Fleas go through a life cycle that includes egg and cocoon stages. Although adult fleas are relatively easy to kill, the egg and cocoon stages are very resistant. The entire life cycle of the flea is the egg to larva, from larva to cocoon, from cocoon to adult. This life cycle can vary from 14 days during warm, moist weather, to several weeks or months under extremes of climatic conditions. So it is best to not stop prevention in the cooler months.