The first week of May, 2014, many gardeners reported seeing this bug on broccoli that had bolted and other plants in the brassica family. This pest is Murgantia histrionica, also known as the Harlequin Bug or Cabbage Bug.
Harlequin bugs come because there are plants that are past their prime. They especially like all the brassica family (cabbage, kale, collard, brussels sprouts, radishes, cauliflower, turnips, mustards, kohlrabi, rutabaga, regular arugula). These plants need to be immediately removed, bagged and disposed of. If they are not removed, the infestation will last into the summer. If some persist until fall planting, which is mostly brassicas, the infestation will carry over.
The solution is not killing by hand or killing by spraying insecticide, it is removing all brassicas at once. It is time to plant other crops.