I first saw this beetle last July but didn’t manage to identify it in time to post it within the season. It is a Galerucella beetle. Browny-green with green legs and a brown stripe down the middle. There are several species looking fairly similar and the way to identify them is to identify the host plant.
The host plant in this case was purple loosestrife, so I think it is Galerucella calmariensis. The adults have overwintered in leaf litter near the purple loosestrife and have now emerged to feed on the leaves and mate. The females will then lay up to 500 eggs on the plant. The first instar larvae feed on the tips of new shoots, and later instars feed on the stems and underside of leaves. When they are ready to pupate the larvae move down to the soil, leaf litter or inside the stem.
(Other possibilities could be Galerucella lineola, the brown willow beetle, or the Galerucella tenella, the strawberry leaf beetle, which is found on meadowsweet and strawberries.)