In this technical fact sheet, we will examine the characteristics, mode of action, and proper use of this natural insecticide.
What pyrethrum is
Natural pyrethrum is extracted from the flowers of plants belonging to the genus Chrysanthemum, among which the most widely used species is Chrysanthemum cinerariaefolium. Pyrethrins are obtained by treating dried flowers with organic solvents, followed by appropriate refining and standardization steps to determine the concentration of the active ingredient.
The term pyrethroids refers instead to the synthetic equivalents of these compounds, which are not permitted in organic agriculture. This natural insecticide acts on the central nervous system, causing insect death, generally by asphyxiation. Due to the low persistence of the compound in the environment and the rapid metabolism of the insecticidal active ingredient by target organisms, maximum insecticidal efficacy is achieved when pyrethrum is used in combination with horticultural oil (white/mineral or vegetable oils), or co-formulated with piperonyl butoxide. These substances act as synergists or slow down the detoxification metabolism in insects. Among the natural compounds commonly used for this purpose is sesamin, contained in sesame oil.
As can be seen, pyrethrum has very low toxicity to mammals and, due to its extreme photolability (it degrades rapidly in sunlight), allows the release of beneficial insects after only 24–48 hours in complete safety. Classic uses of pyrethrum insecticide include control of rose sawfly and large cabbage white butterfly, among others.
Pyrethrum is commercially available in concentrated formulations or ready-to-use products.
Correct use of pyrethrum
This product has strong knockdown activity and a broad spectrum of action. As a contact insecticide, it is active against all insects that come into direct contact with it (hit by the spray) or indirect contact (walking on or feeding from treated plant parts), provided that the correct application rate is used. It is also effective against stored-product insects.
Persistence on vegetation, in addition to mineral oils, can be increased by mixing pyrethrum with soaps (such as insecticidal soap). One of the main limitations of pyrethrum, in addition to photolability, is the pH of the spray solution, which must be below 7. It is therefore important to acidify the solution or check its acidity, especially when using alkaline water, by adding pH correctors to bring it below neutrality.
Best results are obtained when pyrethrum insecticide is applied in the early morning during autumn and winter, or at sunset during the rest of the year, avoiding treatments during periods of maximum sunlight. In addition to solution pH, as with all contact active ingredients, it is important to spray the product evenly over the entire plant and use the correct concentration of active ingredient. Pyrethrum has no systemic activity.
The use of pyrethrum on certain insects must be carefully evaluated and applied only at specific stages of the target insect’s life cycle. This is the case with scale insects, where pyrethrum is effective only at particular life stages corresponding to egg hatch. Any other application timing against scale insects is ineffective.
On some insects, such as the gray apple aphid, its use may be poorly effective and even counterproductive, as it can eliminate beneficial insects that act as natural enemies of the aphid.
Pyrethrum insecticide is permitted in organic agriculture.