Insects will get everywhere. FWIW, we discovered a cluster fly issue at the end of Summer last year and treated all the door frames, external locker doors, cab doors and skylights with repellent / fly killer. We were driving from Cornwall to Herefordshire for a picking weekend at the beginning of November 2019 and got to Taunton when we noticed a few flies buzzing around the cab. By the time we got to Herefordshire this had become twenty or more. We pitched up, hooked up the electric, turned on the gas. Jan then opened the kitchen cupboard to get out the kettle when the air turned black with flies emerging from the cupboard. On investigation they'd got in through the fridge vents and clustered behind the fridge, cooker and kitchen cupboard drawers in their thousands. We ended up with Jan heading over to the venue while I liberally sprayed the affected area and then generally with fly killer and then locking up and heading over to where Jan was. When we came back (close to midnight) it took us over an hour to sweep and hoover up the dead flies and the rest of the long weekend before I could declare the problem properly under control.
Fast forward to this year and we've added taping up the fridge vents to our anti-fly measures!
