As an manufacturer of bifolding doors, this is always a discussion point with our clients - there are a range of choices

Crimsafe make a bifolding security screen door - functions like a typical bifold but with the added security that only Crimsafe can offer

There are retractable insect screens like Freedom, which we have used on several of our bifold installations, through ti Centor

Or if you are really adventurous you would use an air curtain, above the bifold to deter the flies from coming in to the home at all (if you do a search for air curtains on google you will find a vast number, used extensive in commercial applications but creeping more and more in to our homes

Enjoy your wide open space that your bifold will create - www.centralglass,net.au