Flea season varies by climate...
This is from the Dog Owner's Guide website:http://www.canismajor.com/dog/flea01.html
I think the Advantage website used to have a map of the U.S. showing flea seasons, but the site appears to be offline right now.
'Tis the season of renewal, of warmer weather, longer days, flowering trees and shrubs, and bright-colored pansies . . . and fleas.
By late spring, fleas begin to emerge from their pupae as adults and migrate to the nearest dog or cat for blood meals. An adult flea mates shortly after emergence and begins laying eggs within 36 hours. In her brief 50-day lifespan, a single female flea can lay more than 2000 eggs.
Fleas are marvelously adapted for survival. The female lays eggs on the host animal, but the eggs fall to the ground, carpet, sofa, dog bed, owner's bed, or easy chair where they hatch in two-to-five days. The flea larva feeds on organic debris in the environment. Within a week or two, depending on temperature and humidity, the larva spins a pupa (or cocoon) to protect it during metamorphosis to the adulthood.
In the hard-shelled pupa, the larva transforms from a tiny maggot-like creature into a six-legged blood-thirsty super-jumper able to leap 150 times its own body length, and the cycle begins anew.
In the Midwest, the flea life cycle (adult flea Ã:censor: egg Ã:censor: larva Ã:censor: pupa Ã:censor: adult flea) takes about 35-40 days in early spring and 17-21 days in mid-summer. By late summer, cycles slow to two months or more, and they virtually shut down between November and March. In southern and Gulf Coast states, however, fleas complete their cycles in 20 days or less for most of the year and only slow down a bit in mid-winter.
Humidity is critical to flea survival. Eggs need relative humidity of 70-75 percent to hatch, and larvae need at least 50 percent humidity to survive. In humid areas, about 20 percent of the eggs survive to adulthood; in arid areas, less than five percent complete the cycle.
All bets are off when Fido brings fleas in for the winter. Household warmth can keep the cocooned larvae alive until conditions are ripe for emergence of the adults and may even allow life cycles to continue at a snail's pace.