Harmostes serratus is distinguished from other North American species in the genus by the toothed (serrated) edges of the pronotum (the bit behind the head).
This little dude is in the family Rhopalidae, known as “scentless plant bugs” because, although these insects look like and are (distantly) related to stink bugs, they are not actually stinky. They do live on plants, and eat the seeds.
Scentless plant bugs are in the family Rhopalidae. There are approximately 40 species in 10 genera in North America, and approximately 200 species/20 genera worldwide.