You have to remember that a BT trait is no different that spraying BTs.  The worm still have to EAT it to stop feeding and ultimately die.  It is why sprayed corn with pyretheroids is almost always cleaner.  You spray when the silks set then every 5-7 days after until the silks are brown (I am sure you know this already).  The hatching eggs on the silks are killed before they enter the ear and begin feeding.

I am surprised you have a market for GMO sweet corn.  You must be selling to a processor or a large chain.  ALL of my customers (Even the restaurants and wholesalers) ask and either won't take or discount GMO sweet corn.  We are still growing 277a.  Haven't found a replacement that will keep the customers coming quite as well.