Pyrethrin insecticides are derived from two daisies (Chrysanthemum cinerariifolium and C. coccineum) that contain nerve toxins which kill insects on contact. They are approved for organic use, but as with all garden chemicals, you should be careful to apply them according to the label directions and only when necessary.
You can buy pyrethrins in several forms, but the most common are bottled insecticides that contain pyrethrin extract.
The dried flower heads are also available or you can grow your own. Pyrethrin-extract based insecticides are effective against a wide variety of insects, including soft-bodied insects and chewing and sucking insects such as aphids, leafhoppers, mealybugs, spider mites, stink bugs, scale, thrips, and whiteflies.
Although they are organic, pyrethrin-based insecticides are still moderately toxic to mammals, including household pets like cats and dogs. Also be aware that many store-preparations are enhanced with piperonyl butoxide, which makes it more toxic. If you have small children or pets in the house, make sure to limit their exposure to any application of pyrethrin-based insecticides. Pyrethrins are also found sold in combination with a number of other products, including copper and sulfur fungicides.
To use pyrethrin, follow label instructions during application, wear gloves and protective coverings, and repeat applications as needed to control your insect problem.
Depending on the pest target, multiple applications may be necessary. However, you should stop the applications once the pest is under control.
Make Your Own Pyrethrin
You can make your own pyrethrin insecticide from homegrown pyrethrum daisies. Once the plant is blooming, pick off the full blooms and dry the flower heads in a cool, dark and dry place.
Store the dried flower heads in a tightly sealed, airtight container. When it comes time to use them, grind up the flower heads, mix with a little liquid soap to increase their spreadability, and combine with enough water to make a spray. The strength of homegrown pyrethrin varies, so experiment with the proportions until you achieve effective control.
Alternatively, you can use an alcohol extraction process to obtain pyrethrin. Soak 1 cup of packed, fresh flowerheads in 1/8 cup of 70% isopropyl alcohol. Let the container sit overnight (covered). The next day, strain the mixture through a cheesecloth and store the homemade extract in a tightly sealed container. To use, mix with up to 4 quarts of water and spray.