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Dangers of Pyrethrum


Pyrethrum is derived from daisies in the Chrysanthemum family.

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Pyrethrum is the common name given to an insecticide derived from the dried and powdered flowers of pyrethrum daisies (Chrysanthemum cinerariaefolium, C. coccineum and C. marshalli). These flowers contain pyrethrins, which are natural compounds with insecticidal properties. While pyrethrum is considered one of the least toxic insecticides, it has some dangers associated with it.

How Pyrethrum Works

Pyrethrum is a natural insecticide used to kill a range of indoor and outdoor insect pests. It works by attacking the nervous system of the insects, which eventually kills them. Pyrethrum is a contact insecticide, which means the chemical must touch the insect to kill it. Pyrethrum is used in head lice treatments, spray insecticides, bug bombs, foggers and flea sprays. It’s one of the least poisonous commercial pesticides available, primarily because it breaks down fast in the environment and in the human body.

Fire and Combustion

If pyrethrum comes in contact with oxidizers, such as ozone or hydrogen peroxide, combustion is possible. It is also flammable, so should never be near an open flame. If heated, pyrethrum gives off smoke and irritating fumes. Carrier solvents and synergists are also typically used in commercial formulations of pyrethrum-based insecticides, and this can change the pyrethrum’s physical and toxicological properties.

Wildlife Toxicity

Pyrethrum is extremely toxic to aquatic life, such as fish and tadpoles, and mild to moderately toxic to birds. When a fish or tadpole comes in contact with pyrethrums in the water, it enters their skin and affects the organs that regulate balance. Pyrethrum is considered low in toxicity to mammals because it breaks down rapidly in the body.

Reduction of Beneficial Insects

Pyrethrum is highly toxic to beneficial insects such as bees, which are needed to pollinate plants, and also to predatory wasps, which control populations of pest insects. Spraying pyrethrum insecticides in your garden may result in reducing the populations of pollinators and insects that prey on plant pests. For example, if pyrethrum contacts bees and beneficial insects directly, it is fatal. But pyrethrum residues break down quickly, so the risk of eliminating or repelling beneficial insects is limited to how long the residues remain on the plants. The half-life of pyrethrum is typically about 12 days.


Make Organic Pyrethrum Spray at Home for Organic Pest Control

This article is part of our Organic Pest Control Series, which includes articles on attracting beneficial insects, controlling specific garden pests, and using organic pesticides.

What Is Pyrethrum?

One of the oldest pesticides known, pyrethrum is also the strongest insecticide allowed under National Organic Standards guidelines. Made from the dried flowers of a little white daisy now classified as Tanacetum cinerariifolium, pyrethrum insecticides are known for their fast knock-down of unwanted insects. Insects typically become paralyzed as soon as they come into contact with pyrethrum, so it’s often used in wasp sprays. Pyrethrum use in the garden should be undertaken with care and only after cultural methods that might manage a pest have been exhausted. Pyrethrum insecticides are highly toxic to bees, wasps and other beneficial insects, as well as to fish.

Which Pests Does Pyrethrum Control?

Aphids, armyworms, cucumber beetles, cutworms, squash bugs, whiteflies, leafhoppers, thrips and Colorado potato beetles are often brought under control with pyrethrum. Pests that cannot be reached with the spray — for example, corn earworms or leaf miners — should not be treated with pyrethrum products. Additionally, very challenging pests such as cucumber beetles and squash bugs are best managed by excluding them with row covers, with pyrethrum used as a late-season remedy should pests get out of control.

How to Use Pyrethrum

Pyrethrum degrades quickly in sunlight, but precautions still should be taken to protect beneficial insects from exposure. When using pyrethrum to control insects that take flight, such as cucumber beetles, apply pyrethrum early in the morning and then cover the treated plants with row cover or an old sheet to exclude bees and other beneficials for 24 hours. To put a damper on squash bug populations, spray plants as soon as the first nymphs are seen, and again one week later.

Do not use pyrethrum in situations where lady beetles, honeybees and other beneficials are active. Used carelessly, pyrethrum can wipe out these and other beneficial insects.

Home production of pyrethrum pesticides is practical for the resourceful homesteader. Native to current-day Yugoslavia, the Dalmation daisy is a cousin of feverfew, which it closely resembles. Hardy to Zone 6, the plants grow as short-lived perennials and often reseed in hospitable spots. If the dried flowers are soaked in warm water for three hours, the resulting spray is highly toxic to insects for about 12 hours. People who are allergic to other members of the Aster family may react badly to this daisy’s pollen.

Purchased pyrethrum products require less handling and therefore may be safer to use. Look for the OMRI label when choosing a pyrethrum insecticide, because non-listed products often contain piperonyl butoxide, which is considered a possible human carcinogen. Organic pyrethrum products often contain oils or soaps to enhance their effectiveness.

How to Store Pyrethrum

Mix only as much concentrate or infusion as you will need. If not used within one day, place the container in the sun for a few hours and dispose of unused solution by pouring it out in the sun. Sunlight rapidly degrades pyrethrum, and the half-life of pyrethrum in soil is only one to two hours. Store pyrethrum products in their original containers on a high shelf, out of the reach of children and pets, in a dark place at cool room temperatures. Under good storage conditions, the shelf life of pyrethrum is about 1 year. Dried pyrethrum daisies can be stored in the freezer in an airtight container for at least six months.

More information on pyrethrum is available from Cornell University .


Pyrethrum Farming in Kenya Price, By Products, Fertilizer, Pests, Clones, Varieties, Diseases, Climate, Rainfall, Counties, Harvesting, Picking, Commercial Companies, Researchers, Nursery

Pyrethrum Farming in Kenya is set to become a multi-billion dollar industry in the near future following the change of its governing laws and regulations and enactment of Pyrethrum Act 2012.

What is Pyrethrum

Pyrethrum (Chrysanthemum cinerariaefolium) is a flower that is grown majorly for extraction of its pyrethrin. Pyrethrin is a product that is used in the manufacture of pesticides and insecticides.

Pyrethrum (Chrysanthemum cinerariaefolium) is a flower that is grown majorly for extraction of its pyrethrin.

This pyrethrin is found in the seed cases in the yellow part of the flower. When the flower is dried the pyrethrin is extracted using solvents like hexane and sold in the form of an oleoresin commonly refereed to as the pale yellow extract.

Pyrethrins are considered to be very effective because they attack the nervous systems of all insects. It exhibits rapid “knockdown” effect on the insect.

Pyrethrum has advantages over other insecticides in that it is non-persistent in the environment and hence recommended by the World Health Organization ( WHO) for export horticulture and fruit production due to its low Minimum Residue Levels (MRLs).

It is considered to be amongst the safest insecticides for use around food. It is one of the most commonly used allowed non-synthetic insecticides in certified organic agriculture.

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History of Pyrethrum Farming in Kenya

Pyrethrum was introduced in Kenya from Europe by white settlers in 1928. Initially it was grown in large-scale farms but nowadays it is widely cultivated by small-scale farmers. Kenyan pyrethrum was of very high quality. It therefore quickly replaced the Japanese pyrethrum on the world market by around 1941.

During the pyrethrum farming hey days in the 1980s and 1990s, the production of the crop increased steadily and Kenya became a global leader in pyrethrum production, accounting for over 70 percent of the global market.

This lucrative sub-sector supported more than 200,000 small-scale farmers, 3,000 workers directly employed by the Pyrethrum Board of Kenya and over 2 million people deriving their livelihood from the industry either directly or indirectly.

The pyrethrum sector became a major foreign earner contributing approximately KSh. 2.1 billion in 1996 but has since declined to Ksh.130 million in 2010

The number of farmers dwindled from 200,000 in the 1980s to 29,000 (June 2009) and the area under the crop dropped from 40,000 ha during the 1980s to 4,000 ha (2009).

Production has also reduced from a high of 18,000 MT in 1981/82 to 462 MT realized 2009/2010 pool year. The producer price range has stagnated at Ksh. 100 to Ksh. 300 per Kg since 2001 depending on the pyrethrin content.

The decline was caused by among other reasons: unfriendly policy and regulatory environment by the Pyrethrum Board of Kenya; unmet demand; mismatch between market requirements and production, farm level issues such as low productivity and profitability, scarcity of planting materials, high costs of production, delayed payments to farmers, and increased competition from synthetics.

Most of these farmers even up to now still complain that the Pyrethrum Board of Kenya owes them millions of shillings that they had taken as loans and their property is still attached.

Pyrethrum Growing Counties (Areas) in Kenya

Pyrethrum is grown in 19 counties in the country namely;

  1. Nakuru
  2. Kiambu
  3. Nyandarua
  4. Nyeri
  5. Laikipia
  6. Meru
  7. Embu
  8. Baringo
  9. Elgeyo Marakwet
  10. West Pokot
  11. TransNzoia
  12. Bungoma (Mt. Elgon)
  13. Uasin Gishu
  14. Nandi
  15. Kericho
  16. Bomet
  17. Kisii
  18. Murang’a
  19. Nyamira

Pyrethrum Farming Inputs

Pyrethrum is reproduced and propagated by splits/crown division and seed/seedlings through licenced nursery operators and also on individual farmers farms.

The seedlings are grown in a seedbed and when they reach 4 to 5 inches high they are transplanted to the farms. The spacing required for the planting in the main farm is 2 feet ( 60cm) inter- row and 1 feet ( 30cm) from plant to plant ( intra-row)

Recommended Clones and Varieties of Pyrethrum for Growing in Kenya

For high altitude areas the following clones and varieties are recommended

  • SB/66/107 Clones
  • Ma/71/423 Clones
  • Ma/70/1013 Clones
  • Mo/74/223 Clones
  • L/75/477 Clones
  • L/75/487 Clones
  • Varieties are P4

For Medium to Low altitude areas the following clones and varieties are recommended

  • Ks/75/336 clones
  • Ks/70/64 clones
  • Ks/75/313 clones
  • Kr/74/122 clones
  • Varieties: K218, K235

Recommended Fertilizer for Pyrethrum Farming in Kenya

The recommended Fertilizer for Pyrethrum Farming in Kenya is 5g DAP (Diammonium Phosphate) per hole at planting and 250-300kg/ha of TSP (Triple Super-phosphate) per hole after cutting back. Cutting back is done annually near the end of the dry season followed by weeding and earthing up.

Use a table spoon of TSP for every plant when planting. 50kgs of TSP is sufficient for one acre. The fertilizer should be mixed thoroughly with soil in the hole before planting, to avoid scorching the plant roots by fertilizer. Compost manure can also used instead of TSP at the rate of 200 grams per hole.

Manure of 10 ton/ha (a handful per hole) for poor soils 3 months before planting is recommend.

Important pests and their control in Pyrethrum Farming in Kenya

Pyrethrum Farming Climatic Conditions and Ecological Zone

Pyrethrum is best grown in;

Altitude of 1800-3000m above sea level.
A rainfall of above 750mm pa is required.
Loamy volcanic soils with a pH >5.6 are required and are expected to be fertile, deep and well-drained.
The temperature should be less than 180℃ for at least 6 weeks.

Pyrethrum Farming Land preparation

Plough before the onset of rains and make sure you eliminate perennial weeds and parasitic grass that will become a nuisance once pyrethrum seedlings germinate.

Pyrethrum rots when there is too much rain and also dries when there is no rain. So plant when there is normal rain or make sure your land is well drained to eradicate stagnant water.

A farmer sprinkling water to her Pyrethrum Nursery

The clones should be propagated by splitting the mature plants or by planting tissue-culture Raise varieties in a nursery and producing new seedlings. After 4 years, uproot or discard the plants and do crop rotation with cereals such as maize, wheat, oats or barley or grasses such as weeping love grass, guinea grass, Guatemala grass may be used for rotation. If pyrethrum is left in the same field for more than three years, flower yield declines to uneconomical level in subsequent years due to accumulated effect of pathogens, pests and nutrient depletion.

Ideally, an acre should have 22,000 plants and produces approximately 400kgs of dried pyrethrum flowers annually.

Weeding can either be carried out through mechanical or chemical methods.

  1. Mechanical weeding: With forked implements from the first month after establishment until the crop has fully covered the ground. After 2-3 months, earth up to encourage tillering
  2. Chemical weeding: Use herbicides e.g. Sencor, Venzar-Sencor-Ronstar

Topdressing of pyrethrum plants tens to induce them to produce other vegetative stems and leaves that are not important to the farmer. What is important to the farmer is the stems with flower buds. So topdressing is highly discouraged. instead spraying of the BoomFlower is recommended. It is recommended that farmers spray Boomflower two weeks before onset of flower and there after in intervals of two months for profuse flowering.

Pyrethrum Flowers Harvesting/Picking

Pyrethrum flowers should be picked once in 2-3 weeks when the white petals (ray florets) are horizontal and about 3 rows of disc florets are open. This is the time the daisies are in full bloom. This is the time the concentration of pyrethrin is at its peak.

Pyrethrum flowers should be picked once in 2-3 weeks when the white petals (ray florets) are horizontal and about 3 rows of disc florets are open.

Flower harvesting is selectively done by hand. The best picking method is by holding the flower between the first and second finger and then pulling the flower head with the thumb.

Avoid picking flowers with flower stalks because stalks have little pyrethrin and therefore cause reduction in pyrethrin content of the flowers.

Drying of Pyrethrum Flowers

A small scale farmer can dry the flowers by use of the sun. It is less expensive and causes minimal loss of pyrethrin content. However on cloudy weather or when large quantities of flowers need to be dried, artificial methods may be employed. These artificial methods include artificial solar driers.

These solar driers should be used in the right way so as to preserve the pyrethrin content in the flowers. Keep the drying air temperature in the dryer at a maximum of 50℃ to avoid excessive loss of pyrethrins through overheating. During sale of the flowers moisture content should be less than 11%.

Selling of Dry Pyrethrum Flowers

The dry pyrethrum flowers are taken to various designated collection centers in the various growing areas.

The price of dry pyrethrum flowers depends on the pyrethrin content. Different buyers have different parameters they use while determining what to pay the farmer.

The Pyrethrum Processing Company of Kenya (PPCK) previously Pyrethrum Board of Kenya (PBK) pays Ksh. 100 to 375 per kg based on pyrethrins content; whereby the farmer is paid Kshs 100 per every kg of dry flowers on delivery and thereafter a second payment after determination of the pyrethrin content.

HighChem Agriculture pays Ksh. 140 /Kg to the farmer and can rise to over Ksh 220 per Kg as soon as GAP (Good Agricultural Practices) are achieved.

There has been an increase of pyrethrins content in dry flowers from 1.4 to 1.5% in the last few years. The projected national average is 1.9% which is achievable when farmers embrace good Agricultural practices.

Farmers are encouraged to trade in Pyrethrum Farming as cooperative societies since the advantages derived are heavenly as opposed to farming alone.

The Price of Pyrethrum Products and By-Products in the Market

One kg of the product sells at US$ 270 and due to the high demand, the customers always desire to pay up-front once processing is assured.

For every processing of 100 MT of flowers, 1.65 MT of the refined product is produced; refined product, known as pale extract which is mainly exported (80%) and remaining 20% sold to formulators for value addition .

Levels of Pyrethrum Product Portfolio

Pyrethrum Technical Products

  1. Pyrethrum Powder 1.3% w/w AOAC
  2. Pyrethrum Grist
  3. Pyrethrum Extract Crude Oleo-Resin 25% w/w
  4. Pyrethrum Extract Pale 25% w/w, 50% w/w, 50% w/w AOAC pharma, and Conc. w/w AOAC – for export market
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Pyrethrum By-Products

  1. Vegetable Waxy Resins (V.W.R.)
  2. Green oils
  3. Pyrethrum Fine & Coarse Mark (Py-Mack, is a by-product of the pyrethrin manufacturing industry, and is used as cattle feed. Most of the pyrethrins have been extracted, but enough remains that py-mack can be used as cutworm bait, and in treating early attacks of maize stalkborer. Feeding livestock py-mack is also said to reduce incidence of intestinal worms.)

In addition final product, none of the myriad by-products from the factory is a waste material. For example, Pymarc by-product is sold at Ksh. 1,250 per 50 kg bag while sludge is sold at Ksh. 50 per kg. In a nutshell one processing of 100 metric tonnes of flowers earns PPCK over Ksh. 46 million.

End-Use Products by PPCK (Insecticides)

  1. Pyagro 4EC – Crop protection product
  2. Pydust 1% – Grain storage product
  3. Pymos 0.6EC – Adult Mosquito product
  4. Pylarvex 0.5EC – Larvae stage mosquito product
  5. Pynet 5EC – Net treatment product
  6. Pytix 4% – Tick control product.

After value addition the above 3 categories of products confirms that pyrethrum fits well in the economic pillar of vision 2030 as the crop can be converted into a myriad of value added products. Thus, every single product derived from pyrethrum becomes a raw material for other products.

Value Added Pyrethrum By-Products in Kenya by PPCK

PPCK has formulated and registered 6 insecticides for use in public health and sanitation, animal industry and crop protection sub-sectors.

These end use products are more profitable than Pale with Pymos (TM) 0.6EC giving a gross margin of Ksh. 22,000 per kg compared with Ksh. 2,600 per kg from Pale.

The current production of the pyrethrum extracts is inadequate for the Board to venture into mass production of these insecticides.

How different Pyrethrum Product lines can contribute to the revenue from Pyrethrum Farming

Pyrethrum Pale Extract

This is the major product which, accounts for 96% of the business revenue.


Used for animal feeds and accounts for 3% of the sales revenue.

Value added products (End-use products)

There are five end use products developed from value addition whose profitability in relation to the extract (pale) is tenfold. It is currently accounting for 1% of the revenue earned as it is still under market development process.

Traditional markets for pale have remained the same now at the following levels: Americas (20%), Europe (42%), Australia/Asia (21%), Kenya (10%) and rest of Africa (7%).

The Pyrethrum Board of Kenya was split in to two entities after the enactment of the Pyrethrum Act 2013. The two entities are the Pyrethrum Regulatory Authority (PRA) and Pyrethrum Processing Company of Kenya (PPCK). The PRA is the regulatory entity and PPCK is a commercial entity.


Pyrethrum: History of a Bio-Insecticide – Part 1

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Insects and people share a common habitat. From our perspective, many bugs are useful, indeed essential, as for example when they make agricultural crops possible through pollination. We are amazed, for example, by the diligence of bees and extol them in nursery rhymes and make them into movie heroes. But if insects like locusts, potato beetles, cabbage butterflies, grapevine lice, or grain weevils threaten or destroy our crops, then we regard them as parasites. And if common house flies, horseflies, cockroaches, mosquitoes, bedbugs, fleas, or lice badger us where we live, or sting us to the point of drawing blood, then we speak of vermin—and we combat them mercilessly.

This article deals exclusively with household insecticides. We should not ignore the fact that insects may not only be annoying, but can also transmit diseases. For example, the European rodent flea (Xenopsylla cheopis) transmits the plague, tse-tse flies are a source of sleeping sickness, Anopheles mosquitoes carry malaria, clothes lice can bring typhus, and deer ticks can transmit Lyme disease.

1. Chrysanthemums with Insecticidal Effects

One weapon against the annoying scramble proved itself reliable many centuries ago in ancient China and the Near East: finely ground flower heads from two chrysanthemum species: the Caucasian pyrethrum or painted daisy (Chrysanthemum coccineum) from the Caucasus and Persia, and the Dalmatian pyrethrum (Chrysanthemum cinerariifolium) from Montenegro and Dalmatia (see Fig. 1). Precise biological boundaries and names for these species have varied considerably. Botanists today prefer the species name Tanacetum (from «tansies», synonymous with chrysanthemum or pyrethrum).

Figure 1. Two species of the genus Chrysanthemum (or garden pyrethrum) which act as insecticides; left: Caucasian insect-powder plant, or painted daisy (Chrysanthemum coccineum, Tanacetum coccineum, Pyrethrum roseum, or P. carneum); right: feverfew, Dalmatian pyrethrum (Chrysanthemum cinerariifolium, Pyrethrum cinerariifolium, or Tanacetum cinerariifolium). Image sources: Roman Köhler, wikimedia commons, public domain; Kenpei, wikimedia commons, CC BY-SA 3.0.

The greenish-yellow preparation derived from these plants was designated colloquially as «insect powder» or simply «pyrethrum» from the outdated species name, and was sold in drugstores in the 19th and early 20th centuries as «Pulvis insectis». Pyrethrins constitute the powder’s active ingredients: pyrethrin I and II, cinerin I and II, as well as jasmolin I and II. So-called pyrethroids (Greek for «similar to pyrethrin») are synthetic derivatives of the natural pyrethrins.

2. Zacherlin—The «Most Highly Praised Agent Against All Sorts of Insects»

Substantial deliveries of Caucasian insect powder first reached Central Europe in the mid 19th century. The merchant Johann Zacherl (1814–1888) from Munich, Germany, who later moved to Vienna, Austria, was the driving force behind the market launch. He came to know the insect powder in the course of business trips in the Caucasus, where it was used successfully to combat the local pests and garden vermin.

Zacherl recognized its potential and entered into an agreement with rural Caucasian communities to supply him with dried, wild chrysanthemum blossoms. These he arranged to have ground in Tiflis (located now in Georgia) into a somewhat unpleasant smelling powder, which was sent to him in Vienna. The resulting «Zacherl’s Insect Killing Tincture (Zacherlin)» became a tremendous success, thanks especially to his skillful marketing efforts. With its conspicuous trademark of a man in Circassian dress (see Fig. 2) and a Persian cap, carrying a sprayer of his own devising, Zacherlin became the uncontested market leader among insecticides directed toward «cockroaches, bedbugs, fleas, moths, flies, gnats, lice, and other household-, storage-, and hygiene-related pests».

Figure 2. Circassian warrior.

In 1880, Johann Zacherl’s eldest son took over the flourishing firm, changed the supply from Caucasian to Dalmatian chrysanthemums, and in 1888 completed a new building, planned by his father, for an insecticide powder factory in Vienna at Nusswaldgasse 14 (see Fig. 3), with an outer façade in a Persian-Moorish style.

Figure 3. The exterior façade of the Zacherl factory in Vienna’s Nusswaldgasse, completed in 1888 in Persian/Moorish style. It remains one of the most historically remarkable buildings in Vienna from an architectural standpoint. In its significance, it is comparable to the former Yenidze cigarette factory in Dresden, also built in an Orientalized style (see The Chemistry of Tobacco – Part 1). Image source: Geiserich 77, wikimedia commons, CC-BY SA 3.0.

Pyrethrum is not toxic for humans, but it paralyzes insects almost instantly, and is, in some sense, an ideal insecticide. That explains why Zacherlin became such a great commercial success for household purposes. Zacherl’s attempts to extend pyrethrum use to agricultural applications encountered little success, however, since the natural product proved too expensive for wide-area use, and also quickly lost its effectiveness outdoors.

3. First Outdoor Test of Pyrethrum

Around 1900, scientists with the state wine-growing research station in the Swiss canton of Waadt tested nearly 80 different substances for insecticidal effectiveness against the vine moth (Eupoecilia ambiguella), a significant pest for wine growers. In addition to sulfur, copper, arsenic compounds, and soft soap, they studied the use of petroleum emulsions, linseed oil, olive oil, a broad spectrum of plant-based products including spices, as well as pharmaceutically active agents. The best insecticidal characteristics were observed with an aqueous dispersion of 1 % pyrethrum and 3–5 % soft soap [4].

Since caterpillars of the vine moth were unaffected by less expensive insecticides, widespread treatment with pyrethrum was tested in the Waadtland vineyards. To ensure the availability of raw material, a total of 270,000 pyrethrum seedlings were planted in the cantons of Waadt and Wallis. Because of the intense labor involved, and associated costs, there was no chance for pyrethrum to prevail over a wide area. Its potential would pay off only in the case of very expensive grapes, and extremely valuable vines. Nevertheless, the field test was not utterly in vain, since the true potential of the insecticide was now clearly apparent.

Farsighted chemists soon began to take a serious interest in pyrethrum. Since it had been cultivated commercially since the 1880s in Japan, it should not be too surprising that Japanese chemists, in particular, were the first to attempt to isolate the active component. Starting with what was very probably an impure raw material, they managed to show that the active substance must be an ester, because upon cleavage it fully lost its insecticidal powers [2–4].

Japan soon became an important international source of the greenish-yellow insect powder, especially for the United States. This applied also to Germany and Austria, where the Japanese product came to enjoy a high level of esteem in the fight against such pests as lice, fleas, and bedbugs, among others.

On April 17, 1922, Hermann Staudinger (1881–1965) and Leopold Ružička (1887–1976) (see Fig. 4) submitted a ten-part manuscript entitled «Insektentötende Stoffe» («Insecticidal Substances») to the editors of Helvetica Chimica Acta. In it, they describe their work involving pyrethrum [5, 6]. The series of reports, amounting to a total of 160 printed pages, was remarkable from the standpoint of its scope, the experimental expertise demonstrated, and the exceptional originality and creativity with respect to their synthetic approaches.

Figure 4. Left: Leopold Ružička, Nobel Prize for Chemistry in 1939 «for his work with polymethylene and higher terpenes»; right: Hermann Staudinger, Nobel Prize for Chemistry in 1953 «for his discoveries in the field of macromolecular chemistry».

As late as 1995, the renowned English natural products chemist Leslie Crombie (1923–1999) was moved to rave [7, 8]: «Although only two pyrethrin esters were recognized, and the resulting structures were incorrect in certain details, enormous progress was made, particularly when the inadequate techniques of the time are taken into consideration. Staudinger and Ružička’s investigation stands as one of the great chemical classics of its age.»

Already on page 1 of part 1 of the series, «Regarding the isolation and constitution of the effective component of the Dalmatian insecticide powder», the authors astonish us with the following footnote in the work as it appeared in 1924 (!):
«The work described below was carried out in the years 1910–1916 in the chemical departments of the Technical Universities of Karlsruhe and Zürich at the suggestion of, and with financial support from, Mr. P. Immerwahr of Berlin. On this occasion, we wish to express to him our deepest regards for the substantial funding bestowed upon our work. The actual studies were completed in 1916, but could only now be published. St. (for Staudinger, author‘s notation).»

Two questions suggest themselves here. First, who was «Mr. P. Immerwahr of Berlin, the inspirer and financier of the entire research project, and why did publication not occur until eight years after completion of the work?

To begin, it is noteworthy that P. Immerwahr is not cited as a co-author, even though he apparently initiated the project, and for six years financed it with significant funding. Paul Immerwahr (1866–1926) was a Ph.D. chemist and a jurist who occupied leading positions in various real estate and asset management firms. In 1910, he became head of the Auergesellschaft in Berlin. Like his sister, the chemist Clara Immerwahr (1870–1915) [9] and her husband Fritz Haber (1868–1934), he originally came from Wrocław (German: Breslau) (see Fig. 5). The three were, in fact, schoolmates.

Figure 5. Cathedral Island («Ostrów Tumski») with Wrocław Cathedral. Image source: Jar.ciurus, wikimedia commons, CC BY-SA 3.0 PL.

As a businessman versed in chemistry, Immerwahr apparently recognized the commercial potential of pyrethrum, which Germany was importing from Dalmatia, at that time part of the Austro-Hungarian Empire. Presumably, he hoped that his project—through developing a synthesis of the active component of pyrethrum on the basis of domestic coal tar—would contribute to sparing Germany the costly import of natural plant-based material. As a good patriot, he wanted to help the fatherland achieve greater economic autonomy—precisely the goal that lay behind yet another Staudinger project: seeking a synthetic substitute for the import-dependent commodity pepper, for which he had also provided the stimulus [10, 11]. But in the case of the pyrethrum project, Immerwahr had a self-interest as well, because from the very outset he apparently intended to patent the relevant results, and exclusively under his own name.

Before financing the risky pyrethrum research program, Immerwahr must have discussed the matter with his brother-in-law, Fritz Haber, all the more since the proposed investigation involved a completely unexplored field, one that could only be successfully exploited by a competent natural products chemist. Haber, at that time professor of physical and electrochemistry at the Karlsruhe Technical University, Germany, knew of a suitable candidate within his own department: Hermann Staudinger, an associate professor there since 1907, who was 28 years old, very talented, and ambitious. Haber established the necessary contact to Immerwahr, and Staudinger at once recognized that such a collaboration could be a great chance, so he promptly seized upon the opportunity. So it came about that in 1910, Staudinger was able to offer a position as private assistant to his new Ph.D. graduate—a capable colleague from what is now Croatia, by the name of Leopold Ružička—at a starting salary of 100 RM (Reichsmark) per month, if he would be willing to go to work on pyrethrum. Ružička accepted the offer and threw himself into the work.

In the summer of 1912, Staudinger accepted an offer of a professorship at the Swiss Federal Institute of Technology (the ETH) in Zurich as successor to Richard Willstätter (1872–1942). Ružička followed him there and proceeded with his work under a better paying (200 RM), three-year contract. Ružička was also assured of 30 % of Staudinger’s share of eventual net proceeds from any future patent, with an upper limit of 85,000 RM [12]. In those days that amounted to a fortune, and reflected Immerwahr and Staudinger’s high-profit expectations.

The second question raised by the 1924 footnote concerns the sentence: «The studies were completed in 1916, but could only now (1924) be published. St. (for Staudinger, author‘s notation).» This—apparently involuntary—delay was unlikely to have been due to the war, or the postwar chaos, because Staudinger and his coworkers were at the time publishing quite regularly in both Berichte der Deutschen Chemischen Gesellschaft and Helvetica Chimica Acta. Instead, some sort of blocking notice on the part of Staudinger must have caused the long time delay between submission of the work (April 17, 1922), and its appearance in the spring of 1924.

Evidently, all the parties involved had agreed from the outset that the fruits of the research project should be shared as follows: Immerwahr, as financier and source of the ideas, would be the sole holder of any patents, whereas Staudinger and Ružička would be allowed to publish their scientific results. Consistent with patent law, publication could occur only after (!) official issuance of the patent itself, i.e., on October 18, 1923 [13]. The series of publications, so long delayed, became available in printed form at the beginning of 1924.

The agreement struck between Immerwahr and Staudinger had serious negative consequences for Ružička, however, who had carried out all the laboratory work. When the latter informed Staudinger in 1916 that he wanted to use some of his work as a basis for acquiring the official German qualification for a professorship («habilitation»), he learned he could expect little support from his preceptor.

Ružička’s own student, Vladimir Prelog (1906–1998), later described Staudinger’s disappointingly egoistic, thankless behavior quite bluntly [14]: «Staudinger was a truly inspirational teacher, but in his drive to promote his own work, he was also a harsh supervisor. When Ružička expressed his wish to habilitate on the basis of his own research field, he promptly lost his assistantship, and job opportunities became seriously restricted. He, therefore, ended up in a rather ambiguous, conflicted relationship vis-a-vis his mentor, in which his admiration of the latter’s scientific achievements became mixed with a dash of disappointment. The result of his gaining independence was having to look to the chemical industry in search of financial means for both his personal subsistence as well as meeting basic laboratory expenses.»

[1] L. Straumann, Nützliche Schädlinge (in German), Chronos Verlag, Zürich, Switzerland, 2005. https://doi.org/10.3929/ethz-a-004757997

[3] R. Yamamoto, J. Tokyo Chem. Soc.1919, 40, 126.

[4] R. Yamamoto, M. Sumi, J. Chem. Soc. Jpn.1923, 44, 1080.

[7] L. Crombie in Pyrethrum Flowers (Eds.: J. E. Casida, G. B. Quistad), Oxford University Press, Oxford, UK, 1995. ISBN: 978-0195082104

[8] D. A. Whiting, Leslie Crombie, J. Chem. Soc., Perkin Trans. 12000, 2303–2304. https://doi.org/10.1039/B004067N

[10] H. Staudinger, Arbeitserinnerungen (in German), Dr. Alfred Hüthig Verlag, Heidelberg, Germany, 1961, 55.

[12] G. Oberkofler, Leopold Ružička (1887–1976) (in German), Studien‐Verlag, Innsbruck‐Wien‐München, 2001. ISBN: 370651561X

[13] P. Immerwahr, Verfahren zur Herstellung von Mitteln zur Vertilgung und Vertreibung von Schädlingen tierischer und pflanzlicher Natur sowie zur Desinfektion und Vertreibung von Wucherungen (German patent DRP 413871), 1923.

The article has been published in German as:

and was translated by W. E. Russey.

Pyrethrum: History of a Bio-Insecticide – Part 1

Chrysanthemum flowers as an insecticide

Pyrethrum: History of a Bio-Insecticide – Part 2

The chemical structures of pyrethrins and insecticide use between the World Wars

Pyrethrum: History of a Bio-Insecticide – Part 3

Comparing the insecticidal activity of DDT and pyrethrins

Pyrethrum: History of a Bio-Insecticide – Part 4

The comeback of pyrethrin research

Pyrethrum: History of a Bio-Insecticide – Part 5

The fall of DDT and the rise of pyrethrin derivatives


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