The grubs are the second half of the Thread-fighting strategy developed during the early First Pass. The dragons flame Thread out of the sky, and the grubs devour any that reach the ground. They were forgotten about after the Crossing to the Northern Continent, but gradually spread over the Southern Continent and eventually started crossing to the Northern Continent. However, the Northerners misunderstood the grubs, and believed them to be parasites, and continually destroyed them whenever they appeared.
In the Southern Continent, however, by the Ninth Pass (presumably long before then) First Interval they had spread to the scrub land east of Landing, the entire Continent was protected by the grubs, and Threadfall did not harm the vegetation. Any animals that were smart enough to hide in caves or under water during Threadfall were thus safe.
Their purpose was re-discovered in the Ninth Pass, and they began to be spread over the Northern Continent.