Thrips are tiny black insects which fly or jump from leaf to leaf, causing tell-tale silvery streaks.
Thrips cause the worst damage on flowers which become spotted and distorted. They also cause stunted growth.
Many houseplants are affected by thrips but Begonia, Codiaeum and Fuchsia are highly susceptible to Thrips.
If the houseplant has delicate foliage, turn the plant upside down and dip the foliage in a bucket of water to wash the pests off. Repeat as necessary.