In the Midwest, the flea life cycle (adult flea to egg to larva to pupa to adult flea) takes about 35-40 days in early spring and 17-21 days in mid-summer. They thrive on warm weather, from about 70 70 to 80 degrees, and an environment of about 70% humidity.

By late summer, the heat has helped them thrive, and so September and October if there is an Indian summer, can help them thrive.

Fleas virtually shut down between November and March in most places, except the far south.

In southern and Gulf Coast states, 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 your pet 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. They can wrap themselves in the fibers of your household fabrics, and the flea in the pupa stage is almost invulnerable to attack from chemicals.