Quick facts…
- Brown soft scale is a common insect pest of many indoor plants, including Ficus, Schefflera, English ivy, and citrus.
- When brown soft scales feed, they produce a shiny, sticky fluid called honeydew that may cover leaves.
- Insecticidal soaps or horticultural oils can control brown soft scale, but these will require multiple treatments to be effective.
- A systemic insecticide (imidacloprid) that is applied to the soil provides good control of brown soft scale on ornamental plants.
Introduction
Brown soft scale (Coccus hesperidum) is the most important scale insect that occurs on indoor grown plants in Colorado. When feeding, the soft scale excretes a large amount of sticky honeydew that can cover leaves and create serious nuisance problems. Heavy infestations can cause leaves to prematurely shed, and branches to die back.
Brown soft scale occurs worldwide and has an enormous range of recorded host plants, including 125 families of plants in 363 genera. Brown soft scale is only known to occur on indoor-grown plants in Colorado, but occurs year-round on many kinds of landscape plants in areas within the United States that have milder winters. Indoor-grown plants commonly infested in Colorado include citrus, Schefflera, Ficus, English ivy, and bay leaf.
on the underside of a leaf.
Description and Habits
Only female brown soft scales are produced through parthenogenesis. Upon reaching maturity, the adult female internally incubates eggs, which hatch into minute first-instar nymphs, commonly referred to as crawlers, measuring approximately 0.4 mm in length. Crawlers may remain beneath the protective covering of the mother scale for several days before actively dispersing to establish on other parts of the host plant. Stems are typically the preferred sites for settlement, although crawlers may also colonize leaf surfaces. This crawler stage is the most mobile phase in the insect’s life cycle and is primarily responsible for initiating new infestations. Crawlers can be inadvertently transferred through the handling of infested plant material or may disperse over short distances via air currents. After the crawler has settled, it begins to feed with its piercing-sucking mouthparts. Mouthparts are used to reach the sugar-rich fluids present in the phloem vessels of the plant. As they feed, the scale insects excrete excess water and sugars, producing honeydew, a sticky, shiny fluid.
which has collected on a nearby leaf.
darker scales are adults. A tiny first stage nymph
(crawler) may be seen in the upper, center-left.
Within a week or two after birth, the scale will molt to a second stage, which is still quite small, only about 1.0 mm in length. During this period the scale will normally remain in place but can still move. The second stage of brown soft scale may last for about 6-8 weeks followed by a final molt to the adult form.
The adult form grows considerably as it produces and matures eggs, doubling in size within a couple of weeks. Once females begin to reproduce, they do so continuously, so that a few crawlers will be produced daily over 3-4 weeks or more, after which the female dies. On indoor plants, multiple generations will be produced each year. Because the scales are laying eggs over an extended period, generations overlap and there will be no distinct annual peaks in egg production and new crawlers.
petiole of a citrus leaf in the right of the photo. The
honeydew excreted by the insect is on the leaf in the
lower left, a few inches away.
Management of Brown Soft Scale
Monitoring
Early detection can greatly improve the ease of managing brown soft scale. The insects can be difficult to detect since they are small and may blend in well with the plant. However, the presence of honeydew—a sugary excretion produced during feeding—serves as a reliable indicator of infestation.
The honeydew produced by brown soft scale may accumulate near the insect, forming small pools. In many cases, the honeydew is actively ejected from the body, sometimes traveling more than an inch before settling on leaves or other surfaces below or adjacent to the insect. This can lead to the development of sooty mold, a black fungal growth that feeds on honeydew and further indicates active infestation.
By inspecting plants every one to two weeks, early signs of infestation can be detected through the presence of fresh honeydew. Monitoring is particularly important on new growth and plant stems, where scales tend to settle.
Honeydew detection is also useful in evaluating the effectiveness of insecticidal treatments. Since scale insects often remain attached to the plant even after death, the continued production of honeydew can confirm that the insects are still alive and feeding.
(Note: There are other insects occurring on houseplants that also produce honeydew. Aphids are perhaps most common. Mealybugs and other kinds of soft scales, such as hemispherical scale (Saissetia coffeae) and nigra soft scale (Parasaissetia nigra), also excrete honeydew.)
Biological Control
In outdoor settings there are often numerous natural enemies that attack and greatly limit brown soft scale. Most important are several types of small parasitoid wasps that develop within the body of the scale, ultimately emerging through a hole they cut in the back of the body.
Presently there are no reliably effective natural enemies available to control brown soft scale indoors. Generalist predators such as larvae of green lacewings and the predatory beetle Rhyzobious lophanthae will feed on brown soft scale and may help reduce numbers of the scale in some situations. However, if large amount of scales are present and large amounts of honeydew are covering the plant these natural enemies will not be effective.
If it is practical to move plants outdoors then some biological control can occur by some of the generalist natural enemies that are normal residents in yards – lady beetles, green lacewings, various predatory flies and other beneficial insects. Moving plants outdoors also allows plants to be easily sprayed and washed so that the honeydew can be removed.
Insecticide Options (Indoors) (See Table 1a)
Insecticidal soaps (potassium salts of fatty acids) can be used as dilute sprays. Insecticidal soaps will only kill insects that can be covered during application but are usually very effective against the first stage crawlers. Since insecticidal soaps can only kill insects contacted during application, treatments will need to be reapplied to kill insects that were not exposed during application, including later hatched crawlers. Insecticidal soaps can be used on edible plants (e.g., citrus, herbs). Insecticidal soaps are effective against soft-bodied pests but can cause phytotoxicity—such as leaf burn, discoloration, or wilting—especially when used on sensitive plants or applied during high temperatures or direct sunlight. Indoor plants, being shielded from extreme conditions, are generally better suited for soap applications.
Several kinds of horticultural oils are available to be used to control insects and mites on plants. These are designed to be mixed with water (1-2% concentration) and sprayed on the body of the insect. Horticultural oils primarily control insect pests by suffocation, forming a coating that blocks the spiracles—small openings on the insect’s body used for passive diffusion of oxygen. This prevents needed air-flow from reaching the insects tissues which leads to death. This physical mode of action makes oils effective against soft-bodied insects and immobile stages, such as eggs and scales, while also reducing the risk of resistance development. Commonly available horticultural oils are typically derived from either refined mineral oils or neem seed extracts.
Horticultural oils will only kill insects that can be covered thoroughly during application. Stages of the scale will remain that cannot be killed in a single spray, such as crawlers that remained protected under the cover of the mother scale or newly laid hatched eggs. To be effective for control of brown soft scale, oil sprays will need to be repeated several times. Horticultural oil products can be used on edible plants (e.g., citrus grown for fruit, herbs).
Soil-applied systemic insecticides can be used on some plants. The insecticide imidacloprid can move systemically within plants and may provide excellent control of brown soft scale. Houseplant products containing imidacloprid are applied to the soil as granules or a drench, then watered into the soil so that it can be picked up by roots. Water is essential for systemic, soil-applied insecticides to work because it moves the active ingredient into the root zone, where it can be absorbed by the plant and transported throughout its tissues.
At present (January 2025) there are two imidacloprid products labeled for indoor use on houseplants: Bonide Systemic Houseplant Insect Control (2.2% imidacloprid) and Bayer Advanced 2in1 Insect Control Plus Fertilizer (2.5% imidacloprid). Neither of these products allow use on indoor grown food crops – such as citrus being grown for fruit and herbs. Horticultural oils and insecticidal soaps remain the only options for control of brown soft scale when food crop plants are grown indoors.
Insecticide Options for Grown Plants (Outdoors) (See Table 1b)
If plants are moved outdoors during the warmer months, and are then grown as an outdoor ornamental plant, a few other insecticide options exist. These primarily include sprays of various pyrethroid insecticides, such as cypermethrin, cyfluthrin, lambda-cyhalothrin, and permethrin. Pyrethroids can persist on plant surfaces for several days, improving control of young crawler-stage scales. However, they are generally ineffective against mature scales with a protective waxy coating. Alternatively, systemic insecticides containing imidacloprid or acetamiprid can be used on outdoor-grown plants and provide effective control of all stages of brown soft scale. Label instructions for most of these products limit their use to ornamental plants and they cannot be used on plants with fruit or foliage that is edible.
Table 1a. Some insecticides that may be useful to help manage brown soft scale on houseplants when grown indoors.
| Active ingredient | Examples of Trade Names |
|---|---|
| Insecticidal soap | Bayer Advanced Natria Insecticidal Soap, Bonide Insecticidal Soap. Earth-tone Insecticidal Soap, Garden Safe Insecticidal Soap, Safer Insect Killing Soap |
| Horticultural oils (mineral, neem) | Bonide all seasons horticultural & dormant spray oil, Bonide Neem Oil Concentrate, ferti-lome Horticultural Oil Spray Concentrate Garden Safe Neem Oil Extract, ferti-lome Triple Action (plus pyrethrins), Safer EndAll Insect Killer (plus pyrethrins and insecticidal soap) |
| Imidacloprid | Bonide Systemic Houseplant Insect Control, Bayer Advanced 2in1 Insect Control Plus Fertilizer |
Table 1b. Some insecticides that may be useful to help manage brown soft scale on houseplants when grown outdoors.
| Active ingredient | Examples of Trade Names |
|---|---|
| Acetamiprid | Ortho Rose and Flower Insect Killer |
| Imidacloprid (plus cyfluthrin) | Bayer Advanced Dual Action Rose & Flower Insect Killer |
| Cyfluthrin | Bayer Advanced Rose & Flower Insect Killer |
| Cypermethrin | Garden Tech Sevin Concentrate Bug Killer |
| Lambda-cyhalothrin | Spectracide Insect Killer for Lawns and Landscapes |
| Permethrin | Bonide Eight Insect Control Vegetable, Fruit & Flower Concentrate |
Reference: CSU Fact Sheet 5.599