Scale insects are a diverse group of mostly small external parasites of plants, feedings on sap drawn directly from the plants vascular system. They are a very serious pest and can kill a plant or prevent it from growing. Scale insects can also be vectors of a number of diseases. This group of insects thrive in warm and humid conditions.

There are thousands of species of scale insects known, divided into numerous groups based on appearance, etc. There are three main groups: The hard armored, the soft armored, and the un-armored or mealy bugs, scale insects. Of interest to orchid growers are the “soft Scale” (Coccidae) and the mealy bugs (Pseudo-coccidae).

Soft scales and mealy bags excrete large amounts of honeydew which not only attracts ants - Ants protect the scale insects from predators and parasites and in return collect honeydew – but also provides an excellent medium for the growth of a black fungus called “sooty mould”. Sooty mould lives on the honeydew the scales excreted.

Coccidae are commonly known as soft scales, wax scales or tortoise scales. The soft scales differ from the armored scales in that they do not secrete a waxy covering that is separate from the body. Soft scales are larger (2 - 6 mm). Females have a flat to almost spherical body, which may be covered with wax. In some genera they possess legs but in others, they do not. Antennae may be shortened or missing. Soft scales may reproduce sexually or asexually (partheno-genetically). Females can produce progeny without fertilization and either lay eggs or give live birth, depending on the species.

Mealy bugs are insects in the family Pseudococcidae.

Control: Prevention is the best cure. Monitoring weekly throughout the year is a good start. Carefully check the undersides of leaves and stems for scales. Stop the ants from entering the area you keep your orchids. Over fertilizing helps the scales, especially Nitrogen (survive better on new growth and lay more eggs). Encourage natural predators such as ladybirds, green lacewings, certain wasps, etc. Good air circulations also help.

Insecticides: There are many “oil” based insecticides labeled for scale insect control, make sure the one you use does not harm orchids.

Oil based insecticides are “contact” insecticides and do not penetrate the "cover" of the scales. Therefore, spraying should take place when the scales are going through their “Crawler stage”. That means, the infested orchids must be sprayed at least a couple of times, two weeks apart.

It must be clear that although insecticides will kill the scales, the scales will not drop and must be “brushed off” by hand. Make sure the scales are dead, dead scales are dry. I use a hard toothbrush and slowly but systematically brush of the scales from the leaves and branches. It is impossible to remove them all.

There are good systemic insecticides that also kill scale insects. Check with your local supplier.

Some people are using diluted dish washing liquid successfully. It is harmless, cheap and it has an extra benefit, it also removes “sooty mould”.

Armored Scale Insects:  Very small in size (1 to 3 mm). The body is protected by a cover (the armor) made from wax secreted by the insect and cast skins of previous growth stages. This cover also protects the eggs laid by the female. Most armored scale insects reproduce sexually. The eggs hatch under the protective cover and the “crawlers", migrate to the new growth to settle and feed. Armored scale females lose their legs at the first molt and are sessile for the rest of their lives.

   Young cattleya with "Soft scale"                Same plant showing: "Soft scale"

                                                           and "Boisduval scale" (bottom center)

Dr. George Tsambourakis

Why "oil" based pesticides are used? Because the "oil" suffocates the scales