This insect has seriously damaged my kale, and almost killed my brussel sprouts in less than a week. What is it, and how can I control it. My garden is organic, so a non-chemical solution is preferable. Diatomaceous earth does not appear to phase them.
This insect is the Harlequin cabbage bug. After examining the picture linked to by pnuts, I returned to the garden and found instances of both versions of this bug. I assume that these two colorations represent the adult male and female forms of the insect.
These are highly destructive pests, both in their adult and nymph forms. The simplest way to control them is to pick them off of your plants and either crush them or drown them in soapy water. Here is a longer list of organic ways to control this undesirable insect:
If your infestation is large (like mine), manually dealing with these bugs is more problematic. Here is another list of eradication options, including insecticidal soap.