What are the purple boxes that hang from trees?

They are to kill ash beetles because ash beetles are not native to the USA. ( They come from China), and someone brought an ash beetle or two to the US, and they have been eating all of our ash trees. No animals has adapted to eat ash beetles, so they keep on growing and growing, and our ash trees have been disappearing and disappearing. so people are only putting up those purple boxes because they want their ash tree's back!