Citrus Scale: How To Control These Sap Sucking Pests

Have you noticed small black or reddish brown lumps on the leaves or branches on your citrus tree? Or maybe you’ve seen ants travelling along the branches to feed on a sticky substance. You might have even noticed some sooty mould on your tree or fruit. These can be signs of citrus scale. Find out how to control scale insects and keep your citrus trees healthy and thriving.

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What is citrus scale?

Citrus scale pests are small, sap-sucking insects that attach to the leaves or stems of citrus trees. They can be soft shelled or covered in a hard, waxy coating (armoured). They vary in colour and can be black, brown, red, pink or white.

There are many different species of scale that can affect citrus trees. In the US and Australia, hard scales include Californian Red Scale, Citrus Snow Scale and Black Scale. Soft Scales include Soft Brown Scale and Cottony Cushion Scale.

Cottony cushion scale on citrus — Cottony Cushion Scale. Photo: Daniela Korn

Armoured female scale insects don’t seem to have any wings or legs and have a waxy outer. They don’t need to mate to reproduce. A female scale insect will lay eggs or give birth to tiny live ‘crawlers’ which are laid into the body or under the hard shell. After a few days these crawlers will find a suitable spot on citrus leaves, twigs or fruit to settle. Citrus scale attach to the plant using their piercing mouthpiece. Most hard scale species will stay in the same place, feeding and moulting their outer shall as they grow, while soft scales can move around until they lay eggs.

Scale can affect all types of citrus trees including lemon, lime, orange, mandarin and grapefruit.

Citrus scale on a leaf — Citrus scale. Photo: Andrew Waugh

How to tell if your citrus tree has a scale

To date I have only had a some black scale on one of my mandarin trees, which snuck in on a plant I was gifted by a friend. It was very obvious as there were large adult black scale insects on the underside of some of the leaves. Fortunately I noticed quickly and was able to get the infestation under control before it spread or affected any of my other citrus trees.

The first sign of citrus scale on your tree might not be the insects themselves, especially if you have a soft scale.

You could notice mottling, yellowing and curling of the leaves or sooty mold (or mould) on the leaves, Sooty mould is a type of fungus that forms on sweet honeydew secreted by some scale insects. Another sign that you could have a scale problem is ants. Ants love to feed on the honeydew secretions.

sooty mould on an orange tree leaf — Sooty mold on an orange tree leaf – Photo: Hans Joachim

If you notice any of the signs, you’ll need to carefully look at the underside of the leaves and the branches of your citrus tree for scale insects.

Problems caused by scale

Citrus scale will not necessarily cause a lot of damage to your citrus tree or fruit.

If you have a heavy infestation of citrus scale, your tree leaves may start to turn yellow and drop. A lot of sooty mould can interfere with photosynthesis and affect the health of your tree and how much fruit it produces. It’s also very unsightly.

Unhealthy trees can also be more vulnerable to other citrus pests. Citrus scale can also easily spread to other citrus trees, or other fruit producing trees as well.

How to control citrus scale

There are a few different ways you can control citrus scale on your trees. And you don’t have to resort to chemical insecticides.

Scale pests are usually more active in Autumn (Fall) and Spring and less active in Winter. Make a regular habit of spot checking your citrus regularly during these times. Choose 5 to 10 leaves or twigs and inspect thoroughly for tell tale signs of hard or soft scale insects. That way you can control an infestation early before it causes much damage to your tree.

Physically remove scale

If you have a dwarf or potted citrus tree, and only small number of scale, you might be able to just remove it by hand. Physical ways to manage citrus scale include:

gently scraping off hard scale with your finger nail;
using a soft toothbrush, cotton bud/q-tip or gentle facial sponge soaked in soapy water.

Beneficial pests

Naturally occurring beneficial insects like parasitic wasps are natural enemies of scale. Encouraging beneficial insects in your garden *(and avoiding insecticides) can help keep the natural balance. Commercial citrus growers sometimes use tiny parasitic wasps from the Aphytis species to target citrus scale eggs and crawlers and control scale. These can only be used if no insecticides have been sprayed on the trees.

Avoid using insecticide sprays if possible

If you do decide to spray your citrus trees that have scale, please think carefully about using an insecticide. Insecticide can also kill the good insects that pollinate your flowers and help keep pests under control naturally.

If you need to spray

For a heavy infestation of citrus scale, you could spray with a horticultural oil. The oil will smother and suffocate the scale crawlers and mature adults. Oil can still affect the beneficial insects too, so use with care.

When using an oil treatment, make sure to spray your citrus trees in the early morning or early in the evening to minimise harm to beneficial insects. Don’t spray when the weather is too hot, windy or when it’s going to rain.

Disclaimer: These notes about spraying are for general information only. If you have any questions about whether a product is suitable for your needs, speak to your local garden centre or contact the manufacturer. Please also make sure to read and carefully follow any instructions on the labels of citrus pest sprays.

Try a homemade alternative to chemical sprays

Instead of buying a commercial horticultural oil, you can easily make your own at home with:

half cup of vegetable oil or neem oil;
quarter of a cup of dishwashing liquid detergent; and
5 litres of water.

homemade citrus leafminer spray in trigger spray bottle in front of mandarine orange tree

If you don’t have a 5 litre container, you can just mix the oil and detergent in a lidded jar to make a concentrate. Use a marker on the jar to divide the concentrate into 5 equal amounts. When you are ready to use it, shake well to mix, and add 1/5th of the jar to 1 litre of water.

For a small or potted tree, fill up a spray bottle and spray the leaves of your citrus tree. Ensure you spray the front and back of the leaves. To spray a larger citrus tree, you’ll need a spray pack.

Spray your tree twice, about 5-7 days apart, to make sure you haven’t missed any scale. Again, only spray in the early morning or evening to minimise spraying beneficial bugs. And don’t spray when the weather is too hot or windy as you will burn your trees.

TIP: Label your spray bottle or pack so you know that it contains pest oil.

Preventing citrus scale problems

Keeping your citrus trees healthy and happy goes a long way to preventing pests like citrus scale. Unhealthy or weak citrus trees are more prone to attack from pests in general. Be sure to look after your citrus trees by feeding and watering regularly.

Another important step is to keep ants under control. Ants can be attracted to trees with citrus scale by the sweet honeydew secretions. They also tend to defend citrus scale to protect their food source. Ants can also spread scale insects to other trees and plants. Removing the ants (manually with water or a soap and water mix) can help stop the spread of scale. And the reverse is also true, as controlling citrus scale also helps to reduce ants.

Encourage beneficial insects in your garden, as they help keep many citrus pests at bay and reduce the need for spraying.

Make a habit of regularly inspecting your citrus trees for any unwanted pests like citrus scale. The sooner you find a problem, the sooner you can take steps to bring it under control and reduce any damage to your trees or fruit.