The "ancient" (how old is ancient, anyway?) Japanese killed insects the same way that other "ancient" peoples killed insects on their trees or crops -- however they could. There are a lot of plants that, when stewed make toxic substances. The "ancients" probably made a lot of other "ancients" sick in the process, too.
We "non-ancients" are no different, though we are slowly learning not to spray indiscriminately -- or so I hope.