Hot summers can retard their growth and damage. When aphids feed, they produce honeydew, a substance that can become so thick it drips from trees and plants onto cars, picnic tables, people and slow-moving pets. Honeydew is just a sugar-water substance and is not toxic to plants or the people it drips on. Aphids suck and feed on the sap of their host. If left unchecked, the leaves they feed on begin to curl, providing the little pests a nice protected area where they can continue to feed uninterrupted.
Severe infestations cause leaves to yellow and drop.
Normally they only weaken the plant, and do not severely damage trees or other woody plants, or cause them to die. The most common and easiest aphid control method is restraint. Be patient and allow natural enemies to come and make a meal of them. But, being patient means that you have to allow a little damage and honeydew dripping before the beneficial insects move in for the attack. These insects can be found naturally, or purchased and then released in the landscape.