There’s naturally occurring genetic diversity in any population, resistance happens when an individual, or group has a small variation that helps them survive better. So, over tens to hundreds of generations, the result is a population better adapted to their environment. If the mosquitoes’ environment includes an insecticide, some part of that population will become resistant over time. There are many ways that these changes could occur, so there are multiple genes that affect insecticide resistance. Some genes and mechanisms are well-characterized, but because Culex pipiens complex mosquitoes prefer what they euphemistically call “organically-enriched” larval habitats (i.e. they love water with lots of animal waste or rotting vegetation) it is thought that genes associated with helping larvae cope with such conditions could help them adapt to the pressures of insecticides. Here are some examples of insecticide resistance in Culex pipiens: Something called kdr (“knock down resistance”) is an example of one mechanism of resistance called Target Site Mutations, and results from a single base pair change in the genetic code for one of the sodium channel genes. The mutation changes the shape of the receptor targeted by the toxin in pyrethroid insecticides, and they are rendered ineffective. The other major mechanism that causes resistance is known as metabolic resistance. Here, genes are either up-regulated or they exist in multiple copies. Either way, more gene product is made, which detoxifies or sequesters the toxin in the insecticide. Genes in the esterase family sometimes exist in multiple copies and are associated with metabolic resistance.