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In this article, author and science editor Lars Fischer discusses the benefits and risks of the use of pesticides in agriculture. Pest control is a topic that is repeatedly discussed in the context of cotton cultivation.
Pesticides are a target of criticism: The agrochemicals are toxic, harmful to the environment and above all, in many cases unnecessary. What is really the problem with these substances?
The broad term “pesticides” refers to natural or artificial substances that kill unwanted organisms in the environment. This includes, for example, wood preservatives that prevent rot and disinfectants in the hospital. But special pesticides are also used against parasites such as lice or mites. Strictly speaking, antibiotics are just as much pesticides as they are active ingredients against malaria and other parasites – but they are rarely described as such.
For millions of years, plants have learned to protect themselves against insects and other pests with toxic chemicals. Substances such as nicotine and caffeine belong to this category, as does the pyrethrin from chrysanthemums, which is popular in organic gardening. Walnut trees form a herbicide in their leaves that prevents other plants from growing on the ground. Artemisinin, which is produced from annual mugwort and has made a career as a malaria drug, probably serves a similar function.
When commonly talking about pesticides, one usually thinks of a very specific application: crop protection in agriculture. More than 280 active ingredients are approved for use in crop protection in Germany. The palette ranges from rapeseed oil to the sometimes very toxic organophosphates. Even greater than the number of active ingredients is the range of preparations – and as far as their safety is concerned, the latter are often poorly investigated.
Even in ancient times it was known that arsenic also worked against pests, while sulphur was probably used against fungi. From the middle of the 19th century, people began to produce inorganic salts from heavy metals such as lead and copper, and later mercury, on an industrial scale for plant protection.
However, the era of modern pesticides began in the 1930’s with the organochlorine insecticides, including DDT and lindane. They were simple and cheap to produce, were effective against a wide range of insect pests and were not washed away from the plants by the rain. In many parts of the world, they increased yields drastically and spread very quickly. However, it soon became apparent that these substances hardly decomposed in the environment and accumulated in the food chain.
Subsequent classes of insecticides, such as the organophosphates, are therefore more readily biodegradable and soluble in water and, most importantly, much more effective. While up to two kilograms per hectare of organochlorides were applied during each spraying action, today this level has fallen to sometimes less than 100 grams per application and acre. Organophosphates come from the same class of substances as the nerve gases sarin and VX – and they are correspondingly toxic. However, in contrast to substances such as DDT, modern insecticides are usually readily biodegradable.
Herbicides which work against unwanted plants are also widely used. The oldest are the synthetic auxins, for example the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D), which has been on the market since 1945. They act like a class of plant growth hormone and kill the plant by causing uncontrolled growth. Other known herbicides such as atrazine and paraquat intervene in photosynthesis, while substances such as glyphosate intervene in metabolic pathways that build up amino acids and other biomolecules.
In addition, there are a whole range of other substances which increase the yields of crops, such as fungicides, mordants for seed treatment or wound closing agents for trees. However, herbicides and insecticides are the most important groups of active ingredients. In Germany, about 2.5 kilograms of pesticides are applied per hectare of agricultural land per year.
The dilemma of agriculture can be summed up as follows: create the best possible conditions for a specific plant in as small a space as possible so that as much rich food as possible for humans – or our animals – grows there. It is unavoidable that for other living things, both the good growing conditions of the field and the large amount of nutritious plants are, above all one thing: a richly set table. The fact that uninvited guests can soon be found there lies in the nature of things. The better the yield, the more pests the field attracts.
Crop protection products significantly reduce this problem, whether it is wild plants competing with the crops, or animals that feed on the crop. Pesticides are an integral part of the green revolution, the shift to industrial agriculture, which has roughly doubled global grain production over the last 40 years. In many countries, the security of supply that has been achieved in this way is still the basis of political and social stability.
The dispute is therefore mainly ignited by the question of whether pesticides are worth the risks – and whether it is possible without or at least with fewer agricultural chemicals. Two developments make this debate explosive: Firstly, pesticides probably contribute greatly to the fact that the populations of important pollinators such as wild bees and bumblebees are collapsing dramatically, although many crops are in urgent need of their services. On the other hand, experts estimate that global grain demand will double again by 2050. The scope for solving the problem is small.
Nonetheless, there are already several methods that reduce the use of pesticides in agriculture and in some cases are already very significant. For example, integrated pest management (IPM) has become widespread, especially in greenhouses. In this process, the aim is to make life difficult for plant pests from the beginning by choosing the right conditions, so that pesticides are not needed at all. This includes, for example, denying access to pests through closed buildings and quarantine measures, as well as biological control of pests using livestock or plant varieties that are less susceptible to predators.
Although the European Union prescribes the IPM principles for all farms that use pesticides, the process is currently mainly widespread in closed greenhouses and orchards. Here, biological pest control is usually much cheaper than using pesticides. With cereals in particular, studies show that IPM is usually more complex and expensive than conventional methods. With fruit, low pesticide methods even increase yield and quality. An important reason why these methods are also only spreading slowly in such cases is probably the effort required to acquire the appropriate skills and learn the procedures.
Modern no-till techniques, where the field is not ploughed before sowing and the natural soil structure is preserved, require even more pesticides. Without the otherwise obligatory tillage, erosion is reduced, earthworms and other soil life are more diverse, and humus forms faster. However, in many cases total herbicides such as glyphosate are needed to kill crops after harvest and control unwanted wild plants.
The use of less, or even no synthetic pesticides at all has, above all, become the mission of organic agriculture – IPM plays a much greater role here than in conventional agriculture. It uses biological pesticides such as the Bt toxin of the soil bacterium bacillus thuringiensis – the active ingredient has a long history as a conventional spray but has become known through its use in genetically modified maize – or natural enemies of the pests. There is now a large commercial market for this strategy, known as biocontrol.
Overall, there is a trend towards agriculture that relies less on pesticides as a stand-alone solution, but more on a combination of measures ranging from mixed crops to biological pest control to high-tech methods such as genetically modified crops. However, not even optimists believe that these modern approaches will make synthetic pesticides totally redundant in the long term.
Pesticides are poisonous – that’s the point of the matter. In the ideal case, the substance kills only a very specific pests, leaves all the others alone and is then also biodegradable with no residue. However, firstly, we do not live in an ideal world, and secondly, with such pesticides, we would have to fight each pest with its own substance. That would cost a lot more money and effort and probably cause more damage.
Thus, it is often accepted that most pesticides, in addition to pests, also kill some uninvolved or even useful organisms. Especially important for us humans are honey bees, as well as wild bees and bumblebees. The crisis caused by the so-called Colony Collapse Disorder in the bee industry has raised awareness of the importance of these pollinators, so that pesticides have now also been withdrawn because of the potential dangers to important wild bees and bumblebees.
How toxic plant protection products are for humans varies from substance to substance and is, in some cases, highly controversial. Some substances, the so-called organophosphates, are also highly toxic to humans. In this substance class we not only find highly effective pesticides, but also chemical warfare agents. Those who work with them should wear protective clothing. However, these substances have the advantage that they do not accumulate in the food chain, and at lower concentrations are considered less dangerous than, for example, chlorinated compounds such as DDT.
This ambivalence – human and environmental hazards on the one hand and the benefits of effective pest control on the other – is characteristic of many people’s discomfort with such toxins. The current case of glyphosate shows just how difficult it is to weigh up such an issue: A sub-organisation of the WHO has classified the herbicide in its category 2A – it is therefore likely to cause cancer, such as mate tee, shift work and mustard gas. However, the classification gives only limited information about the size and importance of the effect used, as experts repeatedly criticise. In other words, it says nothing about the actual risk.
By contrast, the Federal Institute for Risk Assessment (BfR), which is responsible for such statements, considers “not only the hazard-related analysis of a substance but also the estimated exposure, i.e. the actual intake of the substance, and from this information determines the risk of developing cancer”, as the organisation writes. On this basis, glyphosate is not carcinogenic in relation to its intended use. In stark contrast to this is the ruling of the International Agency for Research on Cancer, that glyphosate is “probably carcinogenic”. But what does this mean?
Another example is the age-old herbicide 2,4-D, which has been considered as potentially carcinogenic by the IARC since 1987. The substance can cause fertility problems in men who regularly work with it. This makes it one of several substances that have been found to cause reproductive problems in studies of occupationally exposed people, especially those who do not wear protective clothing when spraying the toxins. However, the substance is still approved.
Finally, it is often not clear, especially in the case of long-term health effects of pesticides, whether the active substance or rather an additive is responsible for the effect; especially since in reality, not only one spray comes into question, but a broad combination of influences and substances. Ultimately, the dispute over the health risk of pesticides is not a scientific, but a political one. Society must decide what degree of risk and uncertainty it is willing to accept for the benefits of plant protection products.
This article first appeared in “Der Spektrum”.