PYRETHRINS
Pyrethrin-based formulations are considered by the EPA to be "general use" insecticides - meaning that homeowners can buy and use them without a license. They are one of the least poisonous insecticides to mammals. In fact, pyrethrins are important ingredients in products commonly found in many homes. Household aerosols and pet shampoos are good examples. But, they are poisons.
While they are low in toxicity to people and pets because the body quickly breaks them down, it is not a good idea to come into close or frequent direct contact with the mist. Inhaling pyrethrins can cause respiratory problems (coughing, wheezing, etc.) and skin contact can cause a rash or itching. Pyrethrins are highly toxic to fish and tadpoles and should be especially carefully applied around water.
MAN-MADE BOTANICALS - PYRETHROIDS
The downside to pyrethrins' natural origin is they can be expensive and may be subject to shortages in supply caused by bad weather. In response, scientists have developed synthetic forms of pyrethrins, called pyrethroids that work in the same way. Like pyrethrins, a synergist is added to them to make them more effective. Pyrethroids are less expensive than pyrethrins and are an important ingredient in the insecticide "cocktail" used by municipalities and counties to control mosquitoes.
An important difference between pyrethroids and pyrethrins is that the man-made pyrethroids are not as readily degraded by sunlight and therefore leave a lasting residue. Another important difference is that mosquito populations are already showing signs of resistance to the man-made pyrethroids. There is no evidence yet of resistance to pyrethrins.