Some plants in the Chrysanthemum genus contain a chemical that's toxic to many insects but considerably less dangerous to mammals, making it an effective and relatively safe insecticide. When concentrated, this chemical can also function as a mosquito repellent.
Pyrethrum Daisy and Dalmatian Chrysanthemum
The pyrethrum daisy (Chrysanthemum coccineum), also known as the painted daisy, is a flowering perennial that's winter hardy in U.S. Department of Agriculture plant hardiness zones 3 to 7. Dalmatian chrysanthemum (_Chrysanthemum cinerariifolium_) is hardy in USDA zones 5 to 9. These plants are the source of the natural insecticide, not the hardy garden mum (_Chrysanthemum morifolium_), which is hardy in USDA zones 5 to 9.
Growing painted daisy or Dalmatian chrysanthemum in your garden may have some minor, general repellent effect, but the plants' effectiveness at repelling mosquitoes is greater when its insecticidal chemicals are extracted and used in localized areas.
Pyrethrum and Pyrethrins
Pyrethrum is a naturally occurring chemical that has a neurotoxic effect on mosquitoes and other soft-bodied insects, and pyrethrins are the active ingredients in pyrethrum. The primary effect of pyrethrins is insecticidal, but there is evidence that they have a repellent effect as well. Dalmatian chrysanthemums typically contain more pyrethrins than painted daisies do.
Pyrethrins are often used in commercial pesticides and repellents, but they are different from pyrethroids, which are synthetic insecticides that act much like the natural chemicals.
To make pyrethrum dust, begin by harvesting flowers during dry, warm weather when the flowers are fully developed. Choose flowers near the center of the plant because they contain more pyrethrum than those on the outside of the plant. Hang the flowers upside down to dry for one or two days to concentrate the pyrethrins before crushing the flowers into a fine powder.
Sprinkle the dust in areas where you wish to repel mosquitoes. Burning the dust produces smoke that may also have a repellent effect; however, this may not be advisable in many circumstances.
To create a sprayable pyrethrin extract, soak about 3/4 ounce of dried flowers in 1 1/2 gallons of water for three hours. Spray the extract directly on surfaces or clothing to repel mosquitoes, but avoid inhaling and ingesting any of it. Pyrethrins are unstable and degrade quickly, so use the spray promptly.