How do aphids reduce crop yield?

These little pests- which can come in a multitude of colors- suck the sap of plants. This both weakens the plant and can deform it. Buds sometimes cannot open. The sticky honeydew they leave behind attracts sooty mold which covers the leaves, looks unsightly and would affect the plant's ability to photosynthesize. Also they can spread viruses as they pierce the plant and allow the disease access.