From my experience and from what I have gleaned here on the Gardenweb, they do not go into the soil. The eggs are laid on the base of the plant and when the borers hatch they bore directly into the stem. They do not hang out in the soil. Many people have good luck with BT (Bacillus Thuringiensis ) sold as Thuricide. I have not done it but they will use a syringe and inject it into the affected stem. This will kill the borer when it comes in contact with it. I have not tried it yet but will next year if need be. This year when I saw the eggs had been laid I wiped them off and then made a paste of the Worry Free Sevin dust and painted it all the way around the stem below and above the soil line. Seems to work quite well although the moths will occasionally lay the eggs elsewhere on the plant. The "Worry Free" is supposedly organic or more natural than the regular Sevin. It is made from Chrysanthemums.

Thanks for the reply. I will definitely give Sevin a try next year, although when round 2 comes in August, the plants will be far too large to paint all the stems. That's my main problem. I can fight them when the plants are small, but Round 2, I have no chance. That said, the plants are surviving round 2 very nicely.

The worms do go into the soil, though. After they eat and grow inside the stem, the emerge and burrow into the dirt where they develop into the moth. That's why I'm wondering if there is some sort of soil treatment that will kill them.

I found this article helpful and thought you might too. I had a friend in your same situation. Good luck!

Here is a link that might be useful: Squash Vine Borer Control

I had forgotten all about that website. Thank you. It doesn't quite answer my question, but I'm guessing that probably means the answer to my question is a "no". I will experiment with these ideas next year as well.