In the last Animorphs book, after the Andalites captured the Pool Ship, the Yeerks didn't stand much of a chance against the Andalites. So some ships fought and were destroyed, others surrendered.
At the end of the war, there were still thousands of Yeerks down on Earth, in various host bodies. They had plenty of Dracon beams, etc.
I don't think there was any functioning Yeerk-pool anywhere on Earth?
Private Kandrona-generators were probably a rare luxury. This no-Kandrona situation would probably mean that the Yeerks down on Earth would have been checkmated and the Andalites/Humans could pretty much do whatever they wanted to them.
So what did the Humans/Andalites do to the thousands of Yeerks on those surrendered ships and down on Earth? I didn't find that the last book really described it in detail.
I'm assuming that none of the surrendered Yeerks were allowed to keep their host bodies.
This leaves me with four possibilities:
- The Yeerks were sent back to their homeworld to live in the pools there, without hosts.
- There was some sort of program to make Yeerks into nothlits by allowing them to attain morphing power (for their own slug-form, without a host) and morph some other creature, under strict Andalite/human supervision to ensure they actually became nothlits. (Just like Aftran.) I know this happened to at least some Yeerks (those who surrendered on the Blade Ship) but did it happen to all the thousands down on the surface, etc? It would have been a huge logistical operation requiring much personnel, security, morphing cubes, etc.
- During the court trial of Visser One, the Andalites constructed a device for Visser One that fed him with Kandrona rays and allowed him a sense of hearing and ability to speak. If this was possible, it might also have been possible to create other senses and locomotion. This could have been an option for other Yeerks as well.
- Genocide of Yeerks. (I don't consider this likely in this setting, I don't believe neither the Humans nor Andalites would exterminate surrendered, non-belligerent Yeerks in peacetime. Except for maybe some isolated cases of pissed off warriors, etc.)
Obviously, there could be some combination of these points.
What happened to the surrendered Yeerks after the war?