A species of leaf beetle
This beetle is a species of leaf beetles belonging to the family Chrysomelidae. It’s German name „Pappelblattkäfer“ emphasizes its habitat: It lives on poplars and pastures, whose leafs build its main food.
But the leafs of these trees offer more than just food to the insect. They contain Salicylic acid enabling the beetle to produce an unpleasantly stinging smelling secretion, which helps to scare off certain enemies, like the predatory larvae of a type of hoverflies (or it tries at least …)