WHY DOES THIS HAPPEN
If you notice many small black or brown bugs on your plants, this may be an infestation of insects called Thrips.
Thrips feed on leaf tissue - particularly young, soft tissue - using their mouthparts to consume the contents of your plant's cells. In a similar way to how mosquitoes suck up blood, Thrips feed beneath the surface of the plant, injecting enzymes that then break down the plant into a thick liquid that they then consume.
This process causes the affected area to flatten and turn a silvery-white color, as the green chlorophyll pigment in the leaves has been removed. The damaged leaf tissue may then start to turn brown as it dries out and becomes further damaged by the sun, for example.
If your plant becomes infected with thrips, you need to act fast! These little insects multiply quickly and can do great harm to your houseplant.
SYMPTOMS OF THRIPS
The first sign of thrips that you will usually notice is that your plant’s leaves will start to look a bit dull and unhealthy. You will often see:
Silvery or bronze discoloration
Black specks - fecal spots (thrips poo!)
Part of the leaves die
Be sure to check both the surface and the undersides of your plant's leaves for signs of pests - note that Thrips can also appear on stems and leaf veins, and that damage on the stems may look different to leaf tissue damage (i.e. it may show brown marks instead of silvery discoloration). Also check on any buds, leaf junctions or bends in the plant. These 'hidden' areas that are often protected from rain or wind are prime locations for pests to take over!