Scale insect is particularly tricky to eliminate as the adults form hard waxy shells that act as a protective armour. Worse still, on indoor plants the environment allows them to continue reproducing all year round add to that the honeydew they secrete  becomes a host for sooty mould

Once established  your plants will start to lookSickly with yellow leaves often covered with a black sticky mould, this is sooty mould  and is the consequence  of scale insect infestation.

You can spray your plants with the  systemic insecticides thiacloprid but my preferred method of dealing with then is to use methylated spirits as there hard waxy shell can make then very difficult to eradicate.

Several generations and stages of scale insect can be present on indoor plants so this method deals with then from eggs through to adults

All species have a shell-like covering over their bodies when mature. The eggs are often laid under the protection of this shell but with the cushion scales (eg Pulvinaria species) the eggs are deposited outside the scale under a mass of white waxy fibres.

The adults are sedentary but newly-hatched nymphs crawl actively over the plant surface and spread the infestation.

Once completed it good practice to check on a weekly basis to make sure any you may have missed haven’t started to reinfest your plants