Scale insects – house plant diseases

March 19, 2016 at 9:04 am

Armoured scale insects, Diaspididae

Armoured scale insects and soft scale insects are pests that very often prey on indoor plants. They are recognized as small immobile scales which are visible on leaves or stems of an infested plant.

How to recognize them

Description of scale insects

Armoured scales (Diaspididae) and soft scales (Coccidae) are families of insects belonging to the scale insects superfamily (Coccoidea).

scale insect familiy

There are a few thousand insect species belonging to the armoured and soft scale families.

With scale insects it is the females that are dangerous to plants. After hatching out from eggs they start to wander looking for a place to prey on (usually the bottom of a leaf). They “settle” in the selected place and, over time, cover themselves with a shell resembling a scale, yellowish, brown or black in colour, from 1 to 5 mm in diameter (depending on the species).

The armoured and soft scale varieties are very similar to each other and it is hard for an amateur to distinguish between them. The main difference is that the shell of the armoured scales is not an integral part of the insect, therefore after cautiously removing it, one can see a bug still attached to the plant. In the case of the soft scales, removing the “shield” causes the bug to be pulled off as well, because the shield is an inseparable part of the insect.

Another characteristic which differentiates the soft and armoured scales is that the armoured scale insects usually have a harder shell and the soft scale insects leave traces of a sticky liquid on the plant (honeydew). Many of these things, however, depend on the species and, as I mentioned at the beginning, there are thousands of these.

I would quickly like to add here that differentiating between one kind and another has little practical significance in the growing of house plants. The most important thing to know is whether we are up against one of these families at all, because there is no real difference in the way we deal with them.

Coccoidea, czerwce, miseczniki, tarczniki

Scale insects (coccoidea) (one of the species).
By Wim bOwn work, CC BY 3.0, via Wikimedia Commons

Appearance of an infested plant

A plant infested by scale insects becomes weakened and its leaves can yellow and drop off. In general this process is not as rapid as in the case of, for example, the spider mite, and very often many weeks will pass before you notice that the flower has become a target for these bugs.

It is not very difficult to tell if a plant has been infested by scale insects. Look at the stems and, above all, the leaves of the plant. If you can see immobile brownish scales attached to the bottom of the leaves, which you can scrape off, then you are probably up against these bugs (remember, scales are immobile, they have no legs, feelers etc.). Apart from the scales you will also very often spot a sticky liquid on parts of the plant (honeydew). This is a very typical phenomenon in the case of a plague of soft scales.

Causes of infection

Where have the scale insects on your plant come from? Maybe you introduced them into your home on a newly bought plant, or maybe they moved over from another, already infested plant. It is worth remembering that the plants that are the most exposed to scale insect (or other bug) infestation are those which are weakened or those which are grown in unfavourable conditions.

Many house plants can be a target for scale insects. In my case the bugs most often appeared on ferns.

Scale insects on a fern's leaves

Scale insects on a fern’s leaves

How to fight them

How to fight them – the traditional method

The chemical agents used to fight the scale insects are often produced in the form of sticks to be placed in the soil. (Insecticides in spray form have to penetrate the protective shield of the insects). Under the influence of the water which we use to water the plant, the sticks dissolve and free the insect killing substance which is then absorbed by the plant together with the water. The poison circulating in the plant’s sap is ingested by the bugs and causes their death.

The sticks I use to fight scale insects

The sticks I use to fight scale insects
Photo from www.substral.pl

The big advantage of these sticks is the simplicity of their usage, especially in the case of bushy plants with many leaves (like for example ferns). To cover all of the parts of a plant properly by sprinkling it would be very difficult, whereas the agent absorbed from the soil is spread evenly inside it. In addition, it is present in the plant for many weeks, thanks to which one can be sure that the infestation will be completely eliminated.

Another plus of using these sticks is the absence of any harmful vapours circulating in the air we breathe.

Of course every rose has its thorn. The sticks do not start to work immediately. Usually you will have to wait about 3 weeks for the first effects to appear after placing them in the soil. In the case of scales this is not generally a big problem, because the process of their damaging the plant is usually quite slow.

I would like to add here that the scales on plants can remain many months after the death of the bugs, unless you scrape them off.

Insecticides are not usually without effect on people and animals, so take precautions while using them, particularly if there are children or pets in your home. You should follow the instructions that come with the treatment strictly.

Shields of dead scale insects a few months after using an insecticide

Shields of dead scale insects a few months after using an insecticide

How to fight them – natural methods

If you have a lot of patience and are a fan of using natural methods of fighting pests, you can try to remove scale insects by hand. Just scrape all the insects off with a fingernail, toothpick or sharp stick. Repeat the treatment a few times, every 1-2 weeks.

Unfortunately, simply removing the scales will probably not be enough. Because, apart from the females preying on the leaves under their shields, there are also larvae on the plant. Not only are they much smaller, but they also move around. In addition, the eggs laid earlier by the females can remain on the plant. To remove both the larvae and the eggs of the insects, many sources recommend cleaning the plant with a rag moistened with vinegar, alcohol or a solution of tepid water and soft soap.

You have to be prepared to repeat the procedure (scraping and cleaning) many times and, of course, without any guarantee of success.


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Łukasz Dumiszewski

A computer scientist who likes house plants and who once wanted to be a writer. A blogger who writes about his experiences in growing house plants by using the benefits of computer science. 😉

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