Shucking an ear from a plant where you found corn borer damage in the pith and then shucking one from a plant with a clean pith and no sign of corn borer damage can be interesting. It's not scientific – it's more show and tell, but it can make a point. The same would be true if another insect besides corn borer had affected some stalks and not others.
Danny Greene of Greene Crop Consulting, Franklin, recently completed this exercise to answer a question. Does early-season corn borer damage really bother yield? His answer is yes. To illustrate that, he held an ear from a plant where he found a corn borer actively working against an ear form a plant where there appeared to be no corn borer feeding or damage. The second ear from the clean stalk was obviously longer and had more girth, indicating it likely would have a higher yield potential if all its neighboring plants had ears like it, compared to plants that had corn borer damage.
This was actually the second or perhaps the third brood larvae that Greene was finding in the field.
"The damage from the earlier broods that eat inside the stalk is that they remove vascular tissue. It's tissue that moves water and nutrients up through the plant to help make sugars during photosynthesis," Greene says. If there is less vascular tissue left to work, the plant won't be as efficient at making sugars which wind up as starch in the kernels.
Plant diseases often destroy leaves, which destroy the factory where sugars are made. Insects that feed inside the stalk and mess up the transportation system, on the other hand, prevent sugars made in the leaves from getting where they need to be, or prevent enough nutrients and water from getting from the roots up to the factory in time.