Pesticides, herbicides and fungicides (generally considered together as ‘pesticides’ or informally as ‘chemicals’) have a long and controversial history. On the one hand, we need them to keep towns and streets pest-free to 21st century standards, and they also have an often-overlooked, vital environmental function in protecting our National Parks from invasive species like Rhododendron. On the other hand, over the last five hundred years they have also caused significant damage to the health of humans, wildlife and ecosystems. This is the first of a series of posts in which I’m going to take a more historical view of pesticides. I’m going to be looking at the chemicals which we use today, those which we used in the past, and considering whether there are any lessons to be learned about chemical pest control in general. In Part I, we will be looking at modern pesticides.
Theoretically, the world is now supposed to be looking beyond chemical pest control towards Integrated Pest Management (IPM) (Matthews, 2016, p. 1), with its mixture of physical, cultural, biological and chemical controls. That means, to stop rats for example, we need to tidy and rat-proof our buildings and lock down food sources (cultural control), employ rat traps (physical control) and encourage predators like barn owls (biological controls) as well or instead of using rodenticide. However, since chemical control methods are so reliable compared to the other methods, there is some difficulty in moving away from pesticides. In practice chemical pest control remains the norm around the world.
The continued prevalence of pesticides means that a handful of chemicals dominate discussion about pest control. The most well-known pesticide at present is probably glyphosate, which is a systemic herbicide. Glyphosate is owned by Bayer (formerly Monsanto) and is contained in products like Roundup. In the UK, concern about glyphosate has been largely responsible for calls to ban or restrict pesticides by several town and county councils including Glastonbury, Lewes, and Hammersmith & Fulham in London, due to concerns about human health and environmental impact (Pesticide Action Network UK, 2017). Although glyphosate is a broad-spectrum herbicide, some crops have been genetically modified to be resistant to glyphosate (‘Roundup ready’), meaning that they can be sprayed glyphosate to kill all weeds around them. Selective pesticides are also widely used for this purpose, for example, the pesticide called 2,4-D will not harm monocot plants (grasses or cereals) (Krieger et al., 2010, pp. 560–1) so it can be used to control weeds in arable crops, meadows and ornamental lawns and pitches. In Australia and the United States of America, Atrazine is used for this purpose (Krieger et al., 2010, p. 559) but this chemical has been banned in the European Union due to contamination of ground water.
The most widely used insecticide family is the pyrethrins and pyrethroids (Olkowski, Daar and Olkowski, 1995, p. 90). Pyrethrins are organic compounds derived from the flowers of certain kinds of chrysanthemum. They are concentrated and available to gardeners as sprays, for example, Pyrethrum. Pyrethroids are synthetic chemicals which are created to mimic the pyrethrins. Similarly, potassium fatty acids are used to make insecticidal soap, a contact pesticide which is effective against most flies and nymphs. These chemicals carry a low-risk to humans and are amongst the least controversial. But some pesticides are much more toxic. Neonics are still used in some countries, and will be discussed in the second part of this blog series. Other controversial pesticides are metaldehyde (the main chemical used for slug pellets), which is deadly to all domestic animals especially dogs (Krieger et al., 2010, p. 296), and is often passed up the food-chain to birds and amphibians. Rodenticides are the most poisonous to humans and wildlife. The most used of these are second–generation anticoagulants (which cause internal bleeding after only a single dose). For larger animals, shooting, physical controls like guards for trees and traps are more frequently used than pesticides.
Fungicide applications vary depending on region. The oldest and still amongst the most common worldwide are probably two inorganics; compounds of copper and sulphur (Olkowski, Daar and Olkowski, 1995, p. 77). Among synthetic fungicides, the triazoles are the most widely used on plants and on seeds. These are sometimes considered to have a low toxicity, especially considering that one of the most common ways to apply these fungicides is just to dust seeds before planting. However, some fungicides can be just as toxic as the herbicides and insecticides. Myclobutanil and chlorothalonil are considered toxic to small animals (Krieger et al., 2010, p. 761; 1954-63).
In the present day then, there is a paradox around pesticides. Society is focused on finding pesticides with a low environmental and health impact, preferably those which are organic or break down quickly. However, to be commercially successful, pesticides generally need to be broad-spectrum and fast-acting (i.e. highly toxic), so industry scientists are focused on finding more deadly pesticides. To understand these demands, and to see society’s trend in context, we need to look further back into the history of pest control, which will be the focus of the next post.
List of References
Krieger, R. et al. (eds) (2010) Hayes’ handbook of pesticide toxicology. 3rd edn. London: Elsevier.
Matthews, G. (2016) Pesticides: health, safety and the environment. 2nd edn. Chichester: Wiley-Blackwell.
Olkowski, W., Daar, S. and Olkowski, H. (1995) The Gardener’s Guide to Common-Sense Pest Control. Newtown: Taunton Press.
Pesticide Action Network UK (2017) Pesticide-Free Towns Campaign. Available at: http://www.pan-uk.org/pesticide-free/ (Accessed: 6 November 2018).