Worm inside of stem

[SummerSky]

One of my Big Mamas started wilting suddenly, so I pulled it (it was the only plant out of 80 or so that was wilting). When I examined the plant, I noticed a small hole in the stem close the soil line. I split the stem open and noticed there was a hollow cavity and saw a worm burrowed in there pretty good. I threw the whole plant into the burn barrel. A week or so later, one of my Polish Linguisa plants started doing the same thing- pulled it up and again a worm had burrowed into the stem close to the soil line.

I've never seen this before (then again, this is only the fourth garden I've grown). Only two plants out of the 80 some-odd I've planted have fallen victim to this bug, so I'm not overly concerned, but I'm curious what it is and if there is anything I can do to prevent attack on some of my other plants (I only have a couple plants of some varieties, and I want to see them through the season).

Sorry about the picture quality, I only had my cellphone on me at the time.