The damage is usually short term, but consecutive defoliation for a few years takes a toll and may cause the tree to decline.
Authorities at Iowa State University say this: (What's said here about linden also applies to birch.)
"Japanese beetles are defoliating my linden tree. Will they kill it?
Japanese beetles feed on more than 300 different plants. However, lindens are one of their favorites. Defoliation of well established, healthy lindens (and other trees) is usually not fatal. Defoliation is most harmful to recently planted trees (those planted in the last two to three years) and trees in poor health."
Go here to read the complete bulletin:https://www.extension.iastate.edu/article/yard-and-garden-japanese-beetles
Current information here:
That's a relief, thank you so much. I do have another question regarding the River Birch. Every summer the tree has tiny black inesects, smaller then a poppy seed, the birds seem to eat them and when brushed off they release a not pleasant smell.They don't harm the tree, do you have an idea what they could be?
We can only guess what the insects might be without seeing them or sharp photos.
Based on your description, aphids are about the right size. They are very common and come in various colors. But remember, this is just a guess.
Learn more here: