The right answer is that a strongest bugs were able to survive pesticides, reproduce and create more resistant pests.
Pesticide resistance is a hereditary trait that gives an organism the ability to survive pesticide application at lethal doses for most individuals of the same species.
This resistance, manifested by the absence of inhibition, or reduced inhibition, of the development of a population of pests, may be natural or acquired.
* It is natural if the phenomenon is observed from the first application of a pesticide.
* It is called "acquired" if the phenomenon is observed only after several applications of a pesticide, as a result of the selection over several generations of naturally resistant individuals.
Harmful species develop resistance to pesticides through natural selection: the most resistant specimens survive and transmit their genetic traits to their offspring.