Since Skye's arrival - exactly 8 months ago (her rescueversary is today!) - the door to our backyard has been left open 24/7 so she could go in and out as she pleased. It helped tremendously in making her at ease in her new home, and avoided any accidents in the house. But with the warmer weather, the insects are starting to be a pain, so we tried unsuccessfully to get her using the doggie door in the screen door. Skye is still very suspicious of any active training from our part... So we decided to remove the screen of the doggie door to ease the process. I took her couple hours and a few treats to figure out how to use it :)
It's not preventing all insects to come in, but it's much better than having the whole door open.