Helpl Alder Flea Beetle Attack

Help! We planted around 20 alder trees as part of a hedge eight weeks ago. We've just found that they're being munched by what we've identified as Alder Flea Beetles. So far we've been picking them off at the rate of about a dozen a day and dispatching them to a better place. Plus I've discovered tiny bright yellow eggs on the alder leaves which I'm taking to be theirs.Where do they come from + is this just a short-term problem? Or will we have to be picking them off all through the summer? I can't find much about them online so all suggestions and info would be great.

Thanks