What is a good source/brand for BT? I think I'm going to give that a try.Benlan, I have the same problem. I am using BT and it seems to be helping quite a bit. I started using it when I set out the plants and kept applying it faithfully. I haven't been able to grow zucchini until last year when I started using BT. The borers did get the zucchini in the end, but I got a lot of the squash before the plants succumbed. Be sure you remove the plants when they are finished and remove the larva that are in the vines. Yellow summer squash seems to be less attractive to the borers than zucchini, but I don't know about winter squash as I don't have enough space to grow them. I am quite sure that BT would work just as well on winter squash as summer varieties as long as it is consistently applied.
Look for the moths that lay the eggs, they are black and red. I found one on my zucchini, but they are difficult to find (I think they fly at night).
I also cover the plants with remay when I set them out until they start to flower.