Sheep Sorrel Insect

They are most likely beetles in the family Chrysomelidae (Leaf Beetles). Identification to species is difficult with that family even with a specimen under a microscope. It is pretty common to see outbreaks of different species from time to time locally. It is not likely that they will become a pest because they tend to specialize on closely related plants. Rhubarb and buckwheat are in the same family (Polygonaceae) as Sheep Sorrel, so if you are trying to grow either of those plants, then there might be cause for concern.