Scales and whitefly larvae stay on the plant, so examine the branches and leaf undersides with a magnifying glass. Insects with Piercing-Sucking Mouthparts Aphids, mealybugs, scales and whiteflies have needlelike mouthparts to pierce plant tissues and feed on plant sap. Most plant pests with piercing-sucking mouthparts are smaller than the head of a pin, and secrete a sugary product known as honeydew. It is common for foliage infested with these insects to be covered with black sooty mold that feeds on this sugary secretion. Although the mold does not injure plant tissue, it does detract from the plant's appearance.
Sooty mold indicates pests are present.
Ants are also a good indicator of the presence of insects. They feed on honeydew, and often care for and protect the insects that produce it. Piercing-sucking insect pests have numerous parasites and predators. Among these are small parasitic wasps that deposit their eggs on or in the insect pest. The larvae that hatch from the eggs actually parasitize the insect, killing it.