Houseplants are vulnerable to infestation by pests even if it is often difficult to work out how they arrived on the plant in the first place. Infested plants should be isolated to prevent the spread of pests and monitored even after the problem has disappeared. A single surviving aphid or mealybug on a plant will soon reproduce and reinfest the plant. While many pests can be treated by physically removing them from the plant or by using organic methods, persistent infestations will require pesticides.
Aphids or greenfly are small, tender-bodied insects of up to one-eighth of an inch long. They can be green, yellow or black and tend to cluster on the shoots and new leaves of house plants. When disturbed, aphids will scatter. Adults have a small pair of transparent wings. Aphids exude a sweet substance onto their host surface that causes black, sooty mold and also attracts ants.
Mealybugs can be up to a quarter of an inch long. Although there are several different types, all are white or pale pink and covered in a fine white wool. Mealybugs tend to cluster on the undersides of leaves along the veins and in the branches of tender stems. They will scatter if disturbed.
Scale insects are immobile and look like tiny domes attached to the surface of leaves and stems. Some scale insects can be a quarter of an inch long but most are smaller. They can be green, yellow or brown and tend to cluster on the undersides of leaves and in the joints between stems and leaves.
Whiteflies resemble tiny white moths and will flutter up from an infected houseplant as soon as its leaves are disturbed. They are only one-sixteenth of an inch long. Whitefly larvae look Iike small white scales.
Spider mites are tiny and can only be seen using a magnifying glass. They usually resemble tiny red or yellow spiders, though some species appear worm-like. They can be identified thanks to the fine white webbing they produce and the damage they do to plant leaves. A houseplant with mottled yellow patches on its leaves, especially on the undersides, is likely to be infested with spider mites.
Thrips are tiny insects that are black as adults but white or yellow when immature. They are about one-sixteenth of an inch long and are narrow-bodied, with some species winged. They look like tiny threads to the naked eye. Thrips gather on and around the veins and midribs of an infested plant's leaves and damage the leaves by scraping away the surface to feed on the exposed juices.