Pyrethrins used in organic farming

Origin and mode of action of pyethrins

Pyrethrins are neurotoxic esters extracted from Persian chrysanthemum flowers or Dalmatian pyrethrums. Pyrethrins act by contact or inhalation and are more or less toxic to many insects. They are less targeted than some synthetic degradable insecticides such as pyrimicarb, which is known to be safe for entomophagous insects (a). An INRA study showed that Aged « larvae of the ladybird Semiadalia undecimnotata Schneider, treated from moulting onwards with a solution whose concentration corresponds to the commercial dose, do not show abnormal mortality compared to control larvae, but undergo temporary physiological disturbances that affect food consumption, metabolic use of prey and weight growth. These disturbances, which appear within the first few hours after treatment, disappear afterwards, probably due to the existence of an elimination or detoxification process of the insecticide molecule in the larvae. The increase in the average duration of the stage allows treated larvae to reach a final weight comparable to that of control larvae and results in a slightly higher total food consumption than the latter. » (1) (b).

Pyrethrins are very fragile and degrade rapidly when exposed to oxygen in the air, moisture or light. Compared to synthetic insecticides, their price remains high. Some preparations are low in active ingredients and are therefore considered to be repellents. For example, if you use certain preparations against commercially available ants, the ants are paralysed for a few tens of minutes and then quietly resume their activities. This fragility of pyrethrins makes it necessary to increase the number of applications to have a long-term effect. For this reason, their effects on the environment are not negligible when they are used as repetitive treatments.

Many studies have shown that pyrethrins are very effective against a wide range of insects, both bad and beneficial, but they are ineffective on lepidopterans such as polyphagous carrot moths. However, insects can become resistant to the various pyrethrins via the production of enzymatic detoxifiers. This is particularly the case for whiteflies (also resistant to synthetic pyrethroids). For this reason, pyrethrins are not considered satisfactory insecticides for controlling certain agricultural pests (2). As there are few authorised pesticides in organic farming, pyrethrins are often used in this sector, increasing the risk of producing resistance.

Some remarks on pyrethrin preparations

The activity of pyrethrins is enhanced in many formulations by the addition of synthetic additives such as Piperonyl Butoxide (PBO) in order to suppress the detoxification action present in some insects. Piperonyl Butoxide is a “sinergic” added to pyrethrins to increase its toxicity. This mixture is not very popular in some ecological settings due to the lack of toxicity assessment of synergistic effects. This additive is known to be toxic by inhalation, skin contact and ingestion. It is classified as R40 (suspected human carcinogen) and is highly toxic to aquatic organisms. PBO is biodegradable. PBO, which is also present in formulations containing synthetic pesticides, is rapidly degraded when exposed to sunlight. Its half-life is one day compared to 14 days when it is leached into the soil.

A preparation authorised for use in gardens (3) contains pyrethrins and abamectin, which is a mixture of several chemicals of the avermectin family (a group of molecules produced by bacteria). Abamectin can harm the foetus, can be fatal if ingested or inhaled, and is highly toxic to aquatic organisms with long-term effects (4). Other professional formulations contain a mixture of pyrethrins and rapeseed oil.

Although pyrethrins are natural pesticides that have been used for many years, limited data are available on their impact on the environment. It has been shown that the persistence of pyrethrins in soil increases in proportion to the organic matter. Thus, soil amended with organic matter contains twice as much pyrethrin as native soil, and their mobility is reduced by humic acids (5). Studies have shown that pyrethrins have an influence on microbial flora. While some microbial populations are tolerant to pyrethrins, its degradation products lead to a decrease in bacterial and actinomycete populations (6). However, this reduction is temporary. In general, pyrethrins are rapidly degraded by some bacteria and this is also why they are allowed in organic farming. In very high doses, pyrethrins are very toxic, including to humans, causing vomiting, nausea, convulsions and dermal irritation, but not lethal.

Bees are very sensitive to pyrethrins absorbed by contact or ingestion. As bees play an essential role in the pollination of agricultural crops, the use of pyrethrins at flowering time is not recommended.

Following the ban on synthetic pesticides for private use, pyrethrin-based preparations are now very common in garden shops. As bees tend to leave the fields and fruit trees in the evening to return to their hives, it is advisable to use pyrethrins at the end of the day. However, there are no serious scientific studies showing that pyrethrins are sufficiently degraded the next day to prevent bees from being poisoned in turn.

In vegetable growing, as with all synthetic broad-spectrum insecticides, the use of pyrethrins is bound to be very problematic in protecting certain plants that flower constantly, resulting in a succession of fruits, such as cucurbits, tomatoes, etc.

When the industrial production of pyrethrins is contrary to ecological principles.

The way pyrethrins are produced is the opposite of the main principles of the ecological and interglobal movements. The plants are mainly grown in Tanzania (60% of world production), Kenya, Rwanda and Papua New Guinea. It takes 52,000 plants to obtain 25 kg of powder. To meet the demands of the organic lobbies and their European clients, who are better off than African farmers, vast territories are reserved for the cultivation of these plants at the expense of food crops. In Kenya, one of the poorest countries in Africa, pyrethrum, which thrives best between 2200 and 2800m, has replaced beans and maize. The white daisy-like flowers of pyrethrum are hand-picked by many women and distilled to produce pyrethrin. The flowers are also packaged for export. Of course, as yields are low, so are wages. The only winners are the Kenyan exporters who quickly realised that they could exploit the craze for natural pesticides created by environmental movements in rich countries.

Like all other plant crops, pyrethrum and chrysanthemums are attacked by polyphagous insects and fungi (green peach aphid, nematodes, trips, ascochytosis …). In an Australian study published in 2008 in APS Journals (7), it is stated that in Tanzania and Papua New Guinea, farmers do not treat their pyrethrum with organic insecticides, but with synthetic insecticides and fungicides used in conventional agriculture, notably difenoconazole. This fungicide is known to be toxic to mammals and aquatic environments, with a half-life of 1600 days under certain conditions of use.

(a) Parasites or parasitoids that develop at the expense of other insects.
b) Preparations containing pyrimicarb with the pyrethroid, lambda-cyhalothrin, are not recommended for use in an IPM programme because of their residual toxicity to beneficial insects. They should be used outside the periods when beneficial insects are controlled or when IPM fails.