Aphids really suck (literally)! They use their straw-like mouthparts to pierce leaf tissue and suck out the sugars. They excrete excess sugar in the form of honeydew which drips on cars, buildings, walkways or even unsuspecting people. The sugars build up over time and a black fungus called sooty mold grows on the honeydew which leaves behind a black tint on surfaces. Wasps and ants are also attracted to this abundant sugar source which can make living spaces under aphid infested trees really unappealing to gather.
Oftentimes, aphids are kept to undetectable levels by the many insect predators that feed on them like lady beetles, lacewings, minute pirate bugs, parasitoid wasps, etc. However, due to aphids’ ability to reproduce very quickly through parthenogenesis (asexual reproduction) there are times where populations explode and treatment is warranted.
Look for the signs and symptoms of aphids by checking for shiny film on the upper sides of leaves or under trees on objects like stairs, decks, understory plants, etc. as well as black sooty mold, wasps, and ants.