PYRETHRINS VS PYRETHROIDS
Among plant extracts used in organic farming, pyrethrum extracts hold a special place. Indeed, they have a direct and rapid action, comparable to any good synthetic insecticide prior to the 1990s, and they served as a model for the creation of a large number of synthetic molecules, still currently the most used worldwide.
Pyrethrins are active ingredients derived from a plant called Dalmatian pyrethrum (Tanacetum cinerariifolium). They have a high insecticidal capacity thanks to a high neurotoxic action. Their versatility is important, making them a flagship insecticide for organic farming.
Other plants of the same family have similar properties, and also possess pesticidal properties, such as chrysanthemums, especially Persian chrysanthemum (Tanacetum coccineum).
The use of natural pyrethrins is very common in organic agriculture because their action of shock is important. Their versatility makes it possible to fight against numerous Lepidoptera, Diptera, Homoptera, Hemiptera, Thysanoptera, Hymenoptera, Coleoptera, and so on. In short, they are active on aphids, flies, mosquitoes, bugs, wasps, caterpillars, maggots, leafhoppers and others.
Their neurotoxic action is, however, low or zero on warm-blooded animals, which allows their use in the domestic field or for the treatment of fleas in the house and on dogs. But cats, among the rare exceptions, are very sensitive.
They constitute therefore a highly appreciated insecticide, to fight the insects of house.
It may be noted that their degradation is quick, especially by the action of light, which gives them an action that is not very prolonged over time. This is an advantage in terms of side effects on the environment, but it is an agricultural disadvantage, in terms of duration of effectiveness.
To maintain active protection, or in case of repeated or prolonged attacks of harmful insects, the farmer may be led to repeat sprayings more than once a week.
However, their versatility in fighting pest insects comes along with the same versatility in eliminating useful insects. Auxiliary insects such as ladybugs, lacewings, syrphids, anthocorids or bees are eliminated as quickly as pests.
Another disadvantage, and not least, this group of molecules is extremely toxic to aquatic fauna. An incidental spill, even of limited quantities, into a watercourse can kill all fishes for several kilometers.
In short, it is a natural product, it's true, authorized and widely employed in organic farming, but which requires the utmost precautions to be properly used and to avoid extremely undesirable effects on the environment.
Natural pyrethrins derived from industrial extraction may be used, the safest and most regular, but they can also be obtained by maceration of flowers or leaves of concerned plants, as is the case with tansy manure. In the latter case, the use must be rapid because the molecules released by maceration and fermentation can rapidly degrade under the effect of hydrolysis. It should be noted that these liquid manure have a reputation for possessing certain properties of insect repellers and fungicides.
In the 60s and 70s, agrochemical companies were interested in the properties of pyrethrins. DDT and other organochlorines, which largely dominated the market at the time, were having diverse problems (health and environmental issues), which were serious (while we probably not measured yet all the gravity) and more and more frequently. The evolution of measurement techniques and technologies, of problems detected and of mentalities, was clearly showing that it was urgent to find alternatives.
This is the time of the multiplication of the organophosphorus and especially of the onset, then the multiplication, of synthetic pyrethroids.
From the chemical formulas of natural pyrethrins, chemists successfully study the possibility of modifying them to increase their effects. Thus, over the years, we saw the appearance of neighboring molecules whose name are often related to the chemical family to which they belong, such as permethrin, cypermethrin, deltamethrin, allomethrin, cyfluthrin, cihalothrin, but also fenvalerate, fluvalinate, and I forget a lot.
Each molecule brings some special characteristics and allows chemical firms to file patents, thus having technical and commercial exclusivities.
All of these molecules have specificities, but in general, if they retain the main defects of natural pyrethrins (toxicity to aquatic fauna, versatility and toxicity on many useful insects), they bring them some important characteristics:
- They are specific. It is no longer a cocktail of molecules of varying proportions, therefore to efficiency and variable side effects (see my article on neem oil to understand the importance of it https://culturagriculture.blogspot.com.es/2017/02/98-natural-vs-synthetic-2-neem-oil-vs.html).
- They are much more persistent. This results in passing from 4-5 days of persistence, to 2-4 weeks. Because the shock effect is similar and the persistence is greater, they reduce the number of applications for the same protection, thus reducing the negative effects on the environment.
- They operate at much lower doses, thus reducing environmental losses and negative side effects.
Even if their degradation is slower, they are still totally degraded in a few weeks. In fact, they remain one of the families of synthetic pesticides whose degradation is complete in the environment.
However, among these synthetic pyrethroids, there is at least one, tau-fluvalinate, whose side effects are markedly better. Indeed, its toxicity on bees is very low, not to say zero. It is so true that this molecule, a truly synthetic pesticide, is used purely, directly in hives, to control varroa, a micro-parasite of bees, probably the main cause of the decline of hives (and not neonicotinoids, whatever the ecological lobbies suggest).
Finally, we end up with a similar to my article on Neem oil situation. There are, face-to-face, products with similar negative side effects on aquatic fauna or beneficial insects.
It can even be said that some synthetic products are much better than natural products, since tau-fluvalinate is clearly better than natural pyrethrins in order to fight against beehives decline.
This synthetic molecule has been the only available for a long time, and it can be said that it saved millions of beehives from a certain death, during the years 80 to 2000. Since then, other solutions, some of them biological, have been found, with an equivalent or higher level of effectiveness.
On the other hand, while having the same negative effects, synthetic pyrethroids allow a great reduction of the doses and the number of applications. Under these conditions, the same negative effects are expressed less frequently. In other words, a protection based on natural pyrethrins, will have more serious negative side effects because sprayings will be more frequently repeated.
However, once again, for reasons of ideology, of dogma, organic farming will prefer the use of natural pyrethrins.
It would therefore be preferable to cause more serious damage to the environment, rather than change anything to an ideology devoid of any scientific basis.
A sour note, though: the organic farmer will do everything in his power to avoid using natural pyrethrins ...