The advantage of using gravid traps is that they almost exclusively collect mosquito species that are capable of transmitting diseases (i.e. Culex spp., Anopheles, etc.). Furthermore, they almost exclusively collect females of these species that are ready to lay eggs in the stagnant, vegetation-enriched water that these mosquitoes like to develop in. Coincidentally, if one of these mosquitoes is ready to lay their egg then that means that it had to have fed on a host (i.e. birds) for a blood meal. If by chance one of those hosts were infected with WNV/SLE then it will produce a positive vector sample.