Hard scale insect
Hard scale insects are related to the aphid, but unfortunately, much harder to control. Because hard scales have a wax shield around them, they are well protected from pesticides. Fortunately, there are a number of tricks one can use to get rid of hard scales.
Damage to the plant
Like the aphid, hard scales bite into the plant's veins to absorb its nutrients. This leaves the plant lacking nutrition, causing its leaves to turn yellow and eventually fall off. While hard scales are usually not deadly - causing poor growth and reduced vigor - large infestations can overwhelm plants and cause them to die.
Many pesticides have no effect on the hard scale insect because of its protective shield. Nevertheless, there are a number of methods one can use to put a stop to a hard scale infection. If you have a smaller plant, you can simply submerge it in water for 15-20 minutes. This drowns the hard scales, causing them to fall off the plant. Another method is to soak a cotton ball with alcohol (makes sure it's not sweet alcohol, as this will cause the plant to mould) and then place it over the hard scales. After 10-20 seconds you can easily be able to remove the hard scales from the plant's leaves. If none of this works, take a plant sprayer and mix 20ml kitchen oil, 1 tablespoon of dish soap and 250ml of water. Shake it well and add another 2 litres of water to the bottle. After shaking it again, spray it over the hard scales every week until the pest has disappeared.
Hard scales, like aphids, are also difficult to prevent. They move by hitchhiking on people or pets, or through other plants. A plant that is in the direct way of a draft can also be easily infected with hard scales.
Because hard scales are difficult to prevent, it is important that you check your plant regularly for hard scales (and other diseases). To find them, hard scales look like brown spots and are often on the bottom of plant's leaves.