It happens every year around this time. Our hackberry tree becomes heavily infested with aphids - causing everything under the tree to become sticky with honeydew. Just in case you're unfamiliar with that term "honeydew," it's the sap-like stuff that these piercing-sucking insects exude from their posterior parts (gross).
It's so bad that we have to hose off/wash the steps every day to keep from tracking the sticky stuff in to the house. It also makes sitting outside unpleasant. In addition, all of the plants in the yard are covered in honeydew, giving them a shiny appearance.