Blackfly and greenfly (aphids) are very common sap sucking pests affecting nearly all types of plants. Colonies are visible around new buds or shoots in spring onwards. In extreme cases leaves may become twisted or puckered and distorted as they develop.
Not a major pest of spinach but may have some levels of aphid during the growing season.
Product solution: Provado Ultimate Fruit and Vegetable Bug Killer - ready to use/concentrate, Sprayday Greenfly Killer, Provado Ultimate Bug Killer Concentrate 2.
Cultural control: Encourage birds into the garden to feed on the aphids ie Blue Tits.