Likely aphids (plant lice)

Clusters occur on leaf undersides, stems, and tender new growth, especially in gardens and temperate vegetation.

They pierce plant tissue with sucking mouthparts and consume phloem sap, often excreting sticky honeydew.

Aphids reproduce rapidly, commonly parthenogenetically, and may form dense colonies; winged forms disperse when plants become crowded or stressed. Ants may tend them for honeydew.

They can cause curling, yellowing, stunted growth, honeydew buildup, and transmit plant viruses. They also provide food for lady beetles, lacewings, hoverfly larvae, and parasitoid wasps.