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Pyrethrin
Pyrethrin
Pyrethrin
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Chemical structure of some pyrethrins: pyrethrin I (R=CH3), pyrethrin II (R=CO2CH3)

The pyrethrins are a class of organic compounds normally derived from Chrysanthemum cinerariifolium that have potent insecticidal activity by targeting the nervous systems of insects. Pyrethrin naturally occurs in chrysanthemum flowers and is often considered an organic insecticide when it is not combined with piperonyl butoxide or other synthetic adjuvants.[1] Their insecticidal and insect-repellent properties have been known and used for thousands of years.

Pyrethrins are gradually replacing organophosphates and organochlorides as the pesticides of choice as the latter compounds have been shown to have significant and persistent toxic effects to humans. They first appeared on markets in the 1900s 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.[2]

Chemistry

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Physical and chemical properties of some pyrethrins.
Group Pyrethrin I Pyrethrin II
Chemical compound Pyrethrin I[3][4] Cinerin I[5][4] Jasmolin I[6] Pyrethrin II[7][4] Cinerin II[8][4] Jasmolin II[9]
Chemical structure Pyrethrin I Cinerin I Jasmolin I Pyrethrin II Cinerin II Jasmolin II
Chemical formula C21H28O3 C20H28O3 C21H30O3 C22H28O5 C21H28O5 C22H30O5
Molecular mass (g/mol) 328.4 316.4 330.5 372.5 360.4 374.5
Boiling point (°C) 170 137 ? 200 183 ?
Vapor pressure at 25 °C (mmHg) 2.03×10−5 1.13×10−6 ? 3.98×10−7 4.59×10−7 ?
Solubility in water (mg/L) 0.2 0.085 ? 9.0 0.03 ?

History

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The pyrethrins occur in the seed cases of the perennial plant pyrethrum (Chrysanthemum cinerariaefolium), which has long been grown commercially to supply the insecticide. In the 1880s, pyrethrum cultivation began in Japan when Ueyama grew the flowers in Wakayama and promoted their use across the country, primarily for lice control. Inspired by this, his wife Yuki conceptualized the mosquito coil, which became an effective and globally used tool against mosquitoes. Scientific interest followed, with biologist Fujitani publishing the first study on pyrethrum's insecticidal properties in 1909, sparking international chemical research. In 1923, Yamamoto identified that the active compounds in pyrethrum contained a cyclopropane ring, building on interest from Umetaro Suzuki. Later, Staudinger and Ruzicka analyzed the compounds in depth, and in 1944, LaForge and Barthel finally confirmed the full structures of pyrethrins I and II, along with cinerins I and II.[10]

Biosynthesis

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Cyclopropanation reaction producing chrysanthemyl diphosphate, an intermediate in the biosynthesis of chrysanthemic acid. The reaction starts from dimethylallyl pyrophosphate (DMAPP).

Well after their use as insecticides began, their chemical structures were determined by Hermann Staudinger and Lavoslav Ružička in 1924.[11] Pyrethrin I (CnH28O3) and pyrethrin II (CnH28O5) are structurally related esters with a cyclopropane core. Pyrethrin I is a derivative of (+)-trans-chrysanthemic acid.[12][13] Pyrethrin II is closely related, but one methyl group is oxidized to a carboxymethyl group, the resulting core being called pyrethric acid. Knowledge of their structures opened the way for the production of synthetic analogues, which are called pyrethroids. Pyrethrins are classified as terpenoids. The key step in the biosynthesis of the naturally occurring pyrethrins involves two molecules of dimethylallyl pyrophosphate, which join to form a cyclopropane ring by the action of the enzyme chrysanthemyl diphosphate synthase.[14]

Production

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Tanacetum cinerariifolium also called the Dalmatian chrysanthemum

Commercial pyrethrin production mainly takes place in mountainous equatorial zones. The commercial cultivation of the Dalmatian chrysanthemum (C. cinerariifolium) takes place at an altitude of 1600 to 3000 meters[15] above sea level.[16] This is done because pyrethrin concentration has been shown to increase as elevation increases to this level. Growing these plants does not require much water because semiarid conditions and a cool winter deliver optimal pyrethrin production. The Persian chrysanthemum C. coccineum also produces pyrethrins but at a much lower level. Both may be planted in low-altitude zones in dry soil, but the pyrethrin level is lower.[15]

Pyrethrum extracted of the Persian chrysanthemum (painted daisy) was already imported to central Europe from Georgia in the middle of the 19th century. Most of the world's supply of pyrethrin and C. cinerariaefolium today comes from Kenya, which produces the most potent flowers. Other countries include Croatia (in Dalmatia) and Japan. The flower was first introduced into Kenya and the highlands of Eastern Africa during the late 1920s. Since the 2000s, Kenya has produced about 70% of the world's supply of pyrethrum.[17] A substantial amount of the flowers are cultivated by small-scale farmers who depend on it as a source of income. It is a major source of export income for Kenya and source of over 3,500 additional jobs. About 23,000 tons were harvested in 1975. The active ingredients are extracted with organic solvents to give a concentrate containing the six types of pyrethrins: pyrethrin I, pyrethrin II, cinerin I, cinerin II, jasmolin I, and jasmolin II.[18]

Processing the flowers to cultivate the pyrethrin is often a lengthy process, and one that varies from area to area. For instance, in Japan, the flowers are hung upside down to dry which increases pyrethrin concentration slightly.[15] To process pyrethrin, the flowers must be crushed. The degree to which the flower is crushed has an effect on both the longevity of the pyrethrin usage and the quality. The finer powder produced is better suited for use as an insecticide than the more coarsely crushed flowers. However, the more coarsely crushed flowers have a longer shelf life and deteriorate less.[15]

Use as an insecticide

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Pyrethrin is most commonly used as an insecticide and has been used for this purpose since the 1900s.[18] 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 hyperexcitation causes the death of the insect due to loss of motor coordination and paralysis.[19] 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.[20] Synergists make pyrethrin more effective, allowing lower doses to be effective. Pyrethrins are effective insecticides because they selectively target insects rather than mammals due to higher insect nerve sensitivity, smaller insect body size, lower mammalian skin absorption, and more efficient mammalian hepatic metabolism.[21] Also, mammals are able to process pyrethrin quickly and have higher body temperatures which prevent pyrethrin from working effectively [22]

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.[23] 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.

As pyrethrins and pyrethroids are increasingly being used as insecticides, the number of illnesses and injuries associated with exposure to these chemicals is also increasing.[24] However, few cases leading to serious health effects or mortality in humans have occurred, which is why pyrethroids are labeled "low-toxicity" chemicals and are ubiquitous in home-care products.[21] Pyrethrins are widely regarded as better for the environment, and can be harmless if used only in the field with localized sprays, as UV exposure breaks them down into harmless compounds. Additionally, they have little lasting effect on plants, degrading naturally or being degraded by the cooking process.[25]

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.[26]

Toxicity

[edit]

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).[27]

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 shampoo, for its ability to kill fleas, 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.[28]

Chronic pyrethrin toxicity in humans

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Chronic toxicity in humans occurs most quickly through respiration into the lungs, or more slowly through absorption through the skin.[29] Allergic reactions may occur after exposure, leading to itching and irritated skin as well as burning sensations.[30] These types of reactions are rare because the allergenic component of pyrethrin in semi-synthetic pyrethroids has been removed.[31] 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 [citation needed].

Pyrethrum toxicity

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Exposure to pyrethrum, the crude form of pyrethrin,[31] causes harmful health effects for mammals. Pyrethrum also has an allergenic effect that commercial pyrethroids don't have.[31] 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.[29] 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.[32][unreliable source?] A possibility of damage to the immune system exists that leads to a worsening of allergies following toxicity.[29] 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.[33]

Feline Toxicity

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A cat's liver is not able to metabolize pyrethrin making it toxic to cats. Exposure often results from a flea and tick treatment for dogs being used on a cat.[34]

Environmental effects

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Aquatic habitats

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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.[35] 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.[29] 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.[36] The Pyrethroid Spray Drift Initiative updated language for labeling all pyrethroid products to be used on agricultural crops.[36] 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.[37]

Terrestrial Habitats

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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. This persistence in crops can lead to negative effects for meat production.[38]

Bees

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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.[1] 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.[39]

References

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Pyrethrin

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Pyrethrins are a group of six naturally occurring organic compounds extracted from the flowers of certain species, primarily Chrysanthemum cinerariifolium, that exhibit potent insecticidal properties by disrupting the nervous systems of , leading to and death. These compounds, collectively known as pyrethrins I and II (including pyrethrin I, cinerin I, and jasmolin I for the first group, and pyrethrin II, cinerin II, and jasmolin II for the second), have been used as insecticides since the early , with their insect-repelling effects first noted in around 1800. The crude extract from the dried flowers is referred to as pyrethrum powder, which contains pyrethrins along with other plant impurities. Pyrethrins are widely employed in over 2,000 registered pesticide products worldwide, including household sprays, foggers, pet shampoos, and agricultural formulations for controlling pests such as mosquitoes, fleas, ticks, and flies. They are particularly valued in organic farming and public health applications due to their rapid breakdown in the environment—exhibiting half-lives of approximately 11.8 hours in water and 12.9 hours on soil under sunlight exposure—and relatively low toxicity to mammals, including humans. However, pyrethrins are highly toxic to aquatic organisms like fish and beneficial insects such as bees, necessitating careful application to minimize ecological impact. Often formulated with synergists like piperonyl butoxide to enhance efficacy, pyrethrins served as the chemical basis for the development of synthetic pyrethroids, which offer greater stability but are distinct from the natural forms.

Overview

Definition and Natural Sources

Pyrethrins are a class of natural organic compounds classified as esters, comprising six closely related insecticidal substances: pyrethrin I and II, cinerin I and II, and jasmolin I and II. These compounds are primarily extracted from the dried flower heads, particularly the seed casings, of the pyrethrum daisy, scientifically known as Tanacetum cinerariifolium, a perennial plant in the Asteraceae family. The natural occurrence of pyrethrins is limited to specific species in the genus (formerly classified under ), with T. cinerariifolium serving as the principal source due to its high concentration of these esters in the glandular trichomes of its flowers. Commercial cultivation of this plant is concentrated in high-altitude regions, typically between 1,700 and 3,000 meters above sea level, where optimal cool and moist conditions enhance pyrethrin yields; major production areas include , the world's leading producer, and . The term "pyrethrin" derives from the Greek word pyretos, meaning fever, reflecting the historical use of plants as febrifuges for treating fever-related ailments, with later associations in antimalarial efforts through . In contrast to synthetic pyrethroids, which are laboratory-engineered analogs modified for enhanced stability and persistence, pyrethrins are exclusively naturally occurring and rapidly degrade in the environment.

Historical Development

The insecticidal properties of pyrethrum, the source of pyrethrins, were recognized in ancient Persia around 400 BCE, where dried flower powders from species like Tanacetum coccineum were used to repel insects and delouse children. Similar applications for insect repulsion are documented in ancient and the , where ground pyrethrum flowers served as a natural fumigant. These early uses remained localized until the , when pyrethrum flowers began to be imported into from the and regions, initially as "Persian insect powder" for commercial production starting in the . Commercial cultivation expanded in the late , with introduced to around 1881 from , leading to large-scale farming in regions like Wakayama by entrepreneurs such as Eiichiro Ueyama. In , British colonists established cultivation in during the 1920s, transforming it into a major export crop by the 1930s and supporting the global supply for insecticides. A key innovation emerged in in the , when Ueyama invented the spiral-shaped using powder mixed with binders, revolutionizing household insect control and driving demand for cultivation worldwide. Early 20th-century research advanced understanding of pyrethrins, with Japanese chemist Takeo Yamamoto identifying the ring structure in 1923, followed by the complete elucidation of pyrethrin I and II by U.S. chemists F.B. LaForge and W.F. Barthel in 1944. During , pyrethrum demand surged for military use against disease vectors, but postwar introduction of synthetic alternatives like led to a sharp decline in pyrethrum production by the 1950s. Interest resurged in the 1970s following bans on persistent organochlorine pesticides, including in 1972, as pyrethrins offered a safer, biodegradable option amid growing environmental concerns.

Chemical and Biological Foundations

Molecular Structure and Properties

Pyrethrins are a class of six structurally related esters composed of three monoterpenoid acids—chrysanthemic, pyrethric, and jasmolinic—esterified to three alcohols: pyrethrolone, cinerolone, and jasmololone. The core molecular framework consists of a substituted cyclopropanecarboxylic acid linked via an ester bond to a monoterpenoid alcohol bearing a cyclopentenone ring. Pyrethrin I, the ester of (+)-trans-chrysanthemic acid and (S)-pyrethrolone, has the molecular formula \ceC21H28O3\ce{C21H28O3} and a molecular weight of 328.45 g/mol. Pyrethrin II, formed from pyrethric acid and (S)-pyrethrolone, possesses the formula \ceC22H28O5\ce{C22H28O5} and a molecular weight of 372.46 g/mol. These molecules exhibit complex stereochemistry due to multiple chiral centers and geometric isomerism in the side chains. The chrysanthemic acid moiety contains two chiral centers at positions 1 and 3 of the cyclopropane ring, resulting in cis and trans diastereomers; natural pyrethrins predominantly feature the (1R,3R)-trans configuration, which is essential for their biological activity. The pyrethrolone alcohol introduces an additional chiral center at C-3, yielding a total of three chiral centers and up to eight possible stereoisomers per pyrethrin. The pentadienyl side chain in pyrethrolone also allows for E/Z geometric isomerism, further contributing to structural diversity, though natural isolates are primarily the (2Z,4E) form. Physically, pyrethrins manifest as viscous, pale yellow to amber oils at , with low volatility (vapor pressure on the order of 10−710^{-7} mm Hg). They exhibit very low water solubility, typically less than 1 mg/L at 20°C and 7, but are highly soluble in organic solvents such as , acetone, and (up to 250 g/L). Their octanol-water partition coefficients (log KowK_{ow}) range from 4.3 for pyrethrin II to approximately 5.9 for pyrethrin I, reflecting strong and affinity for non-polar environments. Chemically, pyrethrins demonstrate stability in neutral and mildly acidic conditions, remaining intact for over 30 days at 5–7. However, as esters, they are susceptible to alkaline , rapidly degrading in basic media ( >9) to yield the corresponding carboxylic acids and alcohols. Exposure to light induces , primarily converting trans forms to less active cis isomers, followed by oxidative degradation; the under direct is typically 1–3 hours.

Biosynthesis Pathways

Pyrethrins are a class of natural insecticides biosynthesized in the flowers of , primarily through pathways that integrate isoprenoid precursors into complex structures. The biosynthesis begins with the formation of (DMAPP) and isopentenyl pyrophosphate (IPP), the universal building blocks of , which are generated via the mevalonate (MVA) pathway in the cytosol and the 2C-methyl-D-erythritol 4-phosphate (MEP) pathway in plastids. These precursors condense to form (GPP) and (FPP), which serve as substrates for the alcohol and acid moieties of pyrethrins, respectively. A critical early step involves the chrysanthemyl diphosphate (CDS), which catalyzes the head-to-tail condensation of DMAPP and IPP to produce the irregular monoterpenoid chrysanthemyl diphosphate (CPP), the foundational alcohol component of all pyrethrins. This reaction is unique to pyrethrin-producing and is encoded by the TcCDS in T. cinerariifolium, where it exhibits specificity for the trans-isomer of IPP to ensure stereochemical fidelity in the final product. Subsequent of CPP, catalyzed by the bifunctional activity of chrysanthemyl diphosphate (CDS) or a Nudix such as TcNudix1, yields chrysanthemol, the precursor. The acid portions derive from oxidation of chrysanthemol to chrysanthemic acid via alcohol and . Pyrethric and jasmolinic acids involve additional modifications, including P450-mediated s (e.g., CYP77B6v2 for C10 in pyrethric acid), oxidations, and incorporation of polyunsaturated chains from the pathway, mediated by lipoxygenases and acyltransferases. Esterification links the acid and alcohol moieties via acyltransferases, such as those identified in the BAHD (e.g., TcTPs), which transfer the from the acid-CoA to the alcohol, producing the six pyrethrin esters (pyrethrins , cinerins , and jasmolins ). These enzymes show substrate promiscuity, allowing the formation of multiple analogs within glandular trichomes of the plant's capitula. Genetically, the pyrethrin pathway is regulated by a cluster of synthase and genes in the T. cinerariifolium , with key loci like TcCDS and the terpene synthase gene TcTPS2 upregulated in flower heads compared to vegetative tissues. Environmental factors, including high altitude and intense light exposure, enhance expression of these genes by influencing signaling, which activates pathway transcription factors and boosts pyrethrin accumulation up to 2-3% of dry flower weight. Transcriptomic studies have mapped over 20 pathway-related genes, revealing co-regulation that correlates with pyrethrin yield variations across cultivars. Post-2010 advances in have elucidated pathway bottlenecks, with of TcCDS and associated P450s in achieving detectable pyrethrin yields, though limited by precursor flux and enzyme stability; for instance, co-expression of IPP isomerase increased CPP production by 5-fold in strains. These efforts, building on the draft genomic sequencing of T. cinerariifolium in 2019, aim to optimize platforms without fully reconstructing the pathway, focusing instead on rate-limiting steps like esterification efficiency. Recent studies (as of 2023) have identified transcription factors such as TcMYC2, TcbZIP60, and TcWRKY75 that positively regulate pyrethrin genes via signaling, alongside chemical genetics tools targeting the TcGLIP for pathway modulation.

Production and Commercial Aspects

Cultivation and Extraction Methods

Pyrethrum, the source of pyrethrins, is cultivated mainly in equatorial highland regions with cool climates and moderate rainfall, such as altitudes exceeding 1,800 meters above , temperatures ranging from 15–25°C, and annual precipitation of 800–1,200 mm on well-drained soils with a of 6.0–7.0. was once a dominant producer, historically accounting for over 70% of global supply in the due to its ideal conditions, though its share has declined to around 2–10% in recent years amid from other producers and synthetics; as of 2024, leads with approximately 13,941 hectares under cultivation and 8,360 metric tons of dried flowers produced. 's cultivation area has expanded to 9,549 acres (about 3,863 hectares) in 2024, up 2% from 2023, with dry flower production reaching 1,634 metric tons in 2023 and government plans to distribute 1 million seedlings in 2025 to further revive output. Plants are propagated from seeds or cuttings and transplanted at densities of 4–8 plants per square meter to optimize growth and flower production. Harvesting occurs 4–6 months after planting, with flowers picked by hand when fully open to maximize pyrethrin content, which typically yields 0.8–1.5% by dry weight in mature flower heads. After harvest, flowers undergo drying in the sun or shade to reduce moisture content to about 10%, followed by grinding into a coarse powder to facilitate extraction. Traditional extraction employs non-polar s like or in a or immersion process, where the powdered flowers are soaked and the solvent is evaporated to yield a crude containing 20–30% pyrethrins. Modern methods increasingly utilize () extraction, which operates at pressures of 200–400 bar and temperatures of 40–60°C, offering a -free alternative that preserves pyrethrin integrity and achieves higher selectivity without residual solvents. The resulting extract is then purified through or to produce a standardized concentrate of 45–55% total pyrethrins, suitable for commercial formulations. Cultivation faces several challenges, including labor-intensive manual harvesting due to the need for selective picking of individual flowers multiple times per season, which can involve 15–36 pickings annually per plant. Pests such as , , and diseases like ray blight (Didymella tanaceti) pose significant threats, requiring integrated management to prevent yield losses of up to 50% in affected fields. Global acreage under cultivation stands at approximately 25,000 hectares as of 2024, concentrated in , with ongoing efforts to expand through improved varieties and climate-resilient practices. Historically, extraction methods evolved from labor-intensive manual pressing and simple immersion in the early to mechanized post-1950s, incorporating continuous extraction plants and automated drying systems that boosted efficiency and scaled production for global markets.

Modern Production Advances

Recent advances in pyrethrin production have focused on techniques to enable of biosynthetic genes in non-native hosts, aiming to overcome limitations of traditional extraction from . Key enzymes such as chrysanthemyl diphosphate synthase (TcCDS), (TcADH2), and (TcALDH) from pyrethrum have been cloned and co-expressed in (), resulting in the production of trans-chrysanthemic acid precursors at levels up to 1328 nmol/g fresh weight—a 48-fold increase over TcCDS alone. Similarly, the complete pyrethric acid pathway has been reconstituted in through transient expression of TcCDS alongside four oxidoreductases (TcADH2, TcALDH1, TcCHH, and TcCCMT), yielding detectable amounts of pyrethric acid and demonstrating the feasibility of using fast-growing plants as biofactories for pyrethrin components. These lab-scale efforts, reported in studies from 2019 to 2022, highlight the potential for scalable, controlled production independent of field conditions. Sustainable agricultural practices have enhanced pyrethrin yield and environmental compatibility through initiatives in , where remains a key . In , companies like Kentegra support smallholder farmers with planting materials, training in (IPM), and long-term purchase contracts, reducing reliance on synthetic inputs and boosting farmer incomes by up to three times compared to other crops; this has revitalized production, which once accounted for over 80% of global supply. In , the Pyrethrum Company of Tanzania (PCT) collaborates with thousands of organic family farms, investing in IPM-based organic , improved seedstocks, and extension services to enable 10-month annual harvests while minimizing chemical use and soil degradation. These initiatives, active since the mid-2010s, promote and by integrating and natural fertilizers. Alternative production sources include engineering pyrethrin pathways into other and exploring cell cultures to diversify supply. has served as a model host for transient and stable expression of TcCDS and downstream genes, producing chrysanthemol and related esters that enhance pest resistance in the engineered plants. Efforts in ( lycopersicum) fruit-specific expression of TcCDS with Solanum habrochaites-derived ADH and ALDH have achieved trans-chrysanthemic acid levels of up to 183 μg/g fresh weight, suggesting tomatoes as a viable alternative platform. cell suspension cultures of cinerariifolium have also shown promise for pyrethrin synthesis, with ongoing optimizations in media and elicitors to increase yields beyond traditional field levels, though commercial adoption remains limited. Market analyses indicate a growing shift toward such non-agricultural methods, driven by demand for organic pesticides, with the global pyrethrin market projected to reach USD 120.7 million by 2033 at a 6.3% CAGR, partly fueled by biotech innovations. Despite these progresses, challenges in scaling biotech approaches persist, including high engineering costs and suboptimal yields in systems, which currently lag behind natural extraction efficiency. However, these methods offer a reduced environmental footprint by decreasing land requirements, avoiding runoff, and enabling year-round production in controlled settings like greenhouses to buffer against climate variability. Ongoing research emphasizes cost-effective pathway optimization to transition from lab to industrial scales, potentially transforming pyrethrin into a more accessible sustainable .

Applications and Uses

Insecticidal Applications

Pyrethrins exert their insecticidal effects primarily by binding to voltage-gated sodium channels in the nervous systems of , prolonging the open state of these channels and disrupting normal nerve impulse transmission. This binding leads to repetitive firing of neurons, resulting in hyperexcitation, uncoordinated movement, , and eventual death of the target . The compounds are classified into Type I and Type II based on their and physiological impacts: Type I pyrethrins, lacking a cyano group, primarily induce rapid knockdown through tremors and hyperexcitability without prolonged convulsions, while Type II variants, containing a cyano group, cause more severe lethal effects including choreoathetosis and due to enhanced channel modification. This selective arises from differences in isoforms between and mammals, making pyrethrins highly effective against arthropods at low doses. Common formulations of pyrethrins for insecticidal use include aerosols for space sprays, dusts for direct application, and emulsifiable concentrates that can be diluted into liquid sprays for broader coverage. These formulations facilitate quick dispersion and contact with pests in agricultural, household, and settings. To enhance and counteract insect detoxification mechanisms, pyrethrins are frequently combined with synergists such as , which inhibits enzymes responsible for metabolizing the active compounds, thereby prolonging their activity without adding toxicity. Pyrethrins demonstrate broad-spectrum efficacy against a variety of pests, including mosquitoes, flies, , and beetles, making them suitable for both agricultural crop protection and . Historically, pyrethrum-based insecticides, from which pyrethrins are derived, played a key role in military applications during the , particularly in prevention efforts where they were used to target mosquitoes in theaters like the Pacific and Mediterranean, reducing disease incidence among troops through and residual treatments. A primary advantage of pyrethrins lies in their rapid knockdown effect, immobilizing insects within minutes of exposure, which is ideal for immediate in sensitive environments. Additionally, their low environmental persistence—degrading rapidly in and air within hours to days—minimizes long-term residue accumulation compared to more stable synthetic alternatives.

Non-Insecticidal Uses

Pyrethrins are employed in medical applications primarily as pediculicides for treating lice infestations, including head, body, and pubic lice. They are formulated in topical shampoos and lotions, typically at concentrations equivalent to 0.33% pyrethrins combined with as a synergist, which enhances efficacy by inhibiting detoxification enzymes. These products work by disrupting the of lice, leading to and death, though a second application is often required after 9-10 days to target newly hatched nymphs. In , pyrethrins serve as key ingredients in flea and control products for dogs, available in shampoos, spot-on treatments, collars, and dusts. These formulations provide rapid knockdown of ectoparasites, with veterinarians recommending them for their relatively low mammalian compared to alternatives like organophosphates. However, pyrethrins are highly toxic to cats due to their deficient glucuronidation pathway, which impairs , necessitating strict separation of dog and cat products to avoid accidental exposure. Beyond parasiticide roles, pyrethrins find limited use as natural insect repellents in cosmetic and , such as lotions and sprays, where they deter mosquitoes and other biting s through olfactory disruption of insect sensory receptors. Their incorporation remains niche due to rapid and the need for frequent reapplication. The broader pharmaceutical adoption of pyrethrins is constrained by their low water solubility (typically <1 mg/L), which complicates formulation stability and bioavailability in systemic or diverse topical delivery systems. Consequently, non-insecticidal applications represent a small portion of total pyrethrin use, primarily in veterinary ectoparasite control and human pediculicides. Pyrethrins are also registered for non-insecticidal purposes, including as bactericides, disinfectants, and antimicrobials in various formulations.

Safety Profile

Toxicity in Humans

Pyrethrins, natural insecticides derived from chrysanthemum flowers, primarily affect humans through acute exposure via dermal contact, inhalation, or ingestion, with dermal and inhalation routes accounting for the majority of incidents based on poison control data. Acute effects often include skin irritation, such as redness and itching, and respiratory symptoms like coughing or shortness of breath following inhalation of aerosolized formulations. Dermal exposure can also cause paresthesia, a tingling or burning sensation, due to the compounds' action on sodium channels in nerve cells, which is typically transient and resolves without treatment. In rare cases, sensitized individuals may experience anaphylaxis, manifesting as severe allergic reactions including bronchospasm and hypotension, particularly from inhalation or contact with pyrethrin-containing products like shampoos. Notable case studies highlight the risks of acute exposure; for instance, in the 1980s and early 1990s, several fatalities were reported from inhalation exposure to pyrethrin-based dog shampoos during use, leading to bronchospasm, pulmonary edema, and respiratory failure in individuals with underlying asthma or allergies. These incidents underscore the potential for severe outcomes when pyrethrins are inhaled, though such events are uncommon with proper application. Occupational exposure in agricultural or pest control settings can exacerbate acute symptoms, but most cases are mild and self-limiting. Chronic exposure to pyrethrins, typically through repeated occupational handling or household use, may lead to neurotoxicity at high doses, including headaches, dizziness, and fatigue, though evidence is limited and primarily derived from case reports rather than large-scale studies. There are no strong links to cancer, with the U.S. EPA classifying pyrethrins as Group D ("not classifiable as to human carcinogenicity") based on inadequate evidence from animal studies and lack of human data. The World Health Organization has established an acceptable daily intake (ADI) of 0–0.04 mg/kg body weight per day, reflecting low risk from typical dietary or environmental levels. Recent biomonitoring studies from 2020–2025 indicate low urinary metabolite concentrations in the general population, suggesting minimal widespread chronic exposure, but elevated levels in children from household insecticides raise concerns for potential neurodevelopmental effects, such as subtle impacts on cognition and motor skills; a 2025 study confirmed low overall risk but highlighted higher exposures in insecticide-using households.

Toxicity in Animals

Pyrethrins exhibit low acute toxicity in most mammals, with oral LD50 values typically exceeding 1000 mg/kg in rats, indicating a wide margin of safety for species like rodents and . For instance, undiluted pyrethrum extract has an oral LD50 of 2370 mg/kg in male rats and 1030 mg/kg in females. generally tolerate pyrethrins well, allowing safe use in veterinary control products at appropriate concentrations. However, cats demonstrate markedly higher sensitivity due to inefficient hepatic glucuronidation, which impairs the detoxification of pyrethrins and related pyrethroids; toxicity can occur even from low-concentration products (e.g., >0.1% in some formulations), leading to symptoms such as tremors, muscle fasciculations, , and seizures. Case reports document feline fatalities from grooming treated with pyrethrin-based spot-on products, underscoring the need for species-specific formulations and avoiding dog products in multi-pet households. In birds and , pyrethrins pose a moderate risk, with oral LD50 values often above 1000 mg/kg. species, including and sheep, experience low systemic from topical applications used for ectoparasite control, though excessive dosing can cause transient or neuroexcitatory signs like . Interspecies biotransformation differences contribute to varying sensitivities; mammals primarily metabolize pyrethrins via ester hydrolysis and oxidation in the liver, with rapid excretion in , whereas birds and some may have slower clearance rates, prolonging exposure effects. Pyrethrins are highly toxic to aquatic organisms (detailed in Ecological Impacts). Post-2020 veterinary guidelines emphasize safe dosing to mitigate risks, recommending pyrethrin concentrations below 0.2% for cats and avoiding shared use of dog products in multi-pet households, with supportive treatments like lipid emulsion therapy for intoxications. No major outbreaks of pyrethrin in animals have been reported through 2025, reflecting improved labeling and formulation standards. These measures parallel human safety protocols by prioritizing rapid and monitoring for .

Environmental and Regulatory Considerations

Ecological Impacts

Pyrethrins exhibit high toxicity to aquatic organisms, particularly and , with 96-hour LC50 values for typically ranging from 0.01 to 1 µg/L, indicating extreme sensitivity even at low concentrations. This acute lethality arises from disruptions to function in these , leading to rapid immobilization and death upon exposure. , such as crustaceans and , face similarly severe risks, with LC50 values often below 0.1 µg/L, exacerbating population declines in contaminated water bodies. Although in aquatic organisms is low due to rapid in water—often occurring within hours under or neutral conditions—agricultural runoff remains a primary concern, transporting residues into streams and wetlands where they can persist in sediments. In terrestrial ecosystems, pyrethrins harm soil microbes by inhibiting microbial respiration and enzyme activity, which can disrupt nutrient cycling and organic matter decomposition essential for . Earthworms, key indicators of soil quality, experience sublethal effects including reduced burrowing and reproduction at environmentally relevant doses, potentially altering and over time. Persistence in varies from 1 to 30 days, influenced by factors like exposure and microbial activity; half-lives are shorter (0.6–1.9 days) on sunlit surfaces but extend beyond 7 days in shaded or anaerobic conditions, allowing intermittent exposure to biota. Pyrethrins are lethal to pollinators, with an acute contact LD50 for honey bees of approximately 0.04 µg/bee, classifying them as highly toxic and capable of causing direct mortality during foraging. Sublethal exposures impair bee navigation, foraging efficiency, and reproductive success, leading to reduced colony fitness and increased vulnerability to stressors. Studies from the 2020s have linked pyrethroid residues to contributions in colony collapse disorder, where chronic low-level contamination correlates with higher rates of hive abandonment and overwintering losses. Overall, pyrethrins are biodegradable, primarily through and microbial action, minimizing long-term accumulation in ecosystems. However, spray drift during application amplifies risks by depositing residues onto non-target habitats, including bodies and foraging areas, potentially increasing exposure beyond intended sites. strategies, such as establishing vegetated buffer zones around fields, can reduce drift and runoff impacts by 50–70%, effectively lowering transport to adjacent ecosystems.

Regulatory Status and Future Research

Pyrethrins are classified by the U.S. Environmental Protection Agency (EPA) as active ingredients eligible for use in minimum risk pesticides under Section 25(b) of the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA), allowing certain formulations to be exempt from full registration requirements when combined with specified inert ingredients. However, pyrethrins undergo periodic registration review, with the EPA releasing a draft ecological risk assessment in March 2025 to evaluate potential environmental impacts from their use alongside synthetic pyrethroids; as of November 2025, the review process is ongoing with no final decision issued. In the European Union, pyrethrins remain approved as an active substance under Regulation (EC) No 1107/2009, with maximum residue levels (MRLs) set between 0.05 mg/kg and 0.5 mg/kg for various crops such as fruits and vegetables, above the default 0.01 mg/kg limit for unlisted commodities. Restrictions apply in organic farming, where pyrethrins are permitted in some regions like the EU and U.S. due to their natural origin. Pyrethrins are used for applications, particularly in space sprays for prevention, as a natural alternative to synthetic pyrethroids amid rising resistance concerns. Recent reviews from 2023 to 2025 on endocrine disruption potential have yielded inconclusive results, with some studies indicating possible estrogenic effects in vertebrates but lacking definitive evidence of disruption in humans or at typical exposure levels. Future research priorities include addressing gaps in long-term of pyrethrin residues in and , as current data often overlook cumulative effects over extended periods. Investigations into influences on pyrethrin degradation rates are emerging, with studies showing elevated temperatures and altered precipitation may slow breakdown and heighten to non-target . Efforts are also focusing on developing alternatives to synergists like , which amplify efficacy but raise resistance risks, alongside biotechnological approaches to engineer safer pyrethrin analogs with enhanced specificity and reduced environmental persistence. Industry trends reflect a shift toward (IPM) strategies that incorporate pyrethrins to minimize synthetic reliance, supported by their rapid degradation profile. Despite competition from cheaper synthetic pyrethroids, the global pyrethrin market is projected to reach approximately $414 million by 2030, driven by demand for natural insecticides in organic agriculture and applications.

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