harlequins dont show up until august around here and i try to avoid their season. Supposedly insecticidal soaps, sabadilla and pyrethrum are effective.
they overwinter in garden trash (old plants). you can control them by destroying their eggs that are found in early spring on the undersides of early blooming plants.
i have not been able to control them and the best i can do is to hand pick as many as i can.
some of what i wrote here was from The Encyclopedia of Natural Insect and Disease Control that was published in 1984 by the Rodale Press.

What are they and what do they look like?

1eyedJack and the Dawg

Here's a URL with some pics. When you plant intensively, as we have been, it is a nightmare to try to find all the hidden bugs and eggs.

Here is a link that might be useful: Harlequin Bug