Dang stinkers!
Thanks for posting this, incidents like this help all of us to see what possible problems are out there. Sorry you had to pay the service, especially when you did have the screen. I don't see how the bugs got in there with a screen on, maybe you need to re-visit the whole screen idea. Are they the Stainless, small mesh screens?
I don't know how they got in there with screens on, it is a sealed unit. Maybe they crawled in before the screens were on?
Ozz