There are hundreds of different scale species, but one thing that they all have in common is that they are all sap-sucking insects. Generally, there are two classifications of scale insects that commonly infect plants in their home garden. These are:

  • Hard scales which belong to the Diaspididae family
  • Soft scales which belong to the Coccidae family.

While there are others, these are the most common.

Each of these is easily identified by either the presence or absence of a waxy cover. Hard scales produce a waxy cover under which they feed, while soft scales don’t have a waxy cover, but they do tend to have a protective covering of ‘mealy’ secretions. Soft scales also produce large amounts of honeydew which also accounts for their close association with ants; unlike hard scales.

How to Treat Scale Insects

Most scale insects excrete honeydew; a liquid containing concentrated plant sugars.This honeydew promotes the growth of sooty moulds which make the plant look unsightly and can even affect the plant’s photosynthesis. Ultimately, this can lead to a range of plant health issues. Honeydew also attracts ants.

There is a mutualistic relationship between the scale insect and ants - the scale provide honeydew; a food source for the ants and the ant provide some protection to the scale insect from predators - they both benefit from this arrangement.

Treating scale insects requires an understanding of this relationship.

Scale management can involve the following:

  • Prevent ants from climbing into the plant. This will allow the natural predators of the Scale Insects to predate unimpeded on the scale. A band of horticultural glue around the base of the plant will help with this.
Horticultural Glue
  • the application of a horticultural oil. These sprays suffocate small insects such as Scale Insects. A day or so later, all you need to do is hose them off. This method is also effective against hard scales that don’t produce honeydew and have the relationship with ants.

Something to be very mindful of is that some plants are sensitive to oil-based sprays, so please ensure that you do a test spray on a small area first.

What Plants are affected by Scale Insects

There are many different types of scale insects affecting many different types of plants. The following are some examples:

  • Red Scale affects all varieties of citrus and other fruits including fig, olive and grape. They also affect many ornamental plants such as roses.
  • White Louse Scale affects citrus.
  • Soft Brown Scale commonly affects citrus, camellia, ferns, fig, and orchids.
  • Wax Scales affect gardenias, avocado, citrus and lilly pilly, mango.
  • Cottony Cushion Scale affect citrus, grevillea, hakea, and roses.

Symptoms of Scale Insects

  • Honeydew on upper surfaces of leaves.
  • Sooty mould on upper surfaces of leaves.
  • Ants climbing plants and congregating around the scale - feeding on honeydew.
Sooty Mould

How to Prevent Scale Insects Appearing

Monitor

  • Pay attention to the appearance of sooty mould.
  • Notice ants climbing into plants and concentrating around scale insects on the leaves.
  • Look for Scale Insects on leaves, branches and fruit.

Prevent

  • Use horticultural glue to prevent ants climbing up the plant.
  • Manually remove small scale infestations.
  • Prune off heavy infestations.
  • Encourage the presence of natural predators and parasites.

Natural enemies

  • Predators include ladybirds parasitic wasps and lacewing larvae.
About the author: Steve McLean
I am an educator and passionate gardener and traveler. I love helping others to grow productive gardens and am always looking for new ways to develop my own gardening knowledge. I believe that by working together with gardening as a common theme for us all, the world will be a better place.

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