History
The history of pyrethrum usually starts in 1847 with the mention of the species Chrysanthemum cinerariaefolium found growing in Dalmatia (now Yugoslavia). Prior to that it is thought that pyrethrum passed into Europe from Persia along the silk route although details of the particular chrysanthemum flower are not known. During the latter half of the 19th century ‘Dalmatian Insect Powder’ was sold in most pharmacies throughout Europe and in 1885 dried pyrethrum flowers were exported to the USA.

By 1913 some 500 tonnes were shipped to the USA but the advent of World War 1 saw this trade cease and replaced by shipments from Japan where pyrethrum had been grown since 1886. At this time the main uses for the ground pyrethrum flower was the control of body lice on humans and animals and crawling insects in the home. Early methods of extraction was, for example, by percolating the ground flowers with kerosene which was then formulated as space sprays for use against houseflies and mosquitoes.

It was not until 1924 that the first glimpses of pyrethrum chemistry were revealed by Staudinger and Ruzicka who published a now classic series of papers, summarising their discoveries from 1910 onwards, indicating that the insecticidal properties were due to two esters named Pyrethrin I and Pyrethrin II. Their work was studied in the UK by F Tattersfield, working at Rothamsted Experimental Station at Harpenden, who undertake a fundamental study of pyrethrum and published a series of papers between 1924 and 1935. At the same time, in the USA, C B Gnadinger also embarked on a study of pyrethrum and published a series of papers from 1929 to 1936 culminating in the publication of the first pyrethrum book ‘Pyrethrum Flowers’ in 1933, the second edition in 1936 followed by supplements 1936-1945.

Both Tattersfield and Gnadinger provided a firm scientific foundation to pyrethrum research which has continued to the present. Two further books have added to the growing information on pyrethrum: ‘Pyrethrum: The Natural Insecticide’ was published in 1973 as a result of an international symposium on pyrethrum and was edited by J Casida and ‘Pyrethrum Flowers: Production, Chemistry, Toxicology and Uses’ was published in 1995, also as the result of an international symposium on pyrethrum (held in 1992) and is edited by J E Casida and G B Quistad.

Today pyrethrum is available in two forms: a dust produced by grinding the flowers to a standard mesh size and stabilized against degradation by the antioxident butylated hydroxy toluene (BHT) and as a refined concentrate also stabilized by BHT and containing either 25% or 50% pyrethrins. The refining process may use solvents such as hexane or methyl alcohol or a newer process using super critical carbon dioxide. The refining process removes impurities that may cause allergenic reactions on human skin as well as waxes and resins that may clog applicator aerosol nozzles. During processing great care has to be taken to avoid degradation of the esters by oxidation; free radicals must be eliminated and the processing temperature kept below 60°C.

The active ingredient comprises six esters (see Fig. 1) that may vary in their relative amounts depending on the plant variety, geographical source and time of harvest; the average assay is given in Table 1. Extensive biological testing has demonstrated that although extracts from different sources and batches may vary in insecticidal potency, all such differences are eliminated when pyrethrum is used with piperonyl butoxide (PBO) as synergist. At present pyrethrum is used almost exclusively with PBO since it is too expensive to use pyrethrum alone.

Safety to Man and Animals
The first reported complaints against pyrethrum use in humans was during the World War II when allied soldiers found that pyrethrum ointment, used to repel mosquitoes when fighting in tropical areas, produced skin allergies. The ointment was based on crude oleoresin so to overcome the skin reaction the extract was refined to what is now described as ‘pale extract’ being dewaxed and decolourised. This pale extract was developed at the same time as Freon-based aerosols were being developed that required a refined extract. Up until the 1990s pyrethrum was considered a very safe product for use on or near humans and animals.

From 1930-1955 pyrethrum extract was the preferred antihelminthic for young children in France and from 1950 it was added in an alcohol solution to potable water to control the small water crab Ascellus aquaticus found throughout Europe. From 1963 many millions of cans of liquid repellants containing up to 0.5% w/w pyrethrins were applied to human skin in Australia without any significant adverse effects being reported. In the USA up to 1973 household products containing synergised pyrethrum was labelled as ‘non toxic to humans and pets’. In addition, since the availability of pale extract, there has been a continuous use of pyrethrum-based formulations to control head lice in children in the Western world.

More recently, the EPA in the USA, having examined data from a long term feeding study in which extremely large doses of pyrethrum extract was fed to rodents suggested it was ‘likely to be a human carcinogen by the oral route.’ However, in practice the oral dosing each day of up to 5000ppm during a lifetime would never take place. And to put this in context, coffee, alcohol and other foodstuffs can have greater toxic effects at this dosage level (Ames and Gold, 1993).

Following their review of pyrethrum extract the JMPR (FAO/WHO) have issued a report on the safety of pyrethrum (Solecki, 1999). Their response is well reasoned and is reproduced below:

‘In a two-year study of toxicity and carcinogenicity in rats and an 18-month study of carcinogenicity in mice, the NOAEL was 100ppm in both species, equal to 14 and 4mg/kg bw per day in mice and rats, respectively. The liver was the main target. A treatment-related effect on the incidence of lung tumours was seen in mice and increased incidences of benign tumours of the skin, liver and thyroid were observed in rats. The increased incidences of hepatocellular adenomas were associated with persistent induction of cytochrome P450 enzymes and hepatocellular hypertrophy, suggesting that pyrethrins are rodent-specific hepatoproliferative carcinogens. Enzyme induction leading to increased clearance of thyroid hormones would also be consistent with the higher incidence of follicular hyperplasia and follicular adenomas. However, additional studies on the mechanism of formation of the liver and thyroid tumours are required. The Meeting concluded that the increased tumour incidences caused by pyrethrins are threshold phenomena of negligible relevance to the low doses to which humans are exposed (see Appendix I to this monograph addendum).’

Efficacy and Environmental Fate
When dosed with pyrethrum insects react very quickly and onset of paralysis occurs within 2-3 minutes. This ‘knock down’ (KD) effect is very important because the user sees the effect of the insecticide quickly. Pyrethrum has the added advantage that it acts on a wide range of insect species and is a powerful repellent to mosquitoes. Indeed, even micro-doses of pyrethrum, such as from mosquito coils, has the effect of preventing female mosquitoes from biting.

Most pyrethrum formulations include the synergist piperonyl butoxide (Glynne Jones, 1999) which makes them more cost effective since pyrethrum alone is expensive and may be in short supply. These synergised pyrethrum formulations are particularly effective against mosquitoes and midges with the advantage that there is no evidence that these species have developed any resistance to the insecticide.

Pyrethrins are quickly degraded in sunlight and leave no toxic residues and are, therefore, ideal to use as pre-harvest treatments to eliminate insect pests on edible crops up to 24 hours before harvest.

Production
The pyrethrum plant originated from temperate regions and requires exposure to a period of less than 10°C to flower. Commercial production is found in temperate regions such as Tasmania or Norfolk, England as well as elevated areas above 1800-2900 metres near the equator such as Kenya and Ecuador. Yields of dried flowers are highest in temperate regions but there is the disadvantage that with one intense period of flowering harvesting has to be undertaken quickly as soon as the flowers reach optimal pyrethrins content. In areas such as Kenya, where the day length and average temperatures show little variation, flowering and harvesting can be extended over several months. This has the added advantage that extraction and refining costs are reduced as a result of the regular and extended deliveries of dried flowers.

The annual world production of dried flowers has usually not exceeded 20,000 tonnes and with an average pyrethrins content of 1.5% the potential yield, accounting for loses during processing, is about 25kg per tonne. Thus there is a potential world yield of about 800 tonnes of 50% pale extract. During the past 40 years production of pyrethrum extract has often been in short supply and occasionally in surplus. This variation in supply has been a factor in the development of synthetic pyrethroids to maintain a consistent production but none have all the advantages of natural pyrethrum. In an attempt to overcome the variations in supply of pyrethrum it has been suggested that stockpiling of extract should be undertaken with the added advantage that prices would be more stable and predictable.

The Future
There is an increasing demand for a safe organic bioinsecticide and pyrethrum, with proper strategic planning and and stockpiling can meet this demand.

go to top of page