Environmental Problems, Their Causes, and Sustainability 1

A weed is a plant whose virtues have not yet been discovered.RALPH WALDO EMERSONThis chapter addresses the following questions:■■■■What are pesticides, and what types are used?What are the advantages and disadvantages of usingchemicals to kill insects and weeds?How well is pesticide use regulated in the UnitedStates?What are the alternatives to using conventionalpesticides, and what are the advantages and disadvantagesof each alternative?23-1 PESTICIDES: TYPES AND USESHow Does Nature Keep Pest Populationsunder Control? Natural EnemiesPredators, parasites, and disease organismsfound in nature control populations of most pestspecies as part of the earth’s free ecologicalservices.A pest is any species that competes with us for food, invadeslawns and gardens, destroys wood in houses,spreads disease, invades ecosystems, or is simply anuisance. Worldwide, only about 100 species of plants(which we call weeds), animals (mostly insects), fungi,and microbes (which can infect crop plants and livestockanimals) cause about 90% of the damage to thecrops we grow.In natural ecosystems and polyculture agroecosystems,natural enemies (predators, parasites, anddisease organisms) control the populations of about98% of the potential pest species as part of the earth’sfree ecological services and thus help keep any onespecies from taking over for very long.When we clear forests and grasslands, plant monoculturecrops (Figure 6-25, p. 118) and douse fields withpesticides, we upset many of these natural populationchecks and balances. Then we must devise ways to protectour monoculture crops, tree plantations, and lawnsfrom insects and other pests that nature once controlledat no charge.What Are Pesticides? Ways to Repel or KillPestsWe use chemicals to repel or kill pest organismsas plants have done for millions of years to defendthemselves against hungry herbivores.To help control pest organisms, we have developed avariety of pesticides or biocides—chemicals to kill orcontrol populations of organisms we consider undesirable.Common types of pesticides include insecticides(chemicals that kill insects by blocking reproduction,clogging their airways, or disrupting their nervoussystem), herbicides (chemicals that kill weeds by disruptingtheir metabolism and growth), fungicides (funguskillers), and rodenticides (rat and mouse killers).Biocide is a more accurate name for these chemicalsbecause most pesticides kill other organisms as well astheir pest targets.We did not invent the use of chemicals to repel orkill other species; plants have been producing chemicalsto ward off, deceive, or poison herbivores thatfeed on them for about 225 million years. This is anever-ending, ever-changing coevolutionary process:Herbivores overcome various plant defenses throughnatural selection; then new plant defenses are favoredby natural selection in this ongoing cycle of evolutionarypunch and counterpunch.As the human population grew and agriculturespread, people began looking for ways to protect theircrops, mostly by using chemicals to kill or repel insectpests. Sulfur was used as an insecticide well before500 B.C.; by the 1400s, people were applying toxic compoundsof arsenic, lead, and mercury to crops as insecticides.Farmers abandoned this approach in the late1920s when the increasing number of human poisoningsand fatalities prompted a search for less toxic substitutes.Bad news. Traces of these nondegradable toxicmetal compounds are still found in soils dosed withthem long ago.In the 1600s, farmers used nicotine sulfate, extractedfrom tobacco leaves, as an insecticide. In themid-1800s, two more natural pesticides were introduced:pyrethrum (obtained from the heads of chrysanthemumflowers) and rotenone (extracted from theroots of various tropical forest legumes). These firstgenerationpesticides were mainly natural chemicals orbotanicals borrowed from plants that had been defendingthemselves against insects eating them and herbivoresgrazing on them. In other words, we learned tocopy nature.In addition to protecting crops, people have usedchemicals produced by plants to repel or kill insects intheir households, yards, and gardens. Compared withsome commercial insecticides, these chemicals can beless expensive and less of a potential health hazard.See the website for this chapter to find out about naturalchemicals and methods that can be used to controlweeds and to repel or kill common pests such as ants,mosquitoes, cockroaches, flies, and fleas.What Is the Second Generation of Pesticides?Chemistry and Natural Plants to the RescueChemists have developed hundreds of chemicalsthat can kill or repel pests, and they have improvednatural pesticides produced by plants.A major pest control revolution began in 1939, whenentomologist Paul Müller discovered that DDThttp://biology.brookscole.com/miller14519