Mosquito season starts slowly in the spring, when warm weather brings out the first of the bugs, peaks in summer, and tapers off into fall, when humans and animals finally get a break from the annoying and sometimes dangerous bites of these little critters. That's if they live where winters get cold.
Mosquitoes don't go away for good until the first freeze, followed by temperatures consistently below 50 degrees. In the southern continental U.S. and Hawaii, there may be at least a little mosquito activity year round.
In locations where a warm spring follows a cold winter, some species of mosquitoes emerge from hibernation, while others are born from eggs laid last year. Different species have different life cycles, habitats and tolerance to cold, but almost all the females need to start looking for a meal of blood after they've mated, in order to produce eggs. That's when humans begin to notice them.
The exact start of the season depends on both temperature and rainfall. Mosquitoes that hibernate need warm weather to become active, while mosquitoes that spend the winter as eggs need rainfall to flood the eggs and make them hatch.
Since weather makes such a difference, mosquito activity can begin at different times each year, and even in the same year, some species may become active before others. Still, it's possible to estimate roughly when mosquito season will begin and end, depending on what part of the country you live in.
As the hot weather of summer arrives, mosquito season reaches its peak. The warm temperatures make them pass through their life cycle faster, so more are laying eggs and more eggs are hatching.
By the end of the summer, you may notice a decline in bites, since there are fewer mosquitoes around. Those which were born earlier in the summer are gradually disappearing from accidents and predators, and fewer new eggs are hatching.
At the end of the season, mosquitoes species which die off for the winter won't disappear completely until frost, though they become less active as temperatures drop below 50 degrees. Those that hibernate as adults will begin their dormancy when winter weather arrives, but some may come out on warm winter days, so you might see a few any time of year that the temperature is warm enough. Still, you're less apt to be bitten as fall and winter come.
Scientists have tried to predict exactly what makes some mosquito seasons worse, but there are several factors. One study found that Culex mosquitoes were most apt to be infected by West Nile virus when the temperature was warmer, though wetter conditions sometimes also made a difference. Still, it came down to weather more than any other factor. "Overall, 80% of the weekly variation in mosquito infection was explained by prior weather conditions," according to the study. 
Time to Prepare
Though no one can change the temperature or rainfall to control when mosquitoes begin their season, humans can discourage them from breeding by eliminating standing water in backyards, around houses, in gutters and ditches. If you want to keep some shallow standing water, such as in a lily-pond or bird-bath, you can use mosquito control methods such as dunks to prevent mosquito larvae from surviving.
The start of mosquito season is also the time to begin control methods such as traps or sprays, to make sure window and door screens are secure, to put up screened gazebos for outside use or to begin wearing long clothing or using repellents for camping or other outdoor activities, especially at dawn and dusk.
Health Departments and Extension Agents around the country offer advice on how to deal with the local mosquito population. For example, the City of Nashville's health department holds a Backyard Inspection Day at the beginning of May each year, to usher in the start of mosquito season. The goal is to educate people about safe ways to prevent mosquito bites and to encourage them to eliminate breeding sites in backyards, such as anything that will hold stagnant water.
Alaska's infamous mosquito season is short, but not sweet. It peaks from the middle of June to the end of July, when swarms can be severe, but the mosquitoes aren't as bad in cities, above the tree line, or when there's a breeze, according to Alaska.org, which offers other tips for vacationers who don't want to get bit.
Whether you're dealing with mosquitoes at home or planning a vacation, it's useful to be aware of when mosquito season is likely to be at its peak. The website Accuweather.com has a handy customized mosquito activity forecast, where you can type in your zip code and see a prediction of mosquito activity for the next two days. It's based on local weather conditions as well as the time of year, so it's updated regularly for specific local conditions, though of course can only be an estimate.
 "Local impact of temperature and precipitation on West Nile virus infection in Culex species mosquitoes in northeast Illinois, USA."
Photos courtesy of the CDC and Homestead's image bank.