Chemical Pesticides

The mode of action of pesticides is extremely fascinating because the subject covers so many fields of biology and chemistry and has many practical implications. All disciplines of biology have developed greatly since 1,1,1-trichloro- di-(4-chlorophenyl)ethane — better known as DDT — and the other synthetic pesticides were introduced just after the Second World War. At that time, the knowledge of the normal biochemical and physiological processes in organisms was not sufficiently clarified to make it possible for us to understand properly either the mode of action of the pesticides at the target site or their uptake, distribution, and degradation in the ambient environment. The development of resistance of various pests to pesticides should have been possible to predict at that time, even before the use of these pesticides had expanded so much, but how rapidly or to what degree resistance would develop and what biochemical mechanisms where behind the development had to be a matter of experience and research. We now know how nerve impulses are transmitted, how plants synthesize amino acids, and how fungi invade plant tissue. The textbooks in the various biological disciplines have become enormous, but in spite of this, they do not tell us where and why pesticides interfere with the normal processes. Other toxicants are mentioned occasionally, but only when they have been tools for the exploration of the normal processes. The intention of this book is therefore to try to collate some of the knowledge in the respective biological sciences and to explain the points at which the pesticides have an effect. While reading this book students are encouraged to consult textbooks in biochemistry, nerve physiology, plant biochemistry, and so forth, in order to get a more comprehensive explanation of the normal processes disturbed by pesticides. To understand the toxicology of pesticides, it is first necessary to learn organic chemistry, biochemistry, almost all disciplines of plant and animal physiology at the cellular or organismic level, and ecology, as well as the applied sciences within agriculture. This is, of course, impossible, but these disciplines will for many students be much 2 Chemical pesticides: Mode of action and toxicology more interesting when put into the context of an applied science, e.g., pesticide science. At the least, myself and many of my students have become motivated to go back to learn more of organic chemistry and the biological sciences when confronted with the pesticides or other groups of toxicants, curious about why they are toxic or not for different organisms.