Hello Claudia:
Unless grubs can be found feeding on the grass roots, I would reserve that determination. Look in the areas that are still green. The smaller holes with puddled soil does sound like earthworms. They feed on organic matter and there is plenty of that with all the dead grass. Grubs are often blamed for declining lawns where there are other causes. Poor root grwoth will allow you to pull up the grass plants, thinking it is grrubs. Would not rule out lawn disease that may have gotten started in 2012 ith the drought. In our area we have until the middle of September to re seed and know the young plants will be strong enought to overwinter.