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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. Throughout my adult life, gardening has been my passion, therapy, drive and source of purpose. Even as a child I had an intrinsic interest in plants and a desire to understand what makes them grow.

I distinctly remember the moment this began - my family was on one of our regular road trips from Hervey Bay; Australia. We were driving past a field of sugar cane. Dad pulled the car over and we cut a couple of sugar cane stems and brought them home for a treat. To be honest, I didn’t really like the taste, but I did want to try and grow it; and that is exactly what I did. It was then that my fascination, interest and passion for gardening and understanding plants began.

Fast forward a few years and I studied biological sciences and began what would be a 36 year career as a Biology educator. From this, I don’t only love gardening, but I also love helping others learn about gardening. I am also always looking for new ways to develop my own gardening knowledge. I like to think I am truly a life-long learner.

Fundamental to my beliefs about education is that learning is often best done as a part of a community - learning from others, and helping others to learn. It is this type of community that I hope iCultivate will be for its members - a community of gardeners, keen to share their gardening knowledge and wanting to learn about new ways to garden - a community built on the love of gardening.

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