the stuff looks like wet sawdust. When slicing the vine, go up the vine
from the hole. I don't particularly like to wade through these huge
plants, stepping onto loose compost, and the borer is small compared to
the vine, and it is difficult to fish out unless you catch it as soon
as it has entered. I think the best defense is to have a mesh tent up
until the 4th of July. Two things: the zucchini will still get a later
generation of borers, but they will die a month later, early september
instead of early august. Second, the young zucchini plant can not push
successfully against the cloth, so the tent should be high enough to
allow the plant to grow. It only takes three weeks of protection to
miss the first generation.