One of my favorite neighbors stopped by the other day to let me know that he was going to find the borers in the squash stems in his Mom's garden. I walked back to his yard with him to see the damage.
His Mom was there, too, so I got out my pocket knife and handed it to my friend (after asking his Mom if he was old enough, at 9, to use one). The stem was still tough, so getting to the borers wasn't easy, but it turned out that the stem was pretty full of the little guys.
The larval borer, on first glance, resembles a grub more than a caterpiller, but the lifecycle of this particular insect is well-documented. We know what it is.
In my yard, I sprayed the zucchini with Bt (bacterial product that is toxic to caterpillars) once each week through June, but the plants haven't been sprayed since I got back from Texas. The spraying did seem to delay the borers in my yard, but my squash plants are just about done-in, too. Luckily, I have some young squash plants growing under netting right now. They have begun to flower, so the netting will come off soon.
The netting prevents the adult borer moth from laying eggs on my plants, but it also will keep bees and other pollinators away from the flowers.
Right now, the flowers are all male. When I see the first female flowers getting ready to open, I might pull off that netting. I might, though, hand-pollinate those plants until they are just too big to fit under the netting. That strategy would probably give me the most squash.