Congratulations on your trip.
Mosquitoes are a problem along with ticks, gnats and black flies, mostly in the country side. It's not as bad as the American brush, but very annoying.
Being in northern Europe, night temps can drop down to 8 C (45 F), with even flukes that might go below 5 C.
With the route you're planning, I would prepare the same as you would for the northern mid-western states.

You should also consider that anything you may need is also available in Europe.

Have a great trip