Question

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

Answer 1

since the bed bugs became immune to the pesticide they were able to live through it and breed, basically since they were immune throughout time the whole species became immune including the offspring. it was an adaptation. 

Answer 2

Genetic Mutation

Bugs evolved themselves to adapt to this chemical

Step-by-step 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.