Environmental Problems, Their Causes, and Sustainability 1

vegetables even though there is nothing wrong withblemished ones. The only solution to this problem isconsumer education. Would you buy blemished fruitsand vegetables?What Are Other Ways to Control Pests?Copy NatureA mix of cultivation practices and biological andecological alternatives to conventional chemicalpesticides can help control pests.Many scientists believe we should greatly increase theuse of biological, ecological, and other alternativemethods for controlling pests and diseases that affectcrops and human health. A number of methods areavailable.One is the use of various cultivation practices tofake out pest species. Examples are rotating the typesof crops planted in a field each year, adjusting plantingtimes so major insect pests either starve or get eaten bytheir natural predators, and growing crops in areaswhere their major pests do not exist. Also, farmers canincrease the use of polyculture, which uses plant diversityto reduce losses to pests.Homeowners can reduce weed invasions by cuttinggrass no lower than 8 centimeters (3 inches) high.This provides a dense enough cover to keep out crabgrassand many other undesirable weeds. Homeownerscan also avoid growing plants such as roses thatattract a number of insect pests and grow plants suchas chrysanthemums and marigolds that repel many insectpests.Genetic engineering can be used to speed up the developmentof pest- and disease-resistant crop strains (Figure23-6). But there is controversy over whether theprojected advantages of the increasing use of geneticallymodified plants and foods outweigh their projecteddisadvantages (Figure 14-19, p. 292).We can increase the use of biological pest control. Itinvolves importing natural predators (Figures 23-1and 23-7), parasites, and disease-causing bacteria andviruses to help regulate pest populations. More than1,000 species have been introduced to help control pestspecies in North America, with generally favorable results.For example, several species of European beetlesare being used in the United States to help reduce thepurple loosestrife plant that has invaded many U.S.wetlands (Figure 13-5, p. 256).Biological control focuses on selected targetspecies, is nontoxic to other species, and minimizes geneticresistance. Also, it can save large amounts ofmoney—about $25 for every $1 invested in controlling70 pests in the United States. However, biologicalagents cannot always be mass produced, are oftenslower acting and more difficult to apply than conventionalpesticides, can sometimes multiply and becomeMonsantoFigure 23-6 The results of one example of using geneticengineering to reduce pest damage. Both tomato plants wereexposed to destructive caterpillars. The normal plant’s leavesare almost gone (left), whereas the genetically altered plant(right) shows little damage.Figure 23-7 Natural capital: biological pest control. An adultconvergent ladybug (right) is consuming an aphid (left).pests themselves, and must be protected from pesticidessprayed in nearby fields.Another strategy is insect birth control. This involvesraising males of insect pest species in the laboratoryand sterilizing them by exposure to radiation orchemicals. The sterile males are released into an infestedarea to mate with fertile wild females who thenlay eggs that never hatch. This method has been usedto control the screwworm fly, a major livestock pestfrom the southeastern United States (Figure 23-8), andthe Mediterranean fruit fly (medfly) during a 1990 outbreakin California. However, problems include highcosts, difficulties in knowing the mating time and behaviorof each target insect, and the large number ofsterile males needed. In addition, there are few speciesfor which this strategy works, and sterile males mustbe released continually to prevent pest resurgence.Sex attractants can also help control pests. Plantsand animals have evolved a variety of natural attractantscalled pheromones. Scientists have identifiedmany of these natural chemicals and use them to lure528 CHAPTER 23 Pest Management