A student crew was getting the garden ready for planting when they came across sow thistle and burdock plants covered with aphids (probably sow thistle aphids)- a fairly common sight.
The plants were pulled and bagged to prevent movement of these sucking insect pests to lettuce plants. If the infestation had been less severe the plants could have been left in place to serve as food for predators (lady bird beetles, hover fly larvae, lacewing larvae) and wasp parasitoids.