How to Prevent Scale on Plants 2022 [Causes & Treatments]


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How to Prevent Scale on Plants 2022 [Updated]

Scale insects on plant
Scales are plant insects that can often be found on the surfaces of stems or leaves. Gilles San Martin from Namur, Belgium, CC BY-SA 2.0, via Wikimedia Commons

Scale is a collective term for several types of plant insects. They seem to remain stationary, develop a shield or cocoon-like covering for protection, and live right on the surfaces of stems and leaves. Depending on the species, their protective shells may be white or brown and soft or hard. They tend to accumulate close to veins but may also appear throughout the plant, freely sucking the nutritious sap from the most accessible phloem cells.

Beneath their waxy coverings, scale insects are actually soft-bodied. Many are extremely tiny and go undetected until they are present in large colonies. Soft scale insects develop a pliable coat that is permanently attached to their bodies. This type of scale may move from place to place over the course of its life cycle. Hard or armored scale insects are not attached to their tough shells, but they usually remain stationary.

Scale insects are some of the trickiest plant pests to treat because their protective shells make them highly resistant to some insecticides. When left to colonize a plant, they can cause severe and irreversible damage. Yellowing leaves, arrested growth, and the accumulation of mold are just a few symptoms of their spread. To understand how to eliminate them effectively, it’s best to first familiarize yourself with their most vulnerable life stages and with the traits of common species.


The Life Cycle of Scale Insects

Female scale insect with egg case
Scale eggs are stored inside cottony, web-like structures, as seen here. Katja Schulz from Washington, D. C., USA, CC BY 2.0, via Wikimedia Commons

Often, the only time we can effectively detect the presence of scale insects is when they are present in their shelled forms. Unfortunately, the shelled stage of mature scale is the most challenging life stage to treat. A combination of several treatment methods, coupled with timely management, is the best way to permanently get rid of them.

Before scale insects develop their waxy armor, they start off as eggs in cottony, web-like structures. These are usually found on the undersides of leaves, along the base of petioles, and on young growths. The eggs of some species may be hidden underneath the tough armors of female insects, while others remain exposed until they hatch. Once they are born, the juveniles are referred to as “crawlers”. These are tiny enough to explore the plant and travel to nearby ones while remaining undetected.

Once the crawlers find an ideal spot in which to settle and feed, they begin to enlarge in size and develop their protective coatings. Some hard-shell females lose their limbs and can no longer venture in search of other spots if conditions become unsuitable for their survival. Their shells keep them very well-protected, however. Male crawlers remain mobile and are able to mate with females through a flap along the edge of the shell. The males may die after mating; females may likewise die after laying eggs.

In a year, there may be one or multiple life cycles of scale insects on a single plant. Odds are, plants that are infected with scale are home to several life stages at once. This means that, while there are scaled adults, it’s highly likely there are eggs and tiny crawlers as well. Hence, physical removal of the scaled adults may not successfully abolish their young. In just a few days to weeks after initial treatment, you may, unfortunately, find new masses of scale.


Detecting Scale: Common Species

Scale can be found on both indoor and outdoor plants. These insects have a knack for sneakily making their way onto the most protected plants. This is why it’s important to always be on the lookout for them. Weekly inspections of the leaves and shoots are vital to ensuring their colonies are kept in check. Early detection and treatment are key to ridding your garden of these troublesome insects. Once you spot traits of any of the common species below, you’ll need to isolate the affected plant (if possible) and work on eliminating the scale insects in a timely manner.

1) Soft scales (Coccidae)

Calico scale insects
Calico scales (pictured) are known to attract ants because they produce honeydew. R. Gill, Public domain, via Wikimedia Commons
  • Brown soft scale: usually noticeable in summer to fall; females produce yellowish-brown and rounded shells on leaves and twigs; plants may be victimized by up to 5 generations of this scale within a single year
  • Indian wax scale: Shells start off as white (with wax tufts) for young adults and become increasingly pinkish as the insect matures; these favor the branches and foliage of cultivated crops
  • Calico scale: Shells of adult females are black with white mottling (in a calico-like pattern); attract ants due to their production of honeydew; crawlers usually appear in April to May
  • Tuliptree scale: large scale insect that infests many landscape trees; eggs hatch in August to September; the spherical shells are raised and bumpy, giving them a knob-like appearance

2) Hard/armored scales (Diaspididae)

False oleander scales on leaf
False oleander scale infestations can cause leaf drop and chlorotic leaves. James Bailey / CC BY-NC 4.0
  • Tea scale: commonly found on boxwood, camellia, holly, and euonymus; eggs are deposited beneath the brown armor of females; crawlers hatch more frequently in warm conditions
  • Obscure scale: Female scales are grey and have a dirty, oyster-like appearance; usually found on the branches of both young and mature trees; the best way to manage this species is by treating its crawlers in May to June
  • False oleander scale: affects many ornamental plants in warm regions; the shells appear as raised, white specks on the surfaces of leaves; infestations lead to leaf drop and chlorotic leaves
  • Gloomy scale: distinguished by circular shells with a raised, black (like a bulls-eye) center; usually found on the bark of young branches of grapevines, maple, and other large trees

3) Giant scale (Monophlebidae)

Cottony cushion scale
Giant scales can be found in tropical regions as they prefer warmer temperatures. bythepark / CC BY 4.0

The giant scale is a large-sized scale insect, most commonly encountered on ornamental bamboo and cheesewood shrubs. They favor the warm conditions of the tropics.


4) Ground pearls (Margarodidae)

Female Porphyrophora hamelii
Ground pearls feed on root systems, which means that they usually live underground. Vahe Martirosyan, CC BY-SA 3.0, via Wikimedia Commons

Ground pearls are primitive scales that usually live underground as they feed on root systems; they are commonly associated with turfgrass. Symptoms of their infestation are often mistakenly chalked up to droughts or cold shock because they can remain undetected for prolonged periods.


Natural Prevention of Scale

Maple trees
If maple trees are grown close to each other, it’s more likely that some hard scale species will thrive, as they will have a larger surface area to colonize. James St. John / CC BY 2.0

Scale can be very tricky to prevent and treat as their various life stages give them an evolutionary edge. These allow them to survive through unfavorable environmental conditions, including extreme temperatures and the occurrence of potential predators. Even healthy plants are not totally resistant to these pests, especially if they are located close to other infested species. While it may not be possible to ensure your plant is totally immune to scale, you can at least help mitigate infestations.

First, it’s important to keep in mind that the chances of scale infestations are significantly increased when their favored plants are found in large numbers (e.g. in a monoculture). For example, if maple trees are grown close to one another, some types of hard scales are more likely to thrive due to the large surface area for colonization. Moreover, reduced biodiversity of plants leads to fewer animals that may feed on scales.


The Importance of Beneficial Insects

Common wasp
Wasps play an important role in controlling the spread of scales in the wild. Sid Mosdell / CC BY 2.0

Natural scale enemies, such as wasps and flies (known as parasitoids), are responsible for controlling their spread in the wild. These specialized insects rely on the armor of scales as sites where they can metamorphose into their winged forms. They basically feed on scale insects and hijack their protective coatings! Their presence is usually only detectable when they escape from within the scales. For parasitoids to exit the waxy coverings, they must drill an emergence hole.

Other natural predators of scale insects include spiders and mites. Even diverse fungi populations can aid in reducing the success rate of scale reproduction. Maintaining a diverse garden usually leads to having communities of predatory insects and beneficial fungi, so it is the best means of preventing scale infestations without the need for physical labor and chemical treatments.


Physical Removal of Scale

Scale insects on leaf
It may be possible to prune the affected parts of your plant if it is small- or medium-sized. jacilluch, CC BY-SA 2.0, via Wikimedia Commons

Sometimes, all of our efforts at keeping an ecologically well-balanced garden may not be enough to prevent pest infestations. A good management strategy would be to first employ physical removal techniques, especially if the scale population looks to be small or if the plant was just recently infected. For small to medium-sized plants and saplings, it may be possible to remove reproductive scale insects by simply pruning the affected parts.

Plant clippings that are infected with scale should be disposed of properly – not thrown into the compost pit. If the plant is too small to be pruned or if there are a few leftover colonies after the most infected leaves are removed, use a small swab to remove the remaining scales. Dip the swab in rubbing alcohol beforehand. When targeted directly, the protective shells should easily be lodged by the swab and any living insects underneath or around it should be killed by the alcohol.

Dead scale insects and any leftover protective shells can be removed with a microfiber cloth. This method works best for the large leaves of indoor tropical plants. Make sure to be extra gentle as you go over the fragile leaf surfaces, taking care to shake or rinse off the cloth whenever it has collected a lot of scale particles. To lower the chances of a repeat infestation, an organic treatment can be applied afterward.


Chemical Treatments

Scale insects
Chemical treatment may be the only option if you’re dealing with a massive scale infestation. Vera Buhl, CC BY-SA 3.0, via Wikimedia Commons

Insecticidal and pesticidal treatments should only be used on plants with live scale insects and with considerably-sized infestations. Closely inspect the scale beforehand; keep in mind that the protective shells may be remnants of scales that have already been dead for a long time.

If you find that the scale already have emergence holes in them, this is a sure sign that parasitoids have killed the insects and used the shells. Treating these with chemicals may do both the plants and any beneficial insects more harm than good.

While chemical treatments must be treated as a last resort, they may be the most effective means of abolishing all life stages of scale insects. They may also be the only treatment for scale in hard-to-reach areas and for massive infestations affecting medium to large plants. To increase their chances of being effective, it may be necessary to identify the type of scale on your plant.


Windows of Treatment

Scale crawler
The best time to use treatment against scales is when they’re crawlers (pictured) or during their dormant phase. Gilles San Martin / CC BY-SA 2.0

Some species are more prone to being present in multiple generations than others, so there is usually a window during which chemicals should be most effective. Those with armored and impermeable shells are less likely to be affected while they are mature. Aim to treat these during the occurrence of crawlers (molting phase) or during the dormant stage (overwintering phase). Application during the crawler phase, when most of the living scale insects are unprotected by their shells, should require fewer repeat treatments and fewer chemicals in general.

Scale insects that are present in multiple or overlapping generations over the course of a single year will likely require several treatments and regular monitoring. Insecticidal soap or a broad-spectrum pesticide may work. Nonetheless, do some research on the product and on its effect on the type of scale present. Also, look into its effects on certain types of plants. Some ornamental plants are unable to tolerate strong chemicals and should instead be treated with organic oils.


Plant-Based Scale Prevention

Rosemary oil
Plant-based oils, such as rosemary oil, can be used as a non-toxic alternative to chemical scale treatments. Formulate Health / CC BY 2.0

Horticultural oils are often the most ideal and plant-friendly treatments. Some types, particularly organic, plant-based ones, can be used as a pest prevention method even in the absence of scale. Usually derived from neem, rosemary, soybean, and other naturally pesticidal plant extracts, these oils are less harmful to beneficial insects, pets, and humans (compared to chemical insecticides).

Normally sprayed onto all surfaces of the plant for thorough coverage, plant-based oils repel scale and may suffocate any existing ones upon contact. Before systemic insecticides became widely available on the market, these were the most reliable types of pest deterrents. They can be used to repel and kill all life stages of scale, though multiple applications may likewise be necessary as hidden eggs may cause infestations to rebound.

The use of plant-based oils is better for edible plants due to their non-toxic or low-toxic profile. These tend to be safe to apply even up to the date of harvest. What’s more, these oils will also work for mealybugs, which are often found alongside scale insects.

Unfortunately, the use of horticultural oils may not be the best method for serious plant damage. They may also cause some leaves to discolor, and they may harm oil-sensitive plants. If you’re concerned about your plant’s sensitivity, treat a small area first and monitor it for any unfavorable reactions.


Long-Term Scale Management

Oystershell scales on blueberry bush
Don’t panic straight away if you do find scales on your plant — it may just be a small infestation that can naturally die back with the help of beneficial fungi and insects. SB_Johnny, CC BY-SA 3.0, via Wikimedia Commons

The presence of scale does not necessarily spell disaster for plants. It’s normal for these insects to occasionally appear at low infestation levels, after which they should naturally die back in the presence of beneficial insects and fungi. Proper location and plant maintenance are crucial to ensuring the health of both indoor and outdoor plants.

As scale can quickly colonize weak plants, long-term management of their populations involves minimizing all forms of stress for the plants themselves. The healthiest specimens are the least likely to be victimized by infestations. By ensuring your plants are provided with all of their basic needs (cultural management in the form of proper irrigation, protection from droughts and frosts, optimal sun exposure, etc.), you increase their pest resistance and their capacity to survive through unfavorable conditions.

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