My girlfriend's vagina attracts houseflies

My girlfriend's vagina attracts houseflies whenever we would go out grocery shopping or at the lake. Despite her taking a shower in the morning. If I walk behind her they would walk up her leg or fly in while she's wearing a dress. If she wears short pants they would sneak up her legs too.

She's been deciding if to wear no panties the next time she goes out in public since she's tired of the houseflies being on her panties while walking. What could be the cause? Do they like her body oils or vaginal discharge from her vagina?