Ants on Lemon Tree? They're Farming Aphids or Scale

Ants on citrus usually indicate aphids or scale insects. Ants don't harm the tree but "farm" these pests for their honeydew. If aphids are present, blast them off with water or spray insecticidal soap.

Scale insects are harder: they're protected under hard shells most of their lives. Time insecticide sprays for when crawlers (newly hatched) are active, before they form shells.