Insecticides vary in effectiveness, depending on the severity of infestation, preventive use and life stage of the flea. An insecticide might kill adult fleas, flea eggs and larvae or all of these.
Some insecticides are toxic not only to fleas but to pets and humans as well. Foggers can cause respiratory problems. Other insecticides, such as those found in a flea collar or monthly flea repellent solution, are intended for targeted flea removal or prevention.
The best insecticide is the chemical that kills the fleas but does not harm humans and animals. Botanical insecticides such as pyrethrum kill fleas in all life stages with little toxicity to mammals. The chemical imidacloprid is a potent flea killer with little risk to mammals. Diatomaceous earth is a natural substance that dehydrates the flea; it may be mixed with pyrethrin for insecticide purposes.