Soft Scale

Soft scale insects are easy to miss as they suck the life out of your plants. Like their more heavily armored cousins, soft scale insects spend their adult lives protected under a dome.

Brown soft scale (Christian Ferrer) CC BY-SA 4.0

The only real difference between armored and soft scales is that the bodies of armored scale insects are not attached to the dome, whereas soft scale insects are firmly attached. Soft scale domes are usually made of a more cottony, waxy, or powdery material, than the hard plating of armored scale. There are other types of scale insects, but we will leave those for another day.

Types of soft scale insects

There are hundreds of types of soft scale. These are the most common soft scale insects found in California gardens and landscapes:

• Black scale (Saissetia oleae)
• Brown soft scale (Coccus hesperidum)
• Citricola scale (Coccus pseudomagnoliarum)
• Cottony cushion scale (Icerya purchasi)
• Hemispherical scale (Saissetia coffeae)
• European fruit lecanium (Parthenolecanium corni)
• Kuno scale (Eulecanium kunoensis)
• Tuliptree scale (Toumeyella liriodendri)

Soft scales are so successful that several other insects mimic their appearance. These copycats include certain species of aphids, psyllids, mealybugs, and whitefly nymphs. There is even a fungus, called Diamond scale, that copies this successful arrangement.

Soft scale lifecycle

Soft scale has one of nature’s stranger life cycles. One thing that makes soft scales so interesting is that they are mobile only as infants. The initial development stage, or instar, has functional legs, whereas the adults are attached to the inside of a shell, where they feed and lay eggs pretty continuously, until the weather gets too cold or the host plant dies.

​In the photo above, you can see a large number of established hemispherical soft scales. They suck sap from the twigs and leaves of the plant and then excrete a sugary honeydew that ants love. In fact, heavy ant traffic was what notified me of this very subtle, but potentially deadly infestation. As it was, I had to remove several branches.

The problem with infestations such as these is that the excreted honeydew provides a perfect growth medium for fungal diseases, such as sooty mold, and bacterial disease. As ants collect the honeydew and move from plant to plant, they can quickly spread disease throughout your garden.

Symptoms of soft scale infestation

One of the more inconspicuous insects, soft scale can build up a big infestation without anyone noticing. At first, you may simply see what looks like water-stress, leaves may turn yellow and drop earlier than normal, or leaves may die and stay attached.

To avoid the use of chemical treatment against soft scale, regularly monitoring woody plants and stripping any visible vermin away from twigs and leaves by hand are your best bet. You can also use a sticky barrier around the trunks of trees and shrubs to prevent the mobile first instars from establishing themselves on your plants and removing the protection provides by ants.