The mealybug is one of the most common pests of houseplants. A 1/4-inch scale-like bug that's covered with a white flour-like material, the mealybug uses its piercing mouthparts to suck plant juices. The bugs feed in large groups; the first sign of an infestation is what appears to be a cottony mass on the undersides of leaves. If left to their own devices, mealybugs can kill a plant.
Remove the bugs with a cloth or wet paper towel, so you can be sure that you're getting rid of them, not simply moving them to a new feeding place. An insecticidal soap can also be applied; spray the undersides of leaves, the soil surface, and the area around and beneath the plants. Insecticidal soaps are non-toxic to people and pets, but they disintegrate the bugs' "skin," causing them to dry out and die.
The best solution, though, is prevention. Carefully inspect plants before you buy them and bring them home. Keep an eye out for egg masses and crawlers. Also, avoid excessive feeding; mealybugs thrive on juicy, over-fed plant tissue.