Thanks to "Vanessa Cardui" of Am I Bugging You Yet? I now know that these cute bugs are scentless plant bugs, (Niesthrea louisianica). And I found tons of them on a mallow plant at the arboretum. They start out bright red when they hatch, then they develop white spots, and as they continue to grow they become white with dark spots. The adults have wings.

You can click the pictures to see them bigger. For tiny bugs like these, bigger is better.