Controlling Scale Insects and Mealybugs

Scale insects

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

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)

They can also be found in the garden at the end of spring, during summer, and in fall. The damage they inflict is variable, as are also the shapes they take to hide.

→ Delve deeper: Nature & Garden pages about scale

Is my plant infected with scale insects?

Scale insects usually have an oval shape, and a hard or soft waxy shell. In particular, mealybugs are covered in white or pinkish fuzz. They look a bit like cotton, snow or ashes.

  • There are also brown-shelled scale insects that look like black spots on stems and/or leaves.

Symptoms of scale infection

Basic symptoms you’ll notice when scale starts spreading on your plants include:

  • Symptoms of a scale insect infection, here on a kaki treeSmall spots on leaves, leaf joints, stems (especially in hidden areas)
  • Plant growth slows
  • Buds and tips of branches die
  • Heavily infested leaves fall off
  • Leaves are gnarled, twisted, curled and stunted.
  • And of course, the plant is overrun with a) small moving bugs and b) fixed spots with bugs underneath them

When do scale insects usually appear?

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

  • They can appear all year round on indoor plants.
  • Scale is most common in spring, summer and the beginning of fall. They infect all sorts of shrubs.
  • They cannot develop in winter outdoors, but it is nonetheless best to clear infested areas.

What conditions trigger scale?

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.

Scale insects on a plant will often quickly contaminate nearby plants, too.

Will the plant die?

Scale insects and mealybugs must be dealt with immediately because a heavy onslaught can kill your plant.

  • These insects feed on the plant’s sap and weaken it.
  • This hinders proper plant development and growth. It also leads to contamination by a black fungus called sooty mold.
  • On fruit trees, especially citrus, scale insects and mealybugs can destroy entire harvests. It can even kill a whole tree if not dealt with early enough.

Did you know? Some scale insect species are specific to a single plant or plant family. These are being investigated for use as a form of biological control against certain invasive plants.

How to fight scale insects?

You can control scale insects through different means. 100% organic and natural solutions exist and can be very effective.

  • For outside plants, attract beneficial insects: they’ll keep outbreaks from turning into full-blown invasions. If a shrub is overrun, remove and burn all infected branches.
  • Indoors, 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.

A natural and organic scale insect spray recipe

The best recipe contains these ingredients:

  • 1 quart (1 liter) of water and and one teaspoon each black soap (or dishwashing soap), vegetable oil, 90-proof alcohol.
  • Natural spray against scaleSpray 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 (as you’d do for thrips) 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 pests will completely overrun the plant and it will die.

Plants often attacked by scale insects and mealybugs

Most indoor plants such as the ficus tree or hibiscus, orchid, rose, and even succulent plants like Zamioculcas and Aloe vera can become victims.

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

  • Note that all citrus species are particularly vulnerable to scale insects.

An efficient and timely treatment against scale insects and mealybugs will protect other plants in your house or garden.

Different types of scale insects

There are three major families of scale insects. The treatments above work well on all of them because they can break through the scale’s protective shells and fuzz.

Mealybug scale1 / Mealybug (Pseudococcidae): this is the most common.

  • It is fuzzy white and weaves tiny cottony cocoons near where leaves join stems.
  • Its fuzz makes it harder to eat for predators like ladybug and lacewing.
  • There are nearly 2,000 different species.

Armored scale insects2 / Armored scale (Diaspididae):

  • This one protects itself with a rather hard shell.
  • Once in place, it won’t move around.
  • Almost 2,400 species have been identified.
  • Oil-based sprays still work through the shell, but not insecticides.

Soft scale insect3 / Soft scale (Coccidae):

  • Like armored scale, it stays put under a shell.
  • The shell is  softer, it won’t break with a crisp sound.
  • Over 1,000 of these have been documented.
  • Same: oil sprays work, but not water-based treatments.

There are 17 smaller families within the scale insect “Coccoidea superfamily“, totaling over 7,000 species.

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.

Images: adobestock: praphab144, CC BY 2.0: coniferconifer, Katja Schulz, CC BY-SA 4.0: Robert Briggs; Pixabay: Sandeep Handa; Public Domain: Scot Nelson, CC BY 4.0: Kim & Forest Starr