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Ecological Solutions Roundtable
AGRO-BIO - 360 - 02E
Translated by Allyson Gilpin and edited by Stuart B. Hill, Ph.D. in 1995
HOME PRODUCTION OF PYRETHRUM
Pyrethrum is a botanical insecticide produced from Dalmatian Chrysanthemums(1) (Chrysanthemum cinerariaefolium) or, less frequently, from Persian Chrysanthemums(2) (Chrysanthemum coccineum), plants that originated in Eastern Europe and the Caucasus. At present, the Dalmatian Chrysanthemum is commercially cultivated mostly in the mountainous regions of Kenya, Tanzania, and Ecuador. Although commercial production in Canada is not foreseeable, domestic production for the needs of a family vegetable garden or small farm is possible. The purpose of this document is to describe how pyrethrum-based insecticides can be cultivated and produced at home.
The term "pyrethrum" refer to the powder made with the dried flowers of the chrysanthemum, whereas the term "pyrethrins" refer to the six insecticide components occurring naturally in the powder. These six pyrethrins constitute between 0.9% and 1.3% of the dried flowers. The word "pyrethrums" is the common latin name used for ornamental varieties developed from the Persian Chrysanthemum.
The choice of seeds and plants at Canadian seed producers and nurseries is often limited to ornamental varieties of the Persian Chrysanthemum (C. coccineum), which contain less pyrethrins than those of the Dalmatian (C. cinerariaefolium). Unfortunately, these varieties are selected for their appearance rather than for their pyrethrin concentration (plant and seed sources are listed at the end of this document). Ideally, improved strains such as those cultivated in Kenya should be sought, as these varieties were developed for their high pyrethrin levels.
The Persian Chrysanthemum looks like a daisy and produces large white, pink and red flowers. Leaves resemble those of ferns, and plants grow to between 30 and 60 cm in height. The flowering period is June to July in our climates.
The Dalmatian Chrysanthemum looks more like the common daisy. Its flowers, typically white with a yellow centre, grow from numerous fairly rigid stems. Plants have blue-green leaves and grow to between 45 to 60 cm in height.
Other species such as Chrysanthemum balsamita and C. marshalli also contain insecticidal substances, but are less effective than the two previously mentioned species.
Although commercial pyrethrum production is concentrated in mountainous equatorial zones, Dalmatian and Persian Chrysanthemums can grow in our climates. However, their concentration of pyrethrins and numbers harvested may not be as high as in equatorial regions.
Commercial cultivation of the Dalmatian Chrysanthemum is done at altitudes between 1600 and 3000 metres because pyrethrin concentrations increase with altitude, whereas flower production diminishes at altitudes below 1600 metres. Ideal growing conditions are a semi-arid climat with cool winters, ca. 1200 mm rainfall and a two- to three-month dry season. They tolerate temperatures as low as -12oC.
Pyrethrum Chrysanthemums prefer sloped dry, gravelly, high-lime soils. Shaded areas should be avoided because lack of sunshine greatly diminishes pyrethrin concentrations. Chrysanthemums grow well in loamy, humus-rich soil in humid climates, but the pyrethrin concentration will be lower.
Plants can be propagated by plant division or by seeds. It is much easier to start cultivating Chrysanthemums by obtaining plants from a neighbour or a nursery. If seeds are used during the first year, for subsequent years, if more plants are desired, the root ball can be divided in fall or spring when new shoots appear.
Plants are started indoors at the end of February or March about three months before transplanting them outdoors. Seeds must be started in a sterilized medium. They take about two to four weeks to germinate. The ideal temperature for germination is about 20 to 22oC. Seeds can also be planted directly in the ground in August, covering them with a centimetre of fine earth and dry leaves.
Plants are transplanted in rows spaced 30 to 50 cm apart and 35 to 40 cm along the row (seven to nine plants per sq.metre). Expect a 15 to 30% loss of transplanted plants.
Chrysanthemums must be well weeded as they are very sensitive to competition. Although they do not respond to nitrogen or trace elements, phosphorous increases flower production. Except for occasional thrips in the flowers, pests are not usually a problem with Chrysanthemum cultivation.
Timing of harvest is critical for obtaining maximum pyrethrin levels in the flowers. It must be done during dry warm periods. Ideally, the flowers should be harvested at full development and even between the fifth and ninth day following the beginning of flowering. The flowers at the centre of the plant contain more pyrethrins than those on the outside.
Under ideal conditions, a plant can yield 80 to 100 flowers. Ten kilograms of fresh flowers yield between 2.3 and 3.3 kilograms of dried flowers.
Traditionally, in Japan, the flowers were harvested with their stems, and hung upside down for 24 to 48 hours in water before drying. This process slightly increases pyrethrin levels.
The drying method affects pyrethrin concentrations very little, according to Gnadinger et al.(1933). Commercially, the flowers are dried in aerated buildings, in the sun or on drying racks, until moisture content is about 10%.
The fineness of the crushed flowers influences the pyrethrum's efficiency as well as its longevity. The finer the powder the more efficient the pyrethrum will be against insects, but the more rapidly it will deteriorate. Part of the stems can be crushed with the flowers without really reducing the concentration of pyrethrins.
Pyrethrins are highly unstable components. They are quickly altered by light and heat and decompose in air in a matter of days. They decompose almost immediately in alkaline mediums.
Pyrethrin concentrations can be maintained for at least six months by keeping the crushed flowers in a freezer, at temperatures between -2oC and -5oC. At room temperature, freshly harvested flowers will keep for about 10 days in a hermetically sealed container. Dried flowers kept in hermetically sealed containers away from light will keep for several months. Submerging pyrethrum in mineral oil or sesame oil is another form of preservation.
The two ways of using pyrethrum are dusting or spraying. Surprisingly enough, the insecticide effect of pyrethrum for either method is enhanced by cool temperatures. It is therefore recommended to apply in late afternoon so that cooler evening temperatures will improve efficiency, and the sun won't accelerate the degradation of pyrethrins.
Simply apply the dried and crushed flowers to the leaves of plants requiring protection. Commercial formulation contain other substances to facilitate use and adhesion to the leaves. The pyrethrum can be mixed with talc, gypsum, lime or diatomaceous earth to serve this function.
When burned, the uncrushed dried flowers may repel and kill mosquitos. They can also be used crushed in a sawdust mixture and burned for the same purpose. This is the idea behind mosquito coils.
Spraying requires that the active ingredients be extracted. The solutions must be applied immediately following their preparation for maximum efficiency.
Twenty grams of pyrethrum (dried and crushed flowers) are soaked in six litres of warm water(3)
for three hours and then sprayed. It is possible to use fresh instead of dried flowers, but then three to four times more flowers must be used.
Commercially, pyrethrins are extracted with a light petrol-based solvent (kerosene, etc.). For home spraying, soak 500 grams of dried flowers in four litres of kerosene for half a day or more preferably. The kerosene will dissolve 73% of the pyrethrins in 48 hours. After filtering through a cloth, the solution is ready for use.
Dried flowers are soaked in paraffin for one day, and occasionally stirred. The solution must be filtered before use.
Alcohol extraction is possible, but the resulting product has the disadvantage of evaporating very quickly.
The efficiency of pyrethrum can be greatly enhanced through the addition of other products.
The role of the synergist is to increase the potency of the pyrethrum and speed its reaction time by preventing detoxication within the insects. In commercial formulas, piperonal butoxide is usually used for this. It is suspected, however, that this component is mutagenic, and damages liver cells in mammals. Other synergists that are almost as efficient are canola oil, sesame oil, nutmeg oil, parsley oil and sassafras oil, the first being undoubtedly the least expensive. One teaspoon of oil is used per litre of solution.
Soap acts as a surfactant, allowing for better leaf coverage and increasing pyrethrum efficiency by a factor of four. One half tablespoon of soap (Safer insecticide soap or Ivory Liquid) is added to one litre of solution. It will even be better if a little ethyl alcohol is added with this.
Anti-oxidants such as tannic acid, found in the bark of several tree species, slow the breakdown process of pyrethrins when applied under sunny conditions.
The effect of pyrethrum is immediate. Insects are paralysed on contact. It is advisable, however, to ensure that they are not simply stunned, as individuals that receive a low dose may recover several hours later. In this case, treatment must be repeated using a stronger dose. Rotenone and pyrethrum may be combined to obtain a mixture that has a more long-lasting effect.
Pyrethrum provides between 12 and 48 hours protection. It is necessary to reapply after rain.
If the efficiency of the pyrethrum is in doubt, conduct a fly test. This consists of putting some flies in a container with a little pyrethrum powder. If the pyrethrum is fresh, the flies should become paralyzed and/or die within less than a minute. With stale powder, the fly will take about 20 minutes to react.
Although pyrethrum is highly biodegradable and has a short-term effect, it is nonetheless a nonselective insecticide that affects a large number of beneficial species. Pyrethrum is effective against a large number of adult insects, but it is much less effective against larvae. It will not harm bees, or beetle larvae, but may harm a great number of other beneficial insects. It is not harmful to humans and animals, but is to fish and other aquatic organisms. However, some people may have allergic reactions from handling pyrethrum.
These three canadian nurseries carry Dalmatian Chrysanthemums. Plants or seeds of ornemental varieties of Persian Chrysanthemums can be found in most major nurseries.
Anonym. 1987 (May). Plant allies. Mother Earth News American Country, vol. 1(2):81.
Anonym. 1987 (March). Pepping up pesticides naturally. Organic Gardening, 34(3):8.
Casida, J.E. 1973. Pyrethrum: the natural insecticide. Academic Press, New York. 329 pages.
Gnadinger, C.B., I.E. Evans and C.S. Corl. 1933. Pyrethrum investigations in Colorado. Colorado Agricultural College Experiment Station, Bulletin 401. 19 pp.
Grieve, M. 1981. A modern herbal. Volume II. Dover Publications, New York. 902 pages.
Henn, T. and R. Weinzierl. 1989. Botanical insecticides and insecticidal soaps. University of Illinois Cooperative Extension Service, Circular 1296. 2 pp.
Keen, M. 1988 (October). Le retour en force du pyrèthre. Du sol à la Table, No. 166:21.
Stoll, G. 1986. Natural crop protection based on local farm resources in the tropics and subtropics. Josef Margraf, Publisher. Gaimersheim, Germany. 186 pages.
The purpose of this document is to summarize scientific and popular literature available on the subject, in a perspective of organic (biological) agriculture. It does not consist of recommendations or a production guide.
© No part of this publication may be reproduced without prior permission
1. 1 Also called insect flowers, C. cineriifolium in Latin and chrysanthème à feuilles cendrées in French.
2. 2 Also called painted daisy, chrysanthème rouge in French and Chrysanthemum roseum in Latin.
3. 0 Some popular sources suggest using hot water. Since pyrethrins deteriorate rapidly when exposed to heat, this practice cannot be recommended, unless the solution will be used immediately.
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