These are Shieldbug (stink bug) nymphs newly hatched, in the family Pentatomidae.
http://bugguide.net/node/view/...
They usually cause no real harm, and there are predators or parasites which use them for their reproduction which will reduce numbers. They are often wasps which will take them to their nest paralysed, they lay eggs which will use the nymphs as food to grow. Other parasites will utilise the growing nymph.
Nature will look after numbers if you let it.
