Harmful Effects of Pesticides on Non-target Organisms

By: Md. Wasim Aktar

Pesticides are found as common contaminants in soil, air, and water, and on non-target vegetation in our urban landscapes. Once there, they can harm plants and animals ranging from beneficial soil microorganisms and insects, non-target plants, fish, birds, and other wildlife. Table 1 summarizes the harmful effects to non-target organisms of some commonly used urban herbicides.

Table 1 . Harmful effects of selected herbicides on non-target organisms

Source: Sanders, 1969

When pesticides contaminate water they can be harmful to the fish that live there. Insecticides can be particularly toxic to fish. Chlorpyrifos, a common contaminant of urban streams,18 is very highly toxic to fish, and has caused fishkills in waterways near treated fields or buildings.1,2 Diazinon, also commonly found in urban streams,18 is acutely toxic to many species of fish, including salmon.3 Herbicides can also be toxic to fish. According to the EPA, studies show that trifluralin, an active ingredient in the weed-killer Snapshot, "is highly to very highly toxic to both cold and warmwater fish."4 It was also shown in a series of different tests to cause vertebral deformities in fish.5 Oryzalin, the active ingredient of Surflan, also is 'highly toxic' to fish.6 The weed-killers Ronstar and Roundup are also acutely toxic to fish.7,8 The toxicity of Roundup is likely due to the high toxicity of one of the inert ingredients of the product. In addition to direct acute toxicity, some herbicides may produce sublethal effects on fish that lessen their chances for survival and threaten the population as a whole. Glyphosate or glyphosate-containing products can cause sublethal effects such as erratic swimming and labored breathing which increase the fish's chance of being eaten.9,10 2,4-D herbicides caused physiological stress responses in sockeye salmon,11 and reduced the food gathering abilities of rainbow trout.12

Other Aquatic Animals and Plants
In addition to fish, other marine or freshwater animals are endangered by pesticide contamination. 2,4- D or 2,4-D containing products have been shown to be harmful to newts,13 frogs,14 crabs,15 shellfish,16 and other aquatic species.17,18 The weed-killer trifluralin is moderately to highly toxic to aquatic invertebrates, and highly toxic to estuarine and marine organisms like shrimp and mussels. Diuron is also highly toxic to aquatic invertebrates.19 Since herbicides are designed to kill plants, it makes sense that herbicide contamination of water could have devastating effects on aquatic plants. In one study, oxadiazon was found to severely reduce algae growth.20 Algae is a staple organism in the food chain of aquatic ecosystems. Studies looking at the impacts of the herbicides atrazine and alachlor on algae and diatoms in streams showed that even at fairly low levels, the chemicals damaged cells, blocked photosynthesis, and stunted growth in varying ways.20 Another important class of organisms is the cyanobacteria. Cyanobacteria live in aquatic environments as well as soil, and play a crucial role in nitrogen fixation, helping plants convert atmospheric nitrogen into nitrate compounds that the plant can use. Trifluralin was found to inhibit the growth of two common cyanobacteria at all levels of application.21

Insects and Spiders
In addition to killing insect "pests," insecticides obviously have the potential to harm non-target insects such as beneficial natural predators and pollinators. Less obviously, weed-killers can also be harmful to beneficial insects. One study found that exposure to freshly dried Roundup (glyphosate) killed over 50 percent of three species of beneficial insects: a parasitoid wasp, a lacewing and a ladybug. Over 80 percent of a fourth species, a predatory beetle was killed."22 Moderate doses of the herbicide 2,4-D severely impaired honeybee brood production. 23 The herbicide oxadiazon is also toxic to bees, which are pollinators.24 Herbicides may hurt insects or spiders indirectly as well, such as when they destroy the foliage that these animals need for food and shelter. For example, spider and carabid beetle populations declined when 2,4-D applications destroyed their natural habitat.25
The insecticide diazinon is notorious for causing bird kills. Over 50 incidents involving the deaths of up to 1000 birds have been documented in every region of the U.S.95 Diazinon is so lethal to birds that the EPA estimates that between 15 and 80 minutes of grazing on diazinon treated turf is enough to kill a bird.26 Non-target birds may also be killed if they ingest poisoned grains set out as bait for pigeons and rodents.27, 28 Avitrol, a commonly used pigeon bait, poses a large potential for ingestion by non target grain feeding birds. It can be lethal to small seed-eating birds.29 rodifacoum, a common rodenticide, is highly toxic to birds. It also poses a secondary poisoning hazard to birds that may feed on poisoned rodents.30 Herbicides can also be toxic to birds. lthough trifluralin was considered "practically nontoxic to birds" in studies of acute toxicity, birds exposed multiple times to the herbicide experienced diminished reproductive success in the form of cracked eggs. Exposure of eggs to 2,4-D reduced successful hatching of chicken eggs,33 and caused feminization or sterility in pheasant chicks.31 Herbicides can also adversely effect birds by destroying their habitat. Glyphosate treatment in clear cuts caused dramatic decreases in the populations of birds that had lived there.32

Beneficial Soil Microorganisms
One spoonful of healthy soil has millions of tiny organisms including fungi, bacteria, and a host of others. These microorganisms play a key role in helping plants utilize soil nutrients needed to grow and thrive. Microorganisms also help soil store water and nutrients, regulate water flow, and filter pollutants.34 The heavy treatment of soil with pesticides can cause populations of beneficial soil microorganisms to decline. According to soil scientist Dr. Elaine Ingham, "If we lose both bacteria and fungi, then the soil degrades. Overuse of chemical fertilizers and pesticides have effects on the soil organisms that are similar to human overuse of antibiotics. Indiscriminate use of chemicals might work for a few years, but after awhile, there aren't enough beneficial soil organisms to hold onto the nutrients."35 For example, plants depend on a variety of soil microorganisms to transform atmospheric nitrogen into nitrates that plants can use. Common landscape herbicides disrupt this process: triclopyr inhibits soil bacteria that transform ammonia into nitrite36; glyphosate reduces the growth and activity of both free-living nitrogen-fixing bacteria in soil37 and those that live in nodules on plant roots38; and 2,4-D reduces nitrogen fixation by the bacteria that live on the roots of bean plants,39,40 reduces the growth and activity of nitrogen-fixing blue-green algae, 41,42 and inhibits the transformation by soil bacteria of ammonia into nitrates.43,44 Mycorrhizal fungi grow with the roots of many plants and aid in nutrient uptake. These fungi can also be damaged by herbicides in the soil. One study found that oryzalin and trifluralin both inhibited the growth of certain species of mycorrhizal fungi.45 Roundup has been shown to be toxic to mycorrhizal fungi in laboratory studies, and some damaging effects were seen at concentrations lower than those found in soil following typical applications. 46,47 Triclopyr was also found to be toxic to several species of mycorrhizal fungi,48 and oxadiazon reduced the number of mycorrhizal fungal spores.49

Pesticides have contaminated almost every part of our environment. Pesticide residues are found in soil and air, and in surface and ground water across the nation, and urban pesticide uses contribute to the problem. Pesticide contamination poses significant risks to the environment and non-target organisms ranging from beneficial soil microorganisms, to insects, plants, fish, and birds. Contrary to common misconceptions, even herbicides can cause harm to the environment. In fact, weed killers can be especially problematic because they are used in relatively large volumes. The best way to reduce pesticide contamination (and the harm it causes) in our environment is for all of us to do our part to use safer, non-chemical pest control (including weed control) methods.


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