Why do insects become resistant to insecticides over time?

Question

Why do insects become resistant to insecticides over time?

Answer 1

Evolution. When you spray insecticide, most of the bugs die. The few bugs that are strong enough live and reprooduce, creating a second generation that is strong enough to live. After a while, the insecticide has killed off all of the weaker bugs, leaving behind only the stong ones. It's survival of the fittest, the same way bacteria becomes resistant to antibiotics.

Answer 2

Because the insecticide kills off the ones that are not resistant the remaining ones are resistant.

As the insecticde remains  those that are resistant become even more resistant because the breed with other insects carrying resistant genes. This shifts the genetic makup of the species towards resistant.

This is known as genetic drift.