However, bed bugs have become resistant to even this chemical pesticide. what happened that allowed the bug population to increase.?

Genetic Mutation

Bugs evolved themselves to adapt to this chemical

Explanation:

Genetic mutation in bugs has caused such changes in them. There is a high probability that some enzymes must have developed with in the bugs which inhibits the fatal impacts of this chemicals. So , it can be said that the genetics of bugs adapted the chemical due to which it is producing no negative impacts on them. And since this is a genetic change it is being transferred to the next generation.

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