Several insects commonly attack houseplants. Insecticidal soap is an easy-to-use, effective treatment for most soft-bodied pests such as aphids and spider mites. A forceful spray of water from the hose may knock down the population of these pests, too. Rubbing alcohol is effective on insects with waxy coatings such as scale and mealybugs.
No matter what treatment you use, be consistent. For fast-reproducing pests such as aphids and spider mites, you may need to treat plants once a week for a month or so to be rid of the pests.
Fungus gnats are tiny black flies that buzz around the soil; they're often confused with fruit flies. You typically see fungus gnats in large numbers when plants are overwatered. Allow the soil surface to dry between waterings.
Remove and destroy diseased houseplants or affected leaves or stems as they develop to prevent spread of the disease. Some diseases spread by insects, so keeping the insect population in check helps prevent disease problems.