Introduction to integrated methods in the vegetable garden
Chapter : Treatments
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⇒ Organic or conventional treatments against pests.
Before the invention of synthetic pesticides, for thousands of years, famines occurred from time to time due to the emergence of real epidemics that destroyed a large part of the crops. Cereal yields were 10 times lower than today. Nowadays, most people who have never experienced these famines have become accustomed to eating their fill every day, without realising that this progress is due to the use of fertilisers and pesticides in agriculture.
Of course, pesticides do not have only advantages. Their intensive use can produce more resistant pests through selective breeding, they are not selective and they can have a negative impact on biodiversity and especially on beneficial organisms. But the vast majority of farmers know that banning all pesticides is a utopia, especially since everyone in agriculture treats them, including professionals who have converted to organic farming, which many French people do not know. According to a survey conducted by Harris Interactive and distributed by the association Alerte Environnement, 50% of French people do not know that organic farming uses phytosanitary products (1); products that are not, however, harmless because of their chemical properties and the doses commonly used to compensate for their lack of effectiveness.
Treatments are therefore unavoidable if the farmer does not want to find himself with insurmountable losses in yield when, for example, the climatic conditions are unfavourable, producing an explosion of a bio-aggressor. It should never be forgotten that plants are also living beings that can be sick and must be treated from time to time in the same way that one treats one's domestic dog when it is sick. Of course, the use of phytosanitary products is necessary when other solutions such as physical protection or the choice of varieties are no longer effective.
The fear of pesticides, including those used in organic farming, sometimes leads to irrational practices. I have sometimes met home gardeners who prefer to eat worm-eaten vegetables and fruit rather than try to protect them from their pests with effective treatments. Is it good for our health to eat diseased plants? No one would consider eating a farmed animal with an infection or parasites that are not eradicated. Why make an exception for plants? Those who prefer to eat wormy fruit do not know the dangers they are exposed to, as described below.
Worms in fruit or vegetables thrive by digging galleries that facilitate the introduction of particularly pathogenic fungi and bacteria. Don't expect these worms to deposit their excrement outside to please you because you are a fan of organic produce. These droppings deposited in the tunnels are a source of infection and contain dangerous toxins that spread through the fruit. Certain pathogenic fungi (aspergillus sp. - penicillium sp.) that accompany the attacks of the "apple worm" (codling moth) produce patulin, a neurotoxic, teratogenic, immunotoxic and embryotoxic toxin (2), whose admissible daily dose currently tolerated in food is 0.4 µg/kg of body weight (i.e. a very small dose). It is forbidden to market apple juice if its patulin concentration exceeds 50 µg/kg. The presence of patulin cannot be detected by taste or smell.
When bedbugs move into a house, no one would think that they should not try to get rid of them with a suitable pesticide under the pretext of protecting the environment. Plants are frequently victims of biting insects. Why not protect them with appropriate treatments when other means of protection are not effective?
The Acceptable Daily Intake (ADI) for a given substance is the possible daily amount of a xenobiotic substance (foreign to the body) that can be absorbed over a lifetime without risk to health. It is set with a large safety margin, in the order of 100 to 1000, in relation to the no-effect level. The latter establishes the maximum dose that a laboratory animal can absorb on a daily basis throughout its life without producing undesirable physiological effects. The AFIS has published a special dossier on pesticides ("science & pseudosciences" magazine), clarifying what the science says about them in relation to received ideas; accessible en cliquant ici.
Generally speaking, in 60 years, the average doses of registered plant protection products needed to treat 1 hectare have been divided by more than 34. At the same time, the average toxicity of active substances has been divided by 8.5. 75% of the molecules available at the beginning of the 1990s have been withdrawn from the market. (Source UIPP).
Are pesticides dangerous? Certainly they are. Just as the sun is dangerous, bees are dangerous, trains and cars are dangerous... But defining a danger is not enough to measure its real impact in everyday life. Exposure factors must also be taken into account. A hazard can be controlled if the exposure factors are well known and controlled with effective protection.
Danger is everywhere in nature, whether it is the consequence of human activities or natural events such as earthquakes, floods, solar and cosmic radiation... At every moment of the day, we take accepted risks. Cars, motorbikes and even bicycles are responsible for more than 3,000 deaths a year in France and no one is thinking of banning them in application of the precautionary principle enshrined in our constitution, because all those who use them need them to go to work, go on holiday, go shopping... The risk of a road accident exists at every moment, but we believe that the benefits are worth the risk Of course, we would like the risk of road accidents to be zero, but everyone knows that this is an impossible goal.
For the vast majority of plant protection products that we use, the fundamental question is whether exposure to these pesticides presents an acceptable risk. This distinction between dangers (capacity of a substance to produce toxic effects depending on the dose) and risks (exposure to the recommended active dose, recommended means of protection, etc.) is very often ignored by those who call for a ban on all pesticides. Their attitude is most often a manifestation of technophobia. Synthetic products are easily suspected while natural substances are adorned with all the virtues without any link to scientific data.
Extreme environmentalists believe that most synthetic substances used in agriculture have a toxicological classification and should therefore be banned. However, the toxic risk must be estimated in relation to the dose, the number of doses used, the interval between doses, and other factors such as the degradation conditions of each substance. These properties must also be known for metabolites (intermediate substances produced during degradation processes).
There is no such thing as a harmless substance. Everything is toxic, but what is important to know is when the toxicity of a substance becomes unacceptable. In science, the risk-benefit ratio is used to determine whether the use of a synthetic or natural substance is acceptable under specific conditions of use for specific purposes. The substance is accepted when the benefits are considered to outweigh the risks. This way of establishing a "calculated" risk in relation to a hazard is also used for medicines, pesticides used to protect wooden frames, disinfectants such as sodium hypochlorite (bleach), ...
Treatments are triggered when a careful observation of the plants leads to the observation of a partial insufficiency of useful beneficials and after having defined the level of aggressiveness of the bioaggressors. There are also situations where the pest presents a real danger for the environment if no attempt is made to stop its progression quickly by using phytosanitary products. In this case, it is often a case of imported insect pests or new bacterial, fungal or viral diseases. Any lack of treatment leads to a real ecological disaster.
In conventional agriculture, the generalised treatments practised a few decades ago with multi-purpose mixtures of broad-spectrum pesticides are usually abandoned. Spot treatment with very short-lived products reduces their impact on beneficial organisms.
Treatments are stopped when beneficial insects are able to take over. For example, for lettuce, in the north of France, the intervention thresholds usually recommended in the fight against aphids indicate that it is necessary to treat in spring and autumn when 10% of the lettuce is affected by the aphid. This threshold is increased to 20% in summer. However, summer treatments of lettuce have often proved to be inappropriate and may even have a reverse effect by eliminating the beneficial fauna which is particularly effective at this time. Hoverfly larvae can consume up to 400 aphids during their development, making plant protection treatments unnecessary in most cases (3).
For a private individual, synthetic pesticides such as Decis, when it was still allowed, were more than sufficient to treat vegetable pests. Some synthetic pesticides are modified copies of natural molecules used in organic farming to improve their stability and efficacy. Organic agriculture refuses them because they are produced by industry. Deltramethrin (the active ingredient in Decis) was invented by chemists who were looking for a molecule with the same properties as the chrysanthemum pyrethrins allowed in organic farming. Deltramethrin is an insecticide of the pyrethroid family that acts by contact (the pesticide is not introduced into the plant). It acts very quickly on aphids, but has the same disadvantages as the pyrethrins used in organic farming. This insecticide is toxic to bees, fish and freshwater invertebrates. It should therefore not be applied during flowering. As for fish and invertebrates, there is no reason to spray rivers and ponds.
Deltramethrin is metabolised by the microflora when it falls to the soil. But its half-life in the soil is more or less long; from 13 to 72 days (4). In the leaflet of the Société Chimique de France, it is stated that the formulation in the form of a concentrated emulsion of deltramethrin "makes it possible to reconcile good coverage of the plant with high bioavailability" (5). At the doses recommended by the manufacturer, deltamethrin appears to be particularly harmless to birds and mammals. At higher doses, deltamethrin has been shown to be toxic by inhalation (which is also the case for natural pyrethrins registered in organic farming). It is therefore advisable to follow the application conditions specified in the manufacturer's instructions.
Some organic gardeners refer to the simplistic idea that nature should be respected by rejecting the cultivation of any vegetable that is not adapted to the local environment. This principle is also mentioned in some books and press articles to justify the ban on pesticides for private individuals since January 2019 (6). If I take a pest such as the Colorado beetle as an example, such a recommendation for home gardeners leads to a ban on potato cultivation in all regions of France.
The reason often given for banning synthetic pesticides is to accuse individuals of not respecting the doses prescribed by the manufacturers. However, the products considered the most dangerous were already banned before January 2019 for non-professional use.
In the vegetable garden, as for professional farmers, pesticides have never been a major problem. Apart from voluntary intoxication (suicides), regularly reported and proven accidents in agriculture are rare. Finally, the recommendations in the instructions for use or on the packaging complied with the regulations concerning the safety of the applicator and in particular the Maximum Residue Limit (MRL) and the Acceptable Daily Dose (ADI). I would remind you that the ADI corresponds to the quantity of a pesticide that one could absorb daily throughout one's life without risk divided by 100. If a home gardener were to multiply the prescribed dose by 100, he or she would reach the ADI and would not be in danger. The safety margin is therefore very high, which explains the rarity of domestic accidents in agriculture.
The banning of synthetic pesticides for home gardeners is bound to produce plant health problems that are already of great concern. There is no effective organic treatment against certain particularly aggressive pests such as the Clavibacter michiganensis bacterium that causes potato wilt or the Drosophila suzukii fly, which was detected in France in 2009 and which is particularly fond of cherries, raspberries, strawberries, tomatoes, etc., and which can cause a loss of production of up to 80%. This fly « is too well established in the country for it to be possible to eradicate it » (7).
The use of local parasitoids has not yielded convincing results. The amateur gardener will be especially helpless against cryptogamic and bacterial diseases (such as mildew, which often infects tomato and potato crops), forcing him to adopt copper-based preparations approved for organic farming, a metal known to be toxic for humans and the environment and non-biodegradable. However, to combat fungal diseases, there is currently no other credible substitute for copper in organic farming. For this reason, the authorisation to use products containing copper in organic farming was renewed for 7 years on 28 November 2018 by the European Union (with a reduction in the authorised quantities of copper from an average of 6 kg/ha/year to an average of 44 kg/ha/year, with a smoothing mechanism to cope with greater needs in certain years). Consumers of organic produce have not finished absorbing the copper present on organic fruit and vegetables.
The ban on synthetic pesticides targeting 17 million of our fellow citizens has had no effect in reducing the volume of pesticides used in France. 5% of the tonnage of active plant protection substances marketed each year in France was used in non-agricultural areas, the rest being used by farmers. Of this 5%, ½ to 2/3 was used by private individuals (8). It is obvious that this decision was taken more for political than scientific reasons. It is difficult to attack farmers by banning them from using all synthetic insecticides. So it is the private individuals who are sacrificed on the altar of ecologism.
1) Horizons 25 3 2016
2) Université de Bordeaux : Les mycotoxines dans l’alimentation et leur incidence
sur la santé
3) Luttes intégrées contre les pucerons du feuillage de la salade dans le Nord Pas-de-Calais – station d’études sur les luttes biologiques intégrées et raisonnées
7) Le Télégramme N°968 du 4 septembre 2016
8) INRA -portails actus : Cerise : la lutte biologique veut faire mouche contre Drosophila Suzukii - http://www.inra.fr/Grand-public/Sante-des-plantes/Toutes-les-actualites/Lutte-bio-contre-Drosophila-suzukii#.V2pTdd577xU.twitter
9) Ministère de la transition écologique ; Accord cadre relatif à l’usage des pesticides par les jardiniers amateurs