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 1/16th to 1/4th inch (1-5mm)

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

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

Have a special question on scale insects?

Identifying scale insects

What do scale insects look like?

They usually have an oval shape, and a hard or soft shell made from special wax. The smallest are 1/16th of an inch (1mm) but some grow as large as 1/4th inch (5mm).

  • In particular, mealybugs cover themselves in a white or pinkish velvet that gives the infestation a cottony appearance.
  • It is also possible to find brown-shelled scale insects that look like small black hulls attached to stems and/or leaves.

There are thousands of species. Each looks slightly different. Some only attack a single plant species while others feed on many different plants.

Symptoms of scale infection

  • Small spots on leaves, leaf joints, stems (especially in hidden areas)
  • Plant growth slows
  • Leaf buds and tips of branches die
  • Heavily infested leaves die and fall off
  • Leaves are gnarled, twisted, curled and stunted.

What is the usual season for scale insects to appear?

Scale insects and mealybugs don’t necessarily appear in any given season, but rather emerge when a set of conditions is met. Scale insects like heat and high humidity.

  • They can appear all year round on indoor plants.
  • They are common in spring, summer and until the beginning of fall in our gardens.
  • They cannot develop in winter outdoors, but it is nonetheless recommended to clear infested areas.

What conditions help them to reproduce?

As we’ve just mentioned, scale insects love heat and humidity.

  • They particularly like closed and humid environments, and most of all high temperatures.
  • A high relative humidity indoors or regularly spraying water on leaves will accelerate their development.
  • Also, the end of spring or the beginning or fall are seasons when temperatures are fairly high and outside air is moist.

Having scale insects on a plant will often lead to other plants being contaminated as well.

What are the risks for the plant?

Scale insects and mealybugs must be dealt with immediately because an onslaught can very well kill your plant.

  • The insects feed on the plant’s sap and weaken it.
  • This hinders proper plant development and growth, and can even lead to contamination by a black fungus called sooty mold.
  • On fruit trees, especially citrus trees, scale insects and mealybugs can compromise the entire harvest. It can even kill a whole tree if they aren’t dealt with early enough.

Often, scale insects also carry diseases from one plant to another.

  • This is particularly true for viral diseases and for fungal plant diseases.

Since some scale insects are specific to a single plant or plant family, they can be used as a form of biological control.

  • This is currently being experimented to fight certain invasive plants.

How to fight scale insects?

Preventive treatment against scale insects and mealybugs

  • For inside plants, treat against scale insects from fall until the beginning of spring, 3 to 4 times in all.

Curative treatment against scale insects and mealybugs

  • As soon as mealybugs appear, destroy their protective shell with a rag dipped in beer, 90-proof alcohol or soapy water, and then treat with scale insect spray.
  • If a major invasion is ongoing, burn or remove all infected branches.

A natural and organic scale insect spray

The best recipe is to mix following ingredients:

  • 1 quart (1 liter) of water
  • 1 teaspoon dishwashing soap
  • 1 teaspoon vegetable oil
  • and 1 teaspoon 90-proof alcohol.

Spray this solution once a day for 3 to 4 days.

  • There are also white oil-based scale insect sprays that have proven their worth.
    You can find them in specialized stores.
  • A spray based on fermented stinging nettle tea is also very efficient in case of mealybug attacks.
    It is also possible to slow their spread with a powerful blast of water or by cleaning leaves with a rag dipped in 90-proof alcohol and rinsing afterwards with water.

Take note: it is important to treat quickly, otherwise the plant will be entirely invaded and will die.

Different types of scale insects

Since treatments are very similar, usually only the general term “scale” is used. But there are actually three major families of scale insects.

  • One of these is the famous mealybug. This is the white one that weaves tiny cottony cocoons near where leaves are attached to stems.
  • The other two are called scale insects, with the following distinction: armored scale and soft scale, referring to whether the shell is hard or soft.
  • Apart from these three families, there are 17 more within the scale insect “Coccoidea superfamily” which totals over 7,000 species.
  • There are many more species which haven’t yet been described, though!
_
Main families of Scale insects
Soft scaleMealybugsArmored scale
CoccidaePseudococcidaeDiaspididae
Over 1,000 described species
Nearly 2,000 described species
Nearly 2,400 described species
A female soft scale insect on a leaf.A single mealybug insect on a leafThree armored scale insects on a leaf

Plants often attacked by scale insects and mealybugs

Most indoor plants such as ficus tree or hibiscus, orchid, rose, and even succulent-like plants such as Zamioculcas and Aloe vera can contract this pest.

Outdoor plants are also attacked, including citrus, lime tree (or basswood), hortensia, oleander, lilac, olive tree, mulberry tree, red currant, peach tree, and also, albeit less often, the apple tree.

  • Note that citrus are particularly vulnerable to scale insects.

An efficient and timely treatment against scale insects and mealybugs will help contain spreading to other plants in your house or garden.

Smart tip about scale insects

A regular inspection of your houseplants and garden will help you catch infestations at the start. It’s very easy to contain them early on.


Credits for images shared to Nature & Garden (all edits by Gaspard Lorthiois):
Scale infesting branch by Scot Nelson under Public Domain
Brown soft wax scale by Kim & Forest Starr under © CC BY 4.0
Long-tailed mealybug on leaf by Robert Briggs under © CC BY-SA 4.0
Yellow hard wax scales by Katja Schulz under © CC BY 2.0