Large Milkweed Bug (Oncopeltus fasciatus). This insect resembles the Boxelder Bug but this one is found primarily on milkweed plants (Asclepias) rather than on boxelder trees (Acer negundo). We also have a picture of the nymphs.

The Small Milkweed Bug is also found on types of milkweed.

Thanks to Jennie for identifying this plant.