These are the immature life stage (nymphs) of boxelder bugs. These insects sometimes cause moderate damage when they aggregate and feed. The problem should go away within 1-2 weeks, and they are unlikely to completely strip leaves of foliage. Any chemicals that control boxelders would be broad spectrum insecticides with an active ingredient of bifenthrin, cyfluthrin, or lambda cyhalothrin. I would not recommend spraying this insect outdoors, as the insecticide would also affect any other insects (including beneficials) in the applied area.