This is a common error. Many of the "dead by poison" caterpillars were dead zombies yet. Poison kills the caterpillar and the parasitoid wasps that were inside many caterpillars. Result, "Y" caterpillars dead plus "20 Y" butterfly killers dead.
Unless everybody uses poison there, less wasps and more caterpillars will survive in the next years.
But If everybody uses malathion, some people will fall ill by chronic exposition. And if you just stop using poison, will suffer a exponential increase in butterflies by lack of predators the next years, forcing you to use poison again. Is a vicious circle.
I'm tired to explain this again and again, but frustrated people will do it anyway. Outraged people loves the idea of fair revenge and will spend money in tools for that.
Poison means that they are supporting, promoting, increasing the plague suffered in they own plants. Predators are hugely influential. First course of Basic ecology.