The Ecological Impacts of Pesticide Use1
The Risks of Conventional Chemical Pesticides
Overall pesticide usage in the US generally has increased over time. In 1970, the total annual conventional pesticide active ingredient use in the US was approximately 760 million pounds. This number rose to a high of 1,144 million pounds in 1979.2 By 2007, the annual pesticides usage in the US had declined to 857 million pounds, but was still significantly higher than the 1970 number. Pesticide use in the US does not appear to be declining any further. In 1988, the total annual user expenditure on pesticides in the agricultural section was approximately 4.9 million dollars. By 2007, the number had risen to approximately 7.9 million dollars.3 On an amount basis, the agricultural pesticide use was approximately 867 million pounds of active ingredient in 1988 and approximately 877 million pounds in 2007.4
Despite the significant increases in pesticide use in the US during the latter half of the twentieth century, crop damage from pests has actually increased. Between 1945 and 1989, there was a 10-fold increase in insect pesticide use, but crop loss from insect damaged doubled from 7 to 13 percent.5 The development of resistance to pesticides contributes to what appears to be a trend in which, as more pesticides are used, pests actually cause more rather than less crop damage. Studies show that between 1948 and 2001, when US pesticide use increased from approximately 50 million pounds per year to almost 1 billion pounds per year, crop loss due to insect pests actually almost doubled from approximately 7 percent to 13 percent.6 In nature, approximately 1–7 percent of plant biomass is lost to pest damage. When plants are grown in agriculture, the loss from pest damage rises to 30 percent. The dramatically increased loss in the agricultural setting is due to a number of factors including growing crops in monocultures and the effects of breeding for taste, cosmetics, and shipability.7 But, perhaps surprisingly, the large crop loss from pests is in part attributable to the use of pesticides to combat those very pests. A major drawback of broad-spectrum pesticides is that, while the pesticide kills the target pest, it also kills non-target organisms, including predatory and parasitic natural enemies of the pest. In fact, these natural enemies frequently are more sensitive to the pesticides than are the pests themselves. This is due in part to the fact that unlike herbivores, predators have not evolved to be resistant to chemicals found in plants.8 Thus, broad-spectrum pesticides tend to kill higher proportions of predators than pests, resulting in a higher ratio of pest to predator populations.9 Although from an ecological standpoint, narrow-spectrum pesticides are preferable, broad-spectrum synthetic pesticides continue to dominate US pesticide usage.10
Figure 6.1 Unnatural resistance and the development of resistance to insecticides
Note: (a) = Distribution of tolerance before selection; (b) = Distribution of tolerance after selection compared with distribution before selection; x = The dose exerting selection pressure, kills fewer organisms in the population of progeny than in the population of parents; LD50 = Dose lethal to 50 percent of the population. Reprinted with permission. H.F. VAN EMDEN & M.W. SERVICE, PEST AND VECTOR CONTROL 75 (2004).
Figure 6.2 Chronological increase in the number of arthropods (insects and mites), plant pathogen, and weed species resistant to synthetic chemical pesticides since their development in the late 1930s
Reprinted with permission. Gould, F. The Evolutionary Potential of Crop Pests, 79 AM. SCI. 496–507 (1991).
One of the major drawbacks of pesticide use is the tendency of pest populations to develop resistance to the very substances that are intended to control the populations. When a population of pests is exposed to a given pesticide, the pesticide will kill individuals in the population that are susceptible to the particular pesticide, leaving resistant individuals to survive and reproduce. Thus, the next generation of individuals will have a higher proportion of resistant individuals and a lower proportion of susceptible individuals (see Figure 6.1). Another round of pesticide use will have a similar effect, resulting in increasing numbers of susceptible individuals being destroyed before they can reproduce and future populations becoming ever more resistant. This phenomenon creates a treadmill effect, where more and more pesticides are needed to keep up with the ever increasingly resistant pest populations.11 In other words, pesticide efficacy declines as pesticide use increases. When DDT was first employed to combat malaria by killing the disease-carrying mosquito vector, its ultimate failure, despite the initial dramatic success, was due at least in part to the development of resistance by many populations of mosquitoes.12 One of the best-known examples of a pest developing resistance to pesticides is that of the diamondback moth, which has become a much more serious pest post-pesticide use than it ever was pre-pesticide use.13 As of 1990, more than 250 insects were resistant to DDT and organophosphate pesticides.14 Although perhaps counterintuitive, the widespread use of chemical pesticides has not been particularly effective in controlling pests, and in some ways has exacerbated pest problems. Noted pesticide expert David Pimentel estimates that if the US banned all chemical pesticides, we would experience only a 10 percent increase in crop loss.15 Nevertheless, the use of chemical pesticides continues to dominate US agriculture and continues to impose significant ecological risks.
Scientists estimate that as many as 10 million species, or 99 percent of the earth’s wild biodiversity, not including cultivated and weedy species, are in a “precarious condition.”16 Causes and contributors to the decline of this large number of species include indirect habitat destruction through clearing for agriculture and development, the spread of non-native invasive species, pollution, over harvesting of species, and disease. Although there is no doubt that direct habitat destruction is the leading contributor of species loss (estimated as being implicated in 85 of US species decline), pollution, including pesticide pollution, is implicated in 24 percent of US species decline. While the environmental regulation of the past 40 years has resulted in many industries decreasing pollution, agricultural pollution remains high and is considered to be the major source of pollution in many parts of the globe.17 Of course, agricultural pollution is not limited to pesticide pollution and includes many other forms of contamination such as fertilizer runoff, animal waste, and siltation from erosion. Nevertheless, pesticide contamination remains a substantial component of agricultural pollution; and due to the toxic nature of many synthetic pesticides, wildlife, biodiversity, and other environmental impacts from pesticide pollution can be substantial.
Pesticide poisoning of fish and wildlife is a significant factor in species decline. In one study of the decline of fish species in the United States, Canada, and Mexico, it was determined that the destruction of physical habitat was implicated in 73 percent of the declines, the displacement by introduced species was implicated in 68 percent of the declines, the alterations of habitat by chemical pollutants was implicated in 38 percent of the declines, hybridization with other species and subspecies was implicated in 38 percent of the declines, and over harvesting was implicated in 15 percent of the declines. The numbers add up to more than 100 because more than one factor is implicated in many of the fish population declines.18 Thus, while pesticide usage in itself may not directly destroy habitat (although clearing for agriculture certainly does), chemical pesticides may be a significant contributor to species decline; and pesticidal GMOs, which pose risks of spread in the environment similar to non-indigenous species release, may also be important contributors. Habitat destruction, spread of non-natives, pollution, overkill, and disease have been referred to as the five horsemen of the environmental Apocalypse.19
Because pesticides are by definition intended to kill or disrupt living organisms, and because they are intentionally released into the environment, often in large quantities over large areas, it is not surprising that pesticides pose a wide array of risks to individual species as well as to overall ecosystem function. Many pesticides are broad-spectrum, affecting diverse species, including many non-target organisms. Others are more narrowly targeted to pest species. However, even these may have significant impacts on non-target species that are closely related to the intended targets. Some pesticides persist in the environment for weeks, months and even years, while others breakdown relatively quickly. Moreover, living organisms vary significantly in their susceptibility to pesticides. The potential ecological risks of pesticide use depend on a number of factors including toxicity or other hazards of the pesticide, method of application, persistence in the environment, amount used, and susceptibility of non-target organisms. Moreover, there are not many data available on the environmental effects of pesticide usage on many species. Accordingly, the ecological risks of pesticides cannot be easily described or quantified. Nevertheless, some generalizations can be made.
Many pesticides in current use in the US, as well as in other parts of the world, are acutely toxic and are known to cause adverse effects on non-target mammals, birds, reptiles, amphibians, fish, and invertebrates. Birds and other wildlife may be exposed through direct spraying, ingesting pesticide granules, drinking water that has been contaminated by pesticides, or eating prey organisms that have been contaminated by pesticides. While the banning and severe restriction of certain pesticides, such as DDT, over the past 30 years has dramatically reduced certain risks to wildlife, many risks remain. One startling example is that when roughly 10,000 dead birds were tested for the presence of West Nile Virus in 2000, the New York State Department of Environmental Conservation determined that pesticides and other chemicals were responsible for more bird kills than the virus.20 As further evidence of the effects on bird populations, studies have shown substantially higher nesting rates of birds, as well as significantly higher bird abundance and avian species richness, on organic farms as compared to conventional farms that use synthetic pesticides.21 In addition to effects from direct exposure to pesticides, birds and other wildlife may also be exposed to pesticides by ingesting prey animals that have been contaminated. For example, the New York State Department of Environmental Conservation has found a number of different avian species, such as screech owls, red-tailed hawks, American kestrels, and other raptors that have died as a result of eating small rodents that had consumed rat poison.
Other less visible species are also at considerable risk from exposure to pesticides. For example, for the past decade, there has been considerable concern and debate in the scientific community over the worldwide decline of amphibians. There are now significant data to support a conclusion that certain pesticides, such as the herbicide atrazine, may be contributing to the global decline in amphibian populations.22 Until very recently, the results of many studies on the effects of pesticides on amphibians have been puzzling because pesticide levels in nature tend to be much lower than levels found to be lethal in the laboratory setting.23 A recent study sheds new light on this dilemma. Scientists have determined that the combination of the pesticide carbaryl and stress from the presence of predators was more lethal in certain amphibian species than the pesticide by itself. In other words, there appears to be a synergistic effect at work between pesticides and predators, making the combination of the two more lethal than the sum of the parts, and resulting in low concentrations of pesticides in nature being highly lethal to amphibians. Of course, amphibians in nature must cope with other stress, such as the presence of predators, in addition to the stress of pesticides. Accordingly, this study suggests that amphibians in nature may be significantly more sensitive to pesticides than they are in the sterile isolated confines of the research laboratory.24
Although the most obvious adverse effects of pesticide use are those to humans and large animals such as mammals and birds, it is likely that the most significant adverse effects of pesticides are those to invertebrates that are closely related to target pest species. Casualties from this “friendly fire” are widespread in the invertebrate world.25 One example is that the insect Order Lepidoptera contains not only many pest moth species, but also contains many non-pest butterfly species. These butterfly species may be beneficial pollinators and may be aesthetically pleasing, colorful, and interesting species, such as the monarch butterfly. Also, the Order Lepidoptera contains a number of butterfly species that have been listed as threatened or endangered under the federal Endangered Species Act. Pesticides that are used to kill pest moth species generally do not discriminate within the Lepidoptera Order, and will kill non-pest, beneficial butterflies, including endangered species. Mosquito control pesticides have been indicted as one of the threats to the continued survival of the endangered Miami Blue Butterfly over the past few decades. Perhaps equally if not more important than direct acute effects on non-target organisms are the chronic effects upon growth, physiology, reproduction, and behavior. Much less is known about these effects.
Even where a pesticide is not toxic enough to kill an organism, it can have very significant sub-lethal effects on the organism by affecting life span, growth, physiology, behavior, and reproduction. For example, extremely low doses of some pesticides have been determined to disrupt honeybees’ homing flight behavior, thereby adversely affecting pollination.26 Pesticides have been documented to have significant indirect effects on non-target organisms by reducing the populations of animals or plants that serve as food or cover for other species. Moreover, recent studies demonstrate a reduction in the abundance of non-target butterflies on conventional farms as compared to butterflies on organic farms.27 Pesticides have also been implicated in the decline of global pollinator populations.28 Many insects, in addition to bees, play a critical role in pollination in natural ecosystems as well as in agricultural systems.29 These pollinators, which are attracted to flowering crop plants and thus are abundant in certain farm fields at certain times of the year, are vulnerable to insecticide use on these farms. Commercial bees that are brought in to pollinate crops are also vulnerable. Further, pesticides may play a role in “colony collapse disorder,” which is disseminating commercial bees’ colonies.30 Although it is not yet known what the cause of this disorder is, pesticide poisoning, alone or in combine with other environmental pressures, is one of the major suspected causes.31
More generally, insects are critical to the function of virtually all natural ecosystems. As Samways has stated “[t]he ecological grandeur of insects is in their ability as a group to transfer vast amounts of energy. As such, they are determinants of community structure and shapers of habitat.”32 Insects play a “keystone role” in many ecosystems and frequently act as ecosystem engineers and soil modifiers.33 As such, insects can influence the structure and composition of an entire ecosystem. For example, some termites modify hydrology and organic matter in a way that results in increased diversity of tree species, and some unique plants can only exist with the presence of certain ants.34