John etal

U of IL should be reporting shortly on their plot results of roots vs insecticide vs Bt vs seed treatments etc.

I'm suggesting if you don't receive their weekly bulletin that you "subscribe" (free) to it. Some of their data may be relevant to your questions. Here's an article updating their 2006 root digs:

http://www.ipm.uiuc.edu/bulletin/article.php?id=590