Floodwater Mosquitoes

Flooded Field

Where do all these mosquitoes come from?!

No doubt you have noticed that not long after it rains, there are suddenly a whole lot more mosquitoes than usual. Where in the world do they come from? Are they there before the rain and just suddenly more active? Or do they fly in from somewhere?

The answer is more complicated than you might think. As you know, mosquitoes lay their eggs in water, so when it rains, all of the usual varieties of mosquito, the ones that always live in your neighborhood, have a lot more breeding sites after it rains. It's amazing how many things we don't notice day-to-day will start filling up with water. Just a tablespoon of water is enough for dozens of eggs to develop, so you can imagine how many hundreds or thousands can develop in a small area. In warm weather, mosquitoes develop from egg to adult in one to two weeks, or even less, so they really do appear quickly.

But an even bigger impact can come after the rain in the form of floodwater mosquitoes. These species are perfectly suited to areas where heavy rainfall happens infrequently. The adults lay their eggs in areas where it is likely to flood. These hardy eggs can stay there for up to six years until a heavy rainfall causes their resting place to fill up with water. Then the eggs very quickly develop into adults, sometimes in just four or five days. 

Floodwater mosquitoes are also different than the usual mosquitoes near your house in that they are terrific flyers! While your typical backyard mosquito can't fly more than a few feet at a time and only ventures about 1/10 to 1/4 mile from where it hatched, floodwater mosquitoes can travel up to 20 miles! Not only that, but they are hungry and aggressive. Since their lifespans are so short, they are motivated to breed, which means the females are going to be biting. Each female can lay up to 200 eggs, and it has been estimated that there can be 0.7 to 1.3 million eggs in a single acre. So when it seems like there are millions of mosquitoes all of a sudden, that isn't necessarily an exageration!

There are many species of floodwater mosquitoes, and each species is specialized to its environment. There are species that like snowmelt, others that flourish in salt water, and still others that like to lay their eggs in hoofprints. Mosquito control can be a challenge when it comes to floodwater mosquitoes, because they don't stop and rest on things we have treated, as other types of mosquitoes do. They do tend to bite around dawn and dusk however, so it helps to avoid being outdoors during those times. Dressing in light-colored clothing and wearing insect repellent can also help.

The good news is that most floodwater mosquitoes don't usually carry disease, and they don't live very long, usually only about two weeks. So for two to three weeks after a heavy rain, there will be a lot of floodwater mosquitoes around, but they will soon be gone. Until the next heavy rain...