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Scale Bug – How To Control Plant Scale

Scale Bug – How To Control Plant Scale


By: Nikki Tilley, Author of The Bulb-o-licious Garden

Scale is a problem with many houseplants. Scale insects suck sap from plants, robbing them of essential nutrients. Let’s learn more about identifying scale and how to control them.

Identifying Scale Plant Insect

Scale insects thrive in warm, dry environments. The scale bug is small, oval and flat, with a protective tan to brown shell-like covering (scale). Scale generally targets the undersides of leaves and around leaf joints.

The scale plant insect consists of three types:

  • armored scale
  • soft scale
  • mealybug

Scales, both armored and soft, are the most destructive. Armored scales are more difficult to control once mature. Soft scale bugs excrete large amounts of honeydew, which encourages the growth of sooty mold, a black-colored fungus that interferes with photosynthesis. Mealybugs are easier to control. Scales cannot fly so, dispersal depends on the movement of crawlers. Crawlers may be detected by placing double-sticky tape on plant branches.

Scale Insect Control

Scale-damaged plants look withered and sickly. Leaves turn yellow and may drop from the plant. They may also have sticky sap or a black fungus on the leaves and stems. Heavily infested plants produce little new growth. If scale insects are not controlled, death of infested plants is possible. Scale insects are invasive and will infest other plants, so move infested plants away from healthy ones.

Several well-known remedies can be used to eliminate scales from a houseplant. However, there is no easy cure for a scale bug infestation. One possibility is to pick off or gently scrub them loose from the leaves and stems. Dabbing each scale with an alcohol-soaked cotton swab is another possibility for lightly infested plants.

There are also numerous chemical products available for the control of scale bugs. Insecticide sprays, like neem oil, are available at garden centers. Spray applications should be timed to coincide with the crawler stage, which is most susceptible to insecticides. Insecticides must be applied thoroughly each week for a month or more for the greatest results.

For heavy infestations, it is sometimes best to throw away infested plants.

Homemade Control of Plant Scale

Many people prefer to use homemade control of plant scale. Insecticidal soap is a safe and effective alternative to conventional insecticides. You can use bleach-free dishwashing liquid (1 1/2 teaspoons per quart or 7 mL per liter of water) in place of commercial insecticide soaps. Homemade control of plant scale can also be achieved with oil spray. Mix 2 tablespoons (29.5 mL) of cooking oil and 2 tablespoons (29.5 mL) of baby shampoo in 1 gallon (1 L) of water. This can also be mixed with 1 cup (236.5 mL) of alcohol to help penetrate the insect’s shell.

If a fungus is also present, add 2 tablespoons (29.5 mL) of baking soda. Shake well before and during application. Spray every five to seven days as needed, covering both sides of the foliage. Wash the leaves individually with the soap/oil mixture and rinse well.

BEFORE USING ANY HOMEMADE MIX: It should be noted that anytime you use a home mix, you should always test it out on a small portion of the plant first to make sure that it will not harm the plant. Do not spray on hairy or waxy-leaved plants. Also, avoid using any bleach-based soaps or detergents on plants since this can be harmful to them. In addition, it is important that a home mixture never be applied to any plant on a hot or brightly sunny day, as this will quickly lead to burning of the plant and its ultimate demise.

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Scale insects – techniques and organic treatments to avoid them

Scale insects like mealybugs are among the most common parasites on indoor plants.

They can also be found in the garden at the end of spring, during summer, and in the fall.

Quick scale insect facts

NameCoccidae, Pseudococcidae, Diaspididae
Lifespan – up to a year (dormant in winter)
Size – from 1/16 th to 1/4 th inch (1-5mm)

Danger to plant – fatal if infested
Contagious – often
Main treatment – fermented nettle or oil/soap/alcohol mix

The damage they inflict is variable, as are also the shapes they can take to hide.

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Inspection

Inspection is the next step in a treatment program. Once you know what your potential pest looks like, you will want to inspect your plants for signs of activity.

Where To Inspect

Depending on the species, scales will feed on trees, shrubs, or even house plants. On herbaceous plants, you can often find scales settling along stems or leaf veins since those are the biggest sources of moisture on the plant. You can also find scales on fruits or underneath bark.

What To Look For

Be on the lookout for common signs of scale infestation on plants. This will include yellow, dying leaves, honeydew drippings, sooty mold from rotting honeydew, warped or stunted plant growth, and even the scale insects themselves. Some types of scales, like mealybugs, will produce a cotton-like webbing which will make them easier to spot.


Scale insects

Scale insects are sap-sucking insects that gradually weaken plants. They are so called because the actual insect is covered by a hard protective shell or scale.

Description

Scale insects are like little lumps attached usually to stems or the underside of leaves around the main veins. They come in different colours, usually brown but some species are greyish white. They vary in size, anything from 1mm to 6mm.

They don’t lead the most exciting of lives, but therein lies their strength. A newly born scale will make a small effort to crawl a few millimetres to find somewhere to live and will then stay put for the rest of its life laying the eggs for the next generation in the same spot. But this immobility and the thickness of their scale makes them difficult to control and resistant to contact insecticides.

Symptoms

Scale insects are frequently easy to see as the colonies cluster openly on stems, but a little less so if they are feeding on the underside of leaves. Infected leaves may turn yellow or become otherwise discoloured. Heavily infected plants will gradually lose their vigour.

Scale insects, like most other sap feeders, excrete a sugary honeydew that drops onto the foliage below the colony. Such foliage initially develops a sticky sheen, but this can rapidly become home to black sooty moulds.

Treatment and control

Because the hard scale is impervious to contact insecticides, the best control can be achieved using a systemic insecticide. Spraying is often more effective against the newly hatched nymphs in summer, than the shell-covered adults. Small colonies and individual scale insects can be controlled by simply wiping them off the plant with a damp cloth or sponge.


Armored Scales

Armored scales (family Diaspididae) are the smallest scales, measuring 1/25 to 11/100 inch (a few millimeters). They have a hardened waxy coating that covers the insect like armor. This covering can be removed or lifted, exposing the insect underneath (figure 8). Males and females often have different scale coverings (figure 11), which can lead a person to think that two different species of armored scales are present.

Figure 7. Brown soft scale (photo by Chazz Hesselein, Bugwood.org) Figure 8. The covering of armored scales can be removed, exposing the insect underneath.

Armored scales have three basic cover shapes: rounded (figure 9), pupillarial (figure 10), and oystershell (figure 11). As the insect molts, the covering for each consecutive instar (life stage) is added to the previous covering (figure 9). This can help you to estimate the age of the armored scales on the plant.

Besides their hardened coating, armored scales differ from soft scales in several ways:

  • They typically reproduce sexually.
  • Females generally produce only one generation per year, consisting of a small number of eggs (a dozen or fewer).
  • They feed directly on plant cells and xylem (tissues that conduct water), so there is no sugary honeydew or sooty mold produced.
  • They cause plants to decline slowly. Symptoms of infested plants include canopy thinning. Thinning is often mistaken for plant stress, since the scale insects are smaller and blend into the bark. Most are found on woody trees and shrubs, but a few are known to attack grasses.
Figure 9. This armored scale has completed all three of the nymphal instars. The wax covering of armored scales leaves a record of insect development. Figure 10. Euonymus scale has a pupillarial scale form. Males in this scale are white and females are brown. Figure 11. Male armored scales often outnumber female scales. In this species, males are elongated and smaller, where females are oystershell shaped.

The most common species of armored scales in urban landscapes are tea scale on camellias and hollies (figure 12), false oleander scale on southern magnolia (figure 13), obscure scale on oak trees (figure 14), and gloomy scale on maples (figure 15).

Figure 12. Tea scale on camellia Figure 13. False oleander scale on southern magnolia Figure 14. Obscure scale on oak trees (photo by William Fountain, Bugwood.org)

Brown Soft Scale – A Common Insect Pest of Indoor Plants – 5.599

Quick Facts…

  • Brown soft scale is 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.

Figure 1: Brown soft scale, in mixed stages, on the underside of a leaf.

Figure 2: Brown soft scale on a stem of citrus. Most are adults and they have a shiny covering of honeydew.

Figure 3: Honeydew, excreted by brown soft scale, which has collected on a nearby leaf.

Figure 4: Soft brown scale in mixed stages. Larger, darker scales are adults. A tiny first stage nymph (crawler) may be seen in the upper, center-left.

Figure 5: An adult brown soft scale is present on the 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.

Brown soft scale (Coccus hesperidum) is the most important scale insect that occurs on indoor grown plants in Colorado. When feeding it 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 most commonly infested in Colorado include citrus, Schefflera, Ficus, English ivy, and bay leaf.

Description and Habits

Brown soft scale is found on both stems and leaves. When full-grown it is about 4-5mm long (ca. 1/8-in) and normally has an oval body form, although shape can vary depending on the site of the plant where it develops. The body is quite flat, appearing only slightly domed when viewed from the side. Color ranges from yellow-green to yellow-brown and scales may be mottled with some brown spots. Older scales usually become darker brown.

Only females are produced. When mature, the mother scale produces eggs that hatch within her body, producing very tiny (0.4mm) first stage scales known as crawlers. Crawlers may live for a few days under the protective cover of the mother scale, but later move to colonize other areas of the plant. Stems of plants are usually favored sites where brown soft scales will settle, but they also can occur on leaves. The crawler period is the most mobile stage of the insect and is the stage that would infest new plants. Crawlers can be moved and carried while handling infested plants and may be able to be blown short distances.

After the crawler has settled it begins to feed with its piercing-sucking mouthparts. These 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.

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.

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 honeydew they produce while feeding provides an excellent means to easily detect the presence of this insect.

The honeydew produced by brown soft scale may pool next to the insect. Often, the honeydew is ejected, sometimes over an inch or more away from the body, and ultimately lands on a leaf below or aside the insect. By regularly checking plants every couple of weeks, early infestations can be recognized by the presence of the honeydew this insect produces.

Monitoring for honeydew can also be useful to determine if insecticides have been effective. Since the scale insects will often remain attached to the plant after death, the detection of honeydew can be evidence that the insect is alive and continuing to feed.

(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 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 they honeydew can be removed.

Insecticide Options (Indoors). There are several insecticides that can be used to control brown soft scale (Table 1).

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).

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 kill the insect by smothering, blocking the small openings through which it breathes. Commonly available horticultural oils are either mineral oils or neem seed oils.

Horticultural oils will only kill insects that can be covered 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.

At present (January 2019) 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 Outdoor Grown Plants. 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. These have ability to persist on the plant for several days, providing better ability to kill the young crawler stages of the scale. However, pyrethroid insecticides usually cannot control older scales that have developed a protective waxy cover. Alternately, sprays of certain systemic insecticides, containing imidacloprid or acetamiprid as the active ingredient, can also be used on outdoor grown plants, and these can also provide good 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 1. Some insecticides that may be useful to help manage brown soft scale on houseplants when grown indoors and when grown outdoors.

Allowed for use on indoor grown plants

Only allowed for use on plants when grown outdoors

1 Partial list of trade names, based on products found displayed on retail shelves in Colorado during a 2018 survey. Other unlisted products may exist that can also be used. Always check the label instructions when using any pesticide. All pesticides can only be used in a manner that is consistent with the use directions of the pesticide label.


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