural enemies
- Replace a heavily infested houseplant rather than treating it again and again
Chemical, last resort
Sources
UC IPM Pest Notes: Scales, Pub. 7408 (Kabashima & Dreistadt, 2014)
UC IPM Citrus Pest Management Guidelines — Brown soft scale (updated 2017)
UC IPM Floriculture & Ornamental Nurseries — Soft scales (updated 2022)
RHS — Indoor plants: sap-feeding insects
RHS — Scale insects
UC IPM Citrus Pest Management Guidelines — Citricola scale
UC IPM Pest Notes: Mealybugs, Pub. 74174
Rincon-Vitova — Rhyzobius lophanthae product bulletin
Page status
Drafted from the sources below, then checked claim by claim against those sources on 23 September 2026.
Suggest a correction →Brown soft scale by crop
The same pest behaves differently on different hosts. Each page below gives the host-specific control, rate and threshold.