Aphids feed by piercing the phloem tissue of the host plant and sucking up the sap. Unfortunately, sap is mostly sugar, so an aphid must consume a lot of sap to meet its nutritional requirement for protein. Much of what the aphid consumes just goes to waste. The excess sugar is eliminated in the form of a sugary droplet called honeydew. An aphid-infested plant quickly becomes coated in the sticky excretions.