Moved from Galerucini.

Moved from Trirhabda.

...but it turns out they're aren't! ;-)

The simplest character that excludes Trirhabda here involves the relative lengths of the 3rd & 4th antennal segments. In Trirhabda the 3rd is shorter than the 4th; whereas in the specimen here the 3rd antennal segment is longer than the 4th (cf. "See Also" on info page). Below are some genera with 3rd antennal segment longer or equal to the 4th are often mistaken for Trirhabda:

    Galerucella ,     Ophraella ,     and Xanthogaleruca

Apparently there are 14 species of Ophraella in our area, but browsing BugGuide images suggests they're nearly all fairly conspicuously striped. The 1st and last thumbnails above look more like yours, and...though dark forms of the latter resemble your post, Xanthogaleruca have a distinctive "hour-glass" shape for the central spot on the pronotum (which I think is supposed to be diagnostic...but, as we all know, there are often instances of individuals within a species that don't follow the rules ;-)

Happily, both Galerucella and Xanthogaleruca are monotypic...and thus knowing the genus gives you the species too! :-)

I'm thinking this is most likely Galerucella nymphaeae...from the emargination (i.e. slight indentation or "dip") along the hind edge of the pronotum...and also the fairly sharp "teeth" at the anterior edges of the same.

   

Yours seems to have both those characters, whereas (as far as I can see) the other two genera don't have both (in particular the emargination). Also, the 3rd and 4th antennal segments in Xanthogaleruca are roughly equal. But I'm just going by what I see...I don't know if the emargination is a technical character used in the literature for ascertaining the ID.