In resistant wheats, although many eggs and early-instar larvae of the wheat stem sawfly die, most of the effect is in the high mortality of late instar larvae as the stems ripen. In oats, which are immune to sawfly damage, most sawfly eggs hatch but all larvae die in the first or second instar.

The amount and distribution of the pith in the stem determines the sawfly resistance of wheat. The stems of all resistant wheats are at least partially solidly filled with pith in the lower three internodes whereas those of susceptible wheats are hollow. Significant sawfly mortality occurs only in the solid part of resistant stems; in the hollow part survival is as high as in susceptible wheats. Shading the growing plants of the resistant bread wheat Rescue reduces its solidness of stem and at the same time its resistance to sawfly.