In the first of a two-part article, Ralph Early of Harper Adams University discusses the historical and current use of pesticides and focuses on the use of herbicides and insecticides.
Introduction
Over millennia humans have established a particular relationship with the species they categorise as pests and it is a distinctly anthropocentric relationship. Pests threaten the quality of human life and existence, and in this lies the justification for their control and eradication as a common good benefiting humans as individuals and society as a whole. Farmers, more than most people, understand the importance of controlling pests because they present a constant threat to crops, farmed animals, stored materials, processed food products and premises. To illustrate, the FAO (Food and Agriculture Organisation) estimates that the potential world crop yield is reduced by between 20 and 40% by pests and diseases[1]. Therefore pest control in food production is vitally important, but it presents particular challenges in relation to effective and safe methods of use, environmental protection and ecological effects.
This article presents a snapshot of pest control in agriculture by application of pesticides. It is a topic of great importance and one about which a vast literature exists, addressing the wider benefits of pesticides in the context of humans and the environment[2] as well as research of a very specific nature.
Historical perspective
 |
Herbicides are commonly used to
control weeds in maize production
with application ideally before weeds
reach 10 cm high and the crop is at its
4 leaf stage |
Pests compete with human beings for the resources necessary for survival and in some instances people are the resource. Early farmers would have sought the means to control agricultural pests and people generally would have innovated solutions to more personal problems, such as fleas and body lice. Pyrethrum, derived from the flower heads of Chrysanthemums (Chrysanthemum cinerariifolium and Chrysanthemum coccineum), was used as an early form of insecticide and is still in use today. Toxic compounds based on mercury and arsenic were used to combat infestation by the body louse, Pediculus humanus humanus, and head louse, Pediculus humanus capitis. Sulphur compounds were also used to control insects and mites in domestic and agricultural situations. Some 3,500 years ago the Chinese introduced the use of botanical compounds as insecticidal seed treatments and around the same time the ancient Egyptians used cats to control rodents threatening stored grains. In 1200 BCE the Chinese were utilising biological methods of pest control with predatory ants eliminating beetles and their larvae.
The human need to control pests clearly stretches back into history, but scientific pest control and specifically the protection of agricultural produce has progressed most rapidly over the last century and a half, as a consequence of advances in industrial chemistry. As urban populations grew during the 19th century, demands on rural food production increased. Watson records the history of the development of pesticides and the pesticide industry explaining, for example, that many compounds used as early, non-selective pesticides, such as copper acetoarsenite, or Paris Green, a toxic dye, were by-products of industrial processes. Other substances used as pesticides included compounds of arsenic, mercury and sulphur as well as nicotine and hydrogen cyanide. Clearly such pesticides were as potentially lethal to those who used them as to the pests they were intended to destroy.
During the early 20th century a number of notable industrial chemical companies identified the market opportunities that an increasingly mechanised and industrial agriculture offered. These companies were transformed into agro-chemical businesses serving agriculture, first in economically developed countries in Europe and North America, but also other parts of the world as the century’s Green Revolution gathered pace.
It is interesting to note that some of the leading agro-chemical companies of the 20th century developed their expertise in pesticide chemistry as manufacturers of chemical warfare agents. Organophosphorus compounds, for example, act on the nervous system of mammals and insects inhibiting irreversibly the enzyme acetylcholinesterase. This causes the neurotransmitter acetylcholine to overload the nervous systems of affected organisms with constant transmission of signals between motor neurons, resulting in eventual death. The nerve agent Novchok, a topic of international news in 2018, is an organophosphate-based compound. Other examples include Tabun, based on insecticide development by the German chemical company, I.G. Farben in 1936, and VX, a nerve agent developed at Britain’s Porton Down defence laboratory in the 1950s and based on the organophosphate insecticide, Amiton.
Modern pest control
 |
Many vineyards use herbicides
such as glyphosate to clear the ground
between vines, however viticulture is
increasingly moving towards pesticidefree
production |
In relation to modern pest control, Watson[3] defines the period 1930 to 1973 as the Productivist Period, during which many functionally valuable pesticides came into use. The foundations of today’s monoculture agriculture were formed during this time, as engineering and seed companies recognised the commercial opportunities to be gained by collaborating with agro-chemical companies in the intensification of agriculture and food production. Innovations in engineering, plant breeding, fertilisers and pesticides were aligned to support farmers in the challenges they faced, not least in pest control.
Farmers who are engaged in crop production seek to control mainly invasive plant species (weeds), insects, nematodes and the fungal diseases of crops as well as rodents, other small mammals and birds. Those involved in animal production face similar problems when growing e.g. fodder crops, but they also need to control insect and parasitic pests of farmed animals as well as disease-causing microorganisms. The benefits of pest control are primarily increased crop and animal yields with associated increases in harvested product (crops and animals), improved product quality and the control of invasive species. Additionally, the use of pesticides (and drugs) in animal production brings improvements in animal welfare by the alleviation and elimination of animal suffering.
Agricultural pests require treatment with specific pesticides, which are classified according to the pests they eliminate. In the context of British farms this generally means herbicides, insecticides, nematicides, fungicides, bactericides and rodenticides.
The benefits of pest control are primarily increased crop and animal yields with associated increases in harvested product (crops and animals), improved product quality and the control of invasive species.
Herbicides
Herbicides, also known as weed-killers, can be conveniently divided into two groups: selective and non-selective herbicides. They are also defined according to stage of use in the crop cycle: preplanting, pre-emergence, post-emergence and established stands. Different herbicides have different modes of action (MOAs). Some disrupt cell division and are generally used for pre-emergence applications with germinating seeds, while those that disrupt photosynthesis are used post-emergence. Herbicides that target specific enzymes are designed to treat different plant species, from grasses to broad leaved weeds, and are used on established plants, for instance to prepare ground for cultivation and/or seed drilling.
Selective herbicides are formulated such that they are effective against weeds growing among a given crop, e.g. broad leaved weeds or grasses growing alongside maize and pulses. The organic compound 2,4-D (2,4-Dichlorophenoxyacetic acid) functions as a systemic herbicide. It is absorbed into plants where, as a synthetic auxin (growth hormone), it stimulates uncontrolled growth resulting in death. It leaves cereals and grasses unaffected. As a herbicidal compound, it is the active ingredient in many proprietary products and has been used as a defoliant since the 1940s. During the Vietnam war, 2,4-D was weaponised along with 2,4,5-T (2,4,5-Trichlorophenoxyacetic acid) as ‘Agent Orange’ in the USA’s herbicidal warfare programme. Unfortunately Agent Orange was contaminated with dioxins, now linked to abnormally high incidences of cancer, such as leukemia and non-Hodgkin lymphoma, as well as birth defects among the Vietnamese people.
Non-selective herbicides are formulated to kill both grasses and broad leafed weeds. Two examples are, paraquat (N,N'- dimethyl-4,4'-bipyridinium dichloride) and glyphosate (N-phosphonomethyl-glycine). Paraquat obstructs photosynthesis and was used widely for many decades as a very effective non-selective foliar contact herbicide. However, it is extremely toxic to humans and other mammals and has been banned in the EU since 2007, as well as in other countries. Glyphosate, in contrast, is the most widely used herbicide in the world with some market research agencies projecting sales of US$8.5b by 2020. It is as an extremely effective herbicide, favoured by British farmers as a means of controlling blackgrass (Alopecurus myosuroides) and as a desiccant on cereals before harvest, although herbicide resistance is appearing in blackgrass[4]. Matthews[5] provides a detailed review of pesticides and their use, noting that many newer products have entered the market since 2000, including propoxycarbazone-sodium, which is effective against blackgrass, other grasses and some broad-leaved weeds in wheat. Given the need to develop methods of sustainable agriculture, broad-spectrum herbicides have found favour in no-till cultivation, which aims at preserving soil quality and reducing soil erosion and loss caused by cultivations.
Glyphosate functions by blocking the enzyme enolpyruvylshikimate-3- phosphate synthase in the shikimic biosynthetic pathway of plants responsible for the production of aromatic amino acids and other metabolites. Human beings and mammals do not possess the pathway, so glyphosate is considered non-hazardous to human health. In 2015 however, the WHO’s cancer research agency declared glyphosate a probable carcinogen although various studies, e.g. Mink et al.[6], have not revealed an association between glyphosate and site-specific cancer. In 2018, a California court awarded damages against Monsanto in a case concerning a school groundskeeper who developed cancer after years of using the glyphosate-based herbicide, Roundup™[7]. Recent research[8] suggests that glyphosate may be harmful to bees, perturbing gut microbiota and adversely affecting bee health and effectiveness as pollinators.
Insecticides
A wide range of compounds have been and are used as insecticides. Natural insecticides include pyrethrum, nicotine and neem. Synthetic insecticides include or- ganochlorides, such as DDT (Dichlorodiphenyltrichloroethane), organophosphates and carbamates, neonicotinoids and pyrethroids. Different insecticides have different modes of action. For example, DDT interferes with the function of cellular sodium channels; organophosphates interfere with the function of acetylcholinesterase.
As the agricultural use of organochlorine and organophosphate pesticides increased as a consequence of the 20th century’s Green Revolution, concerns were raised about effects on human and animal health. Rachel Carson[9] drew attention to the hazards inherent in the use of pesticides and particularly DDT, which began to focus the attention of public health authorities on the nature and effects of these substances. Studies of global wildlife in the 1960s and 1970s revealed bioaccumulation in body tissues and biomagnification in the biological food chain of various toxic compounds of industrial origin, such as PCBs (polychlorinated biphenyls) and some widely used pesticides. DDT proved to be an environmentally persistent organic pollutant detected in the tissues of many wildlife species, resulting in it being banned or restricted in many countries. It is very effective against the malaria carrying Anopheles mosquito and use is permitted in some countries.
The use of pesticides on food crops involves the intentional application of toxic substances to materials destined for human consumption. Caution must therefore be taken with regard to the toxicity of pesticides, rates of application and the persistence of residues within the environment and on the foodstuffs to which they are applied. In this respect, farmers are provided with explicit recommendations regarding use and application rates in order to ensure that at the time of harvest, Maximum Residue Levels (MRLs) are not exceeded. Even so, history has shown that pesticides that once were presumed to be safe for use were subsequently discovered not to be. Dieldrin, for instance, an organochlorine insecticide used commonly in the mid- 20th century, was banned by the Stockholm Convention on Persistent Organic Pollutants. It proved to be carcinogenic, an endocrine disruptor and harmful to the nervous system of humans, among other effects, and does not readily degrade, remaining active in the environment for many years.
More recently controversy or has arisen in relation to a class of systemic agricultural insecticides resembling nicotine, termed neonicotinoids, which have proven very effective insecticides, for example, in the prophylactic protection of autumn-sown oilseed rape (Brassica napus L.). However, Whitehorn et al[10]. report harm to bumble bee colonies caused by neonicotinoids and similar findings have been obtained by other workers. Consensus on the effects of neonicotinoids is though incomplete as others have been unable to draw definitive conclusions[11]. Concern about the effects of neonicotinoids on insect pollinators, including Apis melifera, the honey bee, has led to the banning of three in the class, imidacloprid, clothianidin and thiamethoxam, for outdoor use in the UK by the end of 2018. Other neonicotinoids, such as thiacloprid, are still permitted.
Conclusion
Pesticides are an important tool in the agricultural production of food. However, they are not without controversy, because of associated possible harms to the environment, wild biodiversity and human beings. As a topic, the use of pesticides in agriculture is complex: scientifically, politically and morally. Part two of this article will consider the other main classes of pesticides - fungicides, nematicides and rodenticides – together with policy perspectives and a view of the future encompassing alternatives to pesticides, precision farming and the ethical aspects of pesticide use.
Ralph Early
Harper Adams University, Newport, Shropshire, TF10 8NB, England
Web harper-adams.ac.uk/ staff/profile.cfm?id=122
References
2. Cooper, J. and Dobson, H. 2007. The benefits of pesticides to mankind and the environment. Crop Protection, 26, 1337–1348.
3. Watson, D. 2018. Pesticides and agriculture: profit, politics and policy. Cambridge: Burleigh Dodds.
4. HGCA. 2008. Herbicide-resistant black-grass: managing risk with fewer options. Information Sheet 03/Summer 2008. London: HGCA.
5. Matthews, G.A. 2018. A history of pesticides. Wallingford: CABI.
6. Mink, P.J., Mandel, J.S. Sceurman, B.K. and Lundin, J.I. 2012. Epidemiologic studies of glyphosate and cancer: A review. Regulatory Toxicology and Pharmacology, 63, 440-452.
8. Motta, E.V.S., Raymann, K. and Moran, N.A. 2018. Glyphosate perturbs the gut microbiota of honey bees.
PNAS. Published ahead of print September 24, 2018.
https://doi.org/10.1073/pnas.1803880115.
9. Carson, R. 1962/2000. Silent Spring. London: Penguin.
10. Whitehorn, P.R., O’Connor, S., Wackers, F.L. and Goulson, D. 2015. Neonicotinoid pesticide reduces bumble bee colony growth and queen production. Science, 336, 351-352.
11. EFSA. 2012. Assessment of the scientific information from the Italian project 'APENET' investigating effects on honeybees of coated maize seeds with some neonicotinoids and fipronil. (PDF). EFSA Journal. 10 (6): 2792. doi:10.2903/j.efsa.2012.2792.
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