Coffee fruit with plant scale and black sooty mould

What are plant scale insects and what should we do about them?

Plant scale insects feed upon the sap of a plant. For this reason they are known as sap sucking insects. Whilst plant scale insects have similar characteristics, different species of scale look slightly different from each other.


What do plant scale insects look like?

Generally, scale looks like small mounds or bumps glued to particular parts of a plant. So in a way, most of them look like small barnacles.  Plant scale insects can be black or brown, shiny or dull, whitish or creamy, light green or pinkish. Some have a hard shell and some are soft and these are known as hard scale and soft scale.

Only the young of the species tends to have legs and move around. These young are called crawlers and are often found moving to the newer growth on an affected plant. The older plant scale insects tend to be stationary and live their lives sucking sap under a protective, waxy coating. In contrast, the young don’t have this protective coating and are therefore more vulnerable.

Plant scale insects can reproduce at a massive rate laying up to 1,000 eggs at a time. Therefore, in a very short period of time, a few scale can become an infestation.

The size of an adult plant scale insect ranges from about 1mm to 6mm or so in length. They are visible to the naked eye but can still be difficult to see.


Plant scale affects plants in three main ways

The plant’s stems, twigs, trunks, leaves, flowers and/or the fruit of plant are susceptible to plant scale insects.  Some scales are generalists, meaning they will attach to almost any plant and some are specialists and are plant host specific.

Scale affects a plant by feeding upon it. Scales have mouths that pierce the cells of the plant and then suck. Imagine being bitten by a mosquito on a continual basis.  Then imagine being bitten by hundreds or thousands of mosquitoes at the same time on a continual basis. That’s pretty much what scale (and other sap sucking insects) do to plants.

When hundreds or thousands of insects are all simultaneously withdrawing sap from a plant the plant is losing hydration and sugars. As a result, the plant is likely to start wilting and yellowing. So the first affect is feeding and withdrawing vital energy from the plant.


The second way plant scale affects plants

Scale insects excrete a substance called honeydew. This substance is apparently quite sweet and full of sugars. For this reason, we often find ants living in synergy with scale and other sap sucking insects. The ants protect the scale from predators and the ants feast on the honeydew.

Over time, the liquid honeydew drips down coating leaves and stems. This excess honeydew starts to go mouldy. This mould is called black sooty mould. It’s a fungus that grows and feeds upon the aged honeydew.  The leaves get coated in a black powdery substance.  This is often  the first thing we see to alert us that there is a sap sucking insect community on a plant.

Black sooty mould coats the leaves preventing sunlight from reaching the leaves.   This hinders photosynthesis and the plants ability to transform sunlight into sugars. The plant’s general healthy spirals downward from here.


The third affect of plant scale insects

The third effect that scale insects have on plants is that they help a plant to die. This perspective is born from the notion that because not all plants attract scale, those that do are in a state of distress and in the process of dying. Sap sucking insects like scale quicken this process and therefore help the plant to die.

Our society tends to shirk away from death and the dying process.  We tend to think that anything that promoted death is bad and should be avoided.  We’ve distanced ourselves from this natural process forgetting that life and death are dancing partners.

From this perspective plant scale insects are mercenary killers. The scale is ‘putting the plant out of its misery’. Just as the bee and the butterfly is killing the flower with pollination. What we do know is that healthy plants do not tend to attract or be attacked by sap sucking insects like plant scale. Sick plants tend to get attacked by sap sucking insects.


How one plant gets scale and other’s don’t

Here’s a video that shows an escalating plant scale insect infestation on one coffee plant. It also shows how other neighbouring coffee plants aren’t affected by the scale. Those two suffered less dehydration and also do not have scale.

All three plants were planted with the same compost and ingredients, in the same soil, at the same time, in the same way by the same gardener a year or two ago. The difference was the amount of water they had during a recent dry spell. The one with the scale had less water and became more dehydrated and distressed.

Therefore, we can see the cycle with plant scale insects.  Scale insects extract nutrients from a plant, their excretion forms a black sooty mould and the consequence of these to things helps the plant to die. The question is: do we intervene?


How do we get rid of scale on our plants?

The number one way to get rid of scale is to prevent the outbreak. We do that by providing optimum conditions for each and every plant in our garden.We ensure our soil is appropriately structured and fertile for the plant. You can read about that in my blog called: How to make healthy soil – transforming dirt into soil.

We also use some of the key methods of organic gardening.  I’ve discussed some methods in the blog called: How to grow plants organically using organic methods.

So in order to have a healthy garden we need to select plants that are suitable for our garden environment. We need to match the soil to the plant and ensure we have suitably fertile soil.

For example there’s no point planting chilli in a clay soil and then watering it daily. The clay is heavy and therefore holds water. Daily watering a clay soil means the soil is constantly wet and that will ensure our plants like chilli develop root rot. Root rot tends to result in death of the plant. That death will likely be met with insects such as plant scale.

However, we can plant eggplant or celery in that soil and water it daily and those plants will tend to thrive… so long as the soil has enough organic matter and is fertile.  Those plants, thriving in their suitable environment won’t tend to attract plant scale insects.

If we plant for the needs of the plant first and foremost, and place our wants second, we are more likely to have healthy plants that don’t attract scale insects.


Encouraging predator insects

If we do have scale insects on our plants and if we do have an otherwise healthy garden then it is likely we have predatory insects in or visiting our garden.  A healthy garden free of pesticides creates a habitat, a sanctuary for these insects.

We can encourage predator insects by planting a diversity of plants in our gardens. Predators also feed on the nectar and therefore flowers are a key element. Of course, in order to encourage predator insects to eat our scale (and other herbivore insects), we need to not kill the scale insects.


How some predators affect scale insects

Plant scale insects have various predators such as wasps, lady beetles, hover-fly larvae. However, I need to add here that these predators tend to prefer the softer sap sucking insects. This might be the soft scale but it most certainly includes aphids and mealy bugs.

Wasps can parasitise adult scale. They lay eggs in the body of the scale insects. The larvae grow and when sufficiently mature they start consuming the scale from the inside. It’s quite gruesome really.  A small hole in the shell of the scale is a key sign that it has been parasitised. This small hole is the exit doorway of the emerged wasp.

Lady beetles also consume scale. This includes the adult lady beetles and their babies.  Lady beetles do not all look alike and the stereotypical lady beetles is not necessarily the most common. We need to get to know what lady beetles look like. Key to this knowledge is knowing what their babies look like in their various forms. It’s truly fascinating. I’ve noticed that it’s the larvae that consume the most.

Hover-flies are one of my favourite insects. They hover right in front of you and look you right in the eyes. I love that! The adults are excellent pollinators second only to bees and their babies are impressive gorgers. The larvae look like maggots. In fact they are maggots because the adults are a fly. These maggots consume many times their own body weight as they gobble up sap sucking insects.

Our garden can provide habitat and sanctuary for these natural predators. To do so we need to ensure are gardens are healthy and free of toxins and pesticides. We also need to ensure there is ample food for them to eat. That means we should not kill their food source – plant scale insects and other sap sucking insects.


Controlling scale by human hand

If you’re not convinced by my arguments so far, you’re probably still looking for killing methods.  We can remove scale by hand by rubbing our fingers along a branch or crushing the plant scale insects.

Soapy water may work for some soft bodied scale or we can prune off the affected parts of the plant.

There are a number of insecticides we can use however in most countries the use of insecticides is based on insecticide product registration. Therefore, unless a particular product’s label states it can be used on the type of scale you have, using it is likely to be unlawful.

For that reason, I can’t inform you what chemical to use.


The effect insecticides have on scale insects

There are many effects of insecticides and you may want to read my blog on the matter.



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