No doubt you are describing harlequin bugs. This is one of the stinkbugs and can be a real problem on cabbage and related plants if left unchecked. At the first sign of problems with this insect, applications of most general-purpose insecticides result in satisfactory control. Contact your local Extension office for insecticide recommendations.
Always remove harvested or overly mature cabbage, broccoli, or cauliflower plants; these serve as excellent breeding and nesting places for harlequin bugs and as a good source of problems for next season's garden.
