Fleas and their larvae live in small nooks and crannies. They live under pile in deep pile rugs, in crevices, under mattresses, on boxsprings, between the boxsprings and bed, and under the cushions on the couch and chairs. They live under the edges of rugs and outdoors under the porch, in dog houses, in crevices in the cement in the cellar, in the garage, and in places you wouldn't dream of living. The chances of a mutant developing in the resulting crowd of offspring is very good. Such a mutant offspring may then metabolize an insecticide being used on a dog or cat in its immediate environment.
Fleas seem to congregate where animals sleep.
At this time the allergy or other immune response caused by the flea is more marked. Fleas want the animals to react. Reactive animals are the ideal host for the flea. Therefore, these areas are the most important when you spraying the house. In the house the odor of such a Malathion solution and the complexity of the sheltered environment call for different measures.