This appears to be the aptly-named Swamp Milkweed Leaf Beetle (Labidomera clivicollis), a native insect that feeds on the leaves and flowers of at least two milkweed species and a couple of milkweed relatives, both as adults and as larvae.

The adults usually don't cause any serious damage to the plants. If the larvae are causing more defoliation or are bothersome, you can simply pluck them off and toss them elsewhere for birds to find. Insecticide use isn't warranted and leaving them alone should still leave plenty of milkweed foliage and blooms for monarchs and the various pollinators that visit.

Several generalist predatory or parasitic insects probably eat these beetles (assassin bugs, mantids, etc.), but we do not have specific information regarding which exact species prey on them. In addition, a range of naturally-occurring insect-specific pathogens (bacteria, fungi, viruses) can kill pest insects as a form of population control.

Miri