We were in the Banff and Jasper late June to early July and there were plenty of mosquitoes around then. I was more used to Greece, where these are only an issue at night but we soon found that we needed repellant on our arms and legs during the day, especially round lake sides. Better safe than sorry. It's a fantastic area with stunning scenery, have a great time.

I was in the Banff, Lake Louise, Jasper area in late August early September a couple of years ago and don't remember midges or mosquitos being a particular problem. But I have read on other sites that there is a big problem with often swarms of the pesky things, so be prepared with repellent.