Aphids are soft-bodied insects that use their piercing sucking mouthparts to feed on plant sap. The most common problem caused by aphids is their excessive production of honeydew. This sticky substance drips onto plant leaves and stems and becomes covered with a black sooty mold. Patios, outdoor furniture, walkways and cars under trees with aphids get covered with this sticky fluid. Even though plants may look bad from the growth of sooty molds, these fungi won't damage the plant tissues. Once the aphids are eliminated, the sooty mold often dries up and falls off the plant.