To eat plant sap, the aphid or scale insect pushes its mouth parts into the plant. Plant sap is under pressure inside the plant. The pressure inside the plant pushes the sap into the insect. Sometimes the pressure is so great that the sap is forced out of the rear of the insect. This is how honeydew is made. The honeydew then drops from the insect and makes a sticky layer on the plant.
Honeydew on plants can cause sooty mould, which is white and appears as powder on the plant.
Honeydew is collected by some insects, such as some wasps and bees. Some honey bees make honeydew into a dark, strong honey, called honeydew honey. Honeydew honey is valuable in parts of Europe and Asia because people think it can be used as a medicine.