Many mosquito populations today are resistant to pesticides that were historically effective. this pesticide resistance arose in these populations because:

Many insects are resistant to pesticides there are many factors that can lead to resistance in the organism.

In the population of mosquitoes some of them carry resistant gene that makes the organism survive even in the presence of pesticide.

This occurs because of mutation that is very rare when it comes to insects. This is the reason that after sometime the mosquito become resistant to pesticides. They are more likely to breed and survive.

56 moles of   lieter in chemistry