The phenomenon is due to natural selection forces.
This refers to the tendency of an adaptive gene to persist and multiply in the environment.
The weak genes in the population are gradually being replaced by strong/adaptive genes. Only organisms with these adaptive genes are able to survive unfavourable changes in the environment.
When they do, they reproduce and pass the adaptive genes to future generations. Thus, new generations will emerge that are well adapted to the environmental condition.
This is why many of the insects targeted by the insecticide were found in the following year.
More on natural selection can be found here: https://brainly.com/question/9830102
#SPJ3