TECHNICAL
Their success was immediate. They were cheap, effective in small quantities, easy to apply, and widely toxic. They seemed to be truly ‘miracle insecticides’
”
switched from coffee to tea production; and the invasion from the Americas of an insect, the grape phylloxera, (Viteus vitifoliae), which almost destroyed the wine industry in France (1848-1878). Not surprisingly, over this period there was a surge of interest in developing pest-control techniques and, by the turn of the 19th century, there were five main approaches to the control of pests which were well established and in common use: (i) biological control; (ii) chemical control (inorganic chemicals, especially sulphur and copper compounds, or plant-derived alkaloids, especially nicotine); (iii) mechanical and physical control (e.g. tree-banding with sticky substances); (iv) cultural and sanitation controls (like crop rotation); and (v) the use of resistant varieties. The first forty years of the 20th century was a period of steady progress in pest control, in which all of these five approaches played an important part. Pest control was revolutionised, however (especially insect pest control), by the Second World War. Driven by the need to control insect vectors of human disease in the tropics, hundreds of manufactured chemicals were screened for insecticidal properties. In the USA, the breakthrough came with dichlorodiphenyltrichloroethane (DDT), manufactured in Switzerland, followed by other chlorinated hydrocarbons. In Germany, another equally toxic group of compounds was developed, the
organophosphates, whilst a third group of synthetic organic insecticides, the carbamates, was also discovered in the 1940s, by Swiss workers. The initial targets of the organic insecticides were the vectors of human disease, but after the war there was a rapid expansion into agriculture. ‘Their success was immediate. They were cheap, effective in small quantities, easy to apply, and widely toxic. They seemed
to be truly ‘miracle insecticides’ (Flint and van den Bosch, 1981). During the 1930s and 1940s too, the first organic selective herbicides were being developed, originally as a result of work on chemicals to regulate plant growth: 2-methyl-4- chlorophenoxyacetic acid (MCPA), 2,4-dichlorophenoxyacetic acid (2,4- D) and 4,6 dinitro-o-cresol (DNOC). The immediate impact was on cereal crops and, to a much lesser extent, on peas and grassland. An expansion into a much wider range occurred from the 1960s onwards (Lockhart et al. 1990).
The period from 1946 onwards has been described as the ‘Age of Pesticides’, divided by Metcalf (1980) into three phases: the Era of Optimism (1946-1962), the Era of Doubt (1962-1976) and the Era of Integrated Pest Management (IPM) (1976-). Whilst it would be easy to quibble with Metcalf’s dating and terminology, this division captures the changes in the scientific climate over this period. In the heady days of the 1940s and the early 1950s, it was widely believed that generous doses of simple organics could entirely eradicate pests and the problems they caused.
Doubts that had been voiced by entomologists Strickland (1945) and Wiggleworth (1945) were decades ahead of the publication of Rachel Carson’s Silent Spring in 1962, which challenged the notion that chemicals brought benefits but negligible risks. These doubts grew steadily until the XVth International Congress of Entomology in 1976 firmly rejected the widespread use of broad- spectrum and persistent pesticides in favour of an IPM approach.
The problems with chemical pesticides
Widespread toxicity
Chemical pesticides are generally intended for particular pests at a particular site, nevertheless, problems arise because they are
Figure 1. Displaying a timeline, drawn to scale, showing the history of the use of pesticides
128 PC February/March 2019
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