They probably just wouldn't eat it at all. Grubs will do the same if you try feeding them "raw" sawdust. I tried mixing un-rotten sawdust in with rotten wood once, and the grub ate all of the rotten wood and left all of the sawdust.
The only time I have found grubs in pine was when it was well decayed - but, I have found Dynastes, Lucanus and "Bess Beetle" grubs in pine before.
Steven