What is this
Those insects are not butterfly larvae. Butterfly and moth larvae are caterpillars. It looks like some type of immature beetle to me. Do you know the name of the plant they are on?
The plant these have collected on is butterfly weed. Just two weeks prior to this I saw four monarch butterfly caterpillars on the plant. At the present time I do not see the caterpillars. Only see these beetles
Could these beetles be harmful to the plant.
They look like Milkweed bug nymphs:
If you want your milkweed to spread more, I would handpick them off your plants. They may do some damage, especially to the seed pods. And if you have plenty of milkweed, you can leave them be.