This outdoor mosquito trap uses a pheromone lure to attract the female culex mosquitoes. Culex are the ones that spread west nile virus and equine encephalitis and are very common in most of the US and Canada. The trap works by utilizing a pheromone that the mosquitoes use to mark a good spot to lay their eggs. They mark the spot to make it easy to find again to lay more eggs. With that pheromone in this trap, it attracts them in and they cannot get out.