The Yeerks have biostasis technology. They put humans into biostasis during "The Experiment", as they searched for a formula to suppress human will.
Why didn't the Yeerks put Yeerks in biostasis during the Great Kandrona Famine (Book 8 "The Alien")? Putting Yeerks in biostasis would have reduced the Kandrona demand enough for the shipboard Kandrona generator to meet it.