Of course when blossoms come you take out the row cover, unless you
want to do it by hand with a paintbrush. I have room in the garden and
I don't like fussing about plants in the summer, if one plant dies I
will remove it and plant kale in its place for the fall. Also, the
first blossoms are all male and go in omelettes (they really are the
most exquisite of all vegetables), so you can leave the cover until you
see the first female blossom (about two weeks after first bloom).
In Michigan the borers attack in waves, with the first wave in June.
All in all I am happy with the arrangement, typically out of four
plants I lose 2-3 in August, with the borer probably getting into the
plant in july, and they give me 15-20 zucchini each before dying. 1-2
get spared and I have a few zucchini for soup later in the season.