I spray weekly with Neem to try and get the eggs before they hatch. But I must not have started early enough. Also if you miss an egg, all it takes is one and they borrow into the stem. A very common problem in squash as I am finding out. I never grew squash before this year so I never knew.
They attack cucurbits as well- think it is the yellow flower the moth spots- traps to cue for arrival include lemon yellow solo plates/bowls with soapy water in them, or vaseline coatings to catch them so you know what to spray for...
SVB will show some poop at the base of the stem, where they have burrowed in. Since I used Tulle as a row cover, they only got 3 plants but one I was able to save by cutting the suckers out and squishing them. Lol The other 2 were too far gone and had 5-6 of the maggots in them. I just hate them, not much bothers me but they are gross.
I would hate for you to cut into the stem and risk loosing the plant if that isn't the problem. You can take a box cutter and slice open the bottom of the stems to see if they are there do it length wise and you won't kill the plant.
Injecting Bt is supposed to work too.
I have only had trouble with them on summer squash, in fact I gave up growing them bc of the SVB. I've never had them on cukes, winter squash, or pumpkins I have no idea why.
I'm going to go with borer, as one leaf was wilted. I can't be back to our garden with syringes for over 10 days, so I sprayed Spinosad directly at the broken off part. I'll report on success/failure when I get back.