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Pyrethrin's are pesticides that can be found occurring naturally in nature. This natural insecticide is a product of the Chrysanthemum cinerariifolium which employs this chemical as a natural defense against foreign insect invaders. They first appeared on markets in the 1900's and have been continually used since then in products such as bug bombs, building insect sprays, and even to spray animals so that they do not get infectious diseases^[1].

The pyrethrins occur in the seed cases of the perennial plant pyrethrum (Chrysanthemum cinerariaefolium), which has long been grown commercially to supply the insecticide. Pyrethrins have been used as an insecticide for thousands of years. It is believed that the Chinese crushed chrysanthemum plants and used the powder as an insecticide as early as 1000 BC. It was widely known that the Chou Dynasty in China widely used pyrethrin for its insecticide properties. For centuries, crushed Chrysanthemum flowers have been used in Iran to produce Persian Powder, an insecticide for household use. Pyrethrins were identified as the potent chemical in the Chrysanthemum plants responsible for the insecticidal properties in the crushed flowers around 1800 in Asia. In the Napoleonic Wars, French soldiers used the flowers to keep away fleas and body lice.^{[dubious – discuss]}

Pyrethrin is most commonly used as an insecticide and has been used for this purpose since the 1900s. In the 1800s, it was known as "Persian powder", "Persian pellitory", and "zacherlin". Pyrethrins delay the closure of voltage-gated sodium channels in the nerve cells of insects, resulting in repeated and extended nerve firings. This hyper-excitation causes the death of the insect due to loss of motor coordination and paralysis. Even though this is extremely effective, insects have developed natural resistance to this toxin which creates problems for users. Resistance to pyrethrin has been bypassed by pairing the insecticide with synthetic synergists such as piperonyl butoxide. Together, these two compounds prevent detoxification in the insect, ensuring insect death. Synergists, which are two chemicals that combine and produce a greater effect, make pyrethrin more effective, allowing lower doses to be effective. Pyrethrins do not harm mammals because it is more difficult to break through the mammals skin than an insects exoskeleton. Also, mammals are able to process pyrethrin quickly and have higher body temperatures which prevent pyrethrin from working effectively ^[2].

Although pyrethrin is a potent insecticide, it also functions as an insect repellent at lower concentrations. Observations in food establishments demonstrate that flies are not immediately killed, but are found more often on windowsills or near doorways. This suggests, due to the low dosage applied, that insects are driven to leave the area before dying. Because of their insecticide and insect repellent effect, pyrethrins have been very successful in reducing insect pest populations that affect humans, crops, livestock, and pets, such as ants, spiders, and lice, as well as potentially disease-carrying mosquitoes, fleas, and ticks.

Specific pest species that have been successfully controlled by pyrethrum include: potato, beet, grape, and six-spotted leafhopper, cabbage looper, celery leaf tier, Say's stink bug, twelve-spotted cucumber beetle, lygus bugs on peaches, grape and flower thrips, and cranberry fruitworm.

Pyrethrins are among the safest insecticides on the market due to their rapid degradation in the environment.

Similarities between the chemistry of pyrethrins and synthetic pyrethroids include a similar mode of action and almost identical toxicity to insects (i.e., both pyrethrins and pyrethroids induce a toxic effect within the insect by acting on sodium channels).

Some differences in the chemistry between pyrethrins and synthetic pyrethroids have the result that synthetic pyrethroids have relatively longer environmental persistence than do pyrethrins. Pyrethrins have shorter environmental persistence than synthetic pyrethroids because their chemical structure is more susceptible to the presence of UV light and changes in pH.^{[citation needed]}

The use of pyrethrin in products such as natural insecticides and pet shampoos^[why?] increases the likelihood of toxicity in mammals that are exposed. Medical cases have emerged showing fatalities from the use of pyrethrin, prompting many organic farmers to cease use. One fatal case of an 11-year-old girl with a known asthmatic condition and who used shampoo containing only a small amount (0.2% pyrethrin) to wash her dog was documented.

Chronic toxicity in humans occurs most quickly through respiration into the lungs, or more slowly through absorption through the skin. Allergic reactions may occur after exposure, leading to itching and irritated skin as well as burning sensations. These types of reactions are rare because the allergenic component of pyrethrin in semi-synthetic pyrethroids has been removed. The metabolite compounds of pyrethrin are less toxic to mammals than their originators, and compounds are either broken down in the liver or gastrointestinal tract, or excreted through feces; no evidence of storage in tissues has been found^[2]

Exposure to pyrethrum, the crude form of pyrethrin, causes harmful health effects for mammals. Pyrethrum also has an allergenic effect that commercial pyrethroids don't have. In mammals, toxic exposure to pyrethrum can lead to tongue and lip numbness, drooling, lethargy, muscle tremors, respiratory failure, vomiting, diarrhea, seizures, paralysis, and death. Exposure to pyrethrum in high levels in humans may cause symptoms such as asthmatic breathing, sneezing, nasal stuffiness, headache, nausea, loss of coordination, tremors, convulsions, facial flushing, and swelling.^{[unreliable source?]} A possibility of damage to the immune system exists that leads to a worsening of allergies following toxicity. Infants are unable to resourcefully break down pyrethrum due to the ease of skin penetration, causing similar symptoms as adults, but with an increased risk of death.

In aquatic settings, toxicity of pyrethrin fluctuates, increasing with rising temperatures, water, and acidity. Run-off after application has become a concern for sediment-dwelling aquatic organisms because pyrethroids can accumulate in these areas. Aquatic life is extremely susceptible to pyrethrin toxicity, and has been documented in species such as the lake trout. Although pyrethrins are quickly metabolized by birds and most mammals, fish and aquatic invertebrates lack the ability to metabolize these compounds, leading to a toxic accumulation of byproducts. To combat the accumulation of pyrethroids in bodies of water, the Environmental Protection Agency (EPA) has introduced two labeling initiatives. The Environmental Hazard and General Labeling for Pyrethroid and Synergized Pyrethrins Non-Agricultural Outdoor Products was revised in 2013 to reduce runoff into bodies of water after use in residential, commercial, institutional, and industrial areas. The Pyrethroid Spray Drift Initiative updated language for labeling all pyrethroid products to be used on agricultural crops. Because of its high toxicity to fish and aquatic invertebrates even at low doses, the EPA recommends alternatives such as pesticide-free methods or alternative chemicals that are less harmful to the surrounding aquatic environment.

Pyrethrin is mainly used on land and can also have impacts in the places that it is used. For instance pyrethrin has the ability to be persistent in the fields that it is sprayed on because it is a organic halide. This persistence in crops can lead to negative effects for meat production.^[3]

Pyrethrins are applied broadly as nonspecific insecticides. Bees have been shown to be particularly sensitive to pyrethrin, with fatal doses as small as 0.02 micrograms. Due to this sensitivity and pollinator decline, pyrethrins are recommended to be applied at night to avoid typical pollinating hours, and in liquid rather than dust form.

http://npic.orst.edu/factsheets/pyrethrins.html

^[1]