Aloha Charles,

Not too familiar with sweet potatos or the weevil itself, but I would say that your guess seems about right =/
As I understand it, the weevils lay their eggs in holes they make in the potato and vine, depending on the time of the year/season. So if you're having problems, a pre-planting insecticide treatment applied to the soil, and once again midway through the season, may do the trick for future crops.

I found a site that you may have already seen, but just in case you haven't, I'll include it here. Hope this helps in some way, just keep doing what works, and let me know what you find! Sometimes all it takes is a little R&D!

Aloha,
Dave
Kauai

Here is a link that might be useful: Cylas formicarius