Pesticides and the Environment
Kelly Hammond
History
As early as 1000 BC mankind has attempted to control pests. The Greek poet Homer cited sulfur as an early insecticide.The Chinese used arsenic as an insecticide by 900 AD.By the mid-nineteenth century there began to be success using chemicals such as pyrethrum and Bordeaux mixture (copper sulfate, lime and water.)The modern era of pesticides began with the advent of DDT during WWII.
Types of pesticides
RodenticidesInsecticidesNematicidesFungicidesHerbicidesInorganicpesticidesPiscicide(fish)Insect and plant growth regulators
AlgicidesBactericidesBiocides (microorganisms)LarvicidesMolluscicidesOvicides(eggs)ViricidesDefoliantsRepellents
Soil characteristics that affect the persistence of pesticides
Particle sizePesticidesremain in the soil longer the smaller the particle size of the soil,dueto the greater amount of surface area. Soils high in organic matter andclay (diameter of 0.002 mm or less) havethe most influence on pesticide retention. Once the pesticide hasboundto the soil it may be unavailable to the target organism.Mineral and organic contentSoil pHMicrobiologicalactivityBacteriaand fungi play a large role in the breakdown of chemicals in thesoil.
Factors that affect microbial breakdown of pesticides
TemperaturepHnutrientavailabilityoxygen(presence or absence)Some invertebrates like mites and earthworms can also break down pesticides.Free enzymes from dying animals, plant roots or excrement also play a role in degradation.
Pesticide entry into the soil from intentional application
Direct application to the soil surfaceIncorporation into the upper profiles of the soilApplication on crops
Pesticide entry into the soil from unintentional application
Drift– when pesticides are applied in a spray form, large amounts can move out of the target area. The smaller the droplet size, the more likely drift will occur. Droplets 2 microns in size can drift over 20 miles ona 3 mphwind current.Atmospheric Fallout– pesticides have been found in rain, snow and atmospheric dust
Once pesticides are in the soil there are several processes that can take place
Adsorption to clay and organic matter–Clayand organic matter are negatively charged creating thecationexchange capacity (CEC). This is how soils are able to bind nutrients as well as chemicals. Pesticides can attach to the negatively charged soil by dipole – dipole attraction, hydrogen bonding, or ionic binding if thepesticideis positively charged. The formulation of the pesticide also influences adsorption. Granular formulations are the most persistent, followed byemulsifiableformulations, thenwettablepowders and dust formulations are the least persistent. Soil moisture also effects adsorption. As water is added to thesoil,polar water molecules compete with the pesticide for adsorption sites and force the pesticide into solution. Soil pH can influence adsorption. In acidic soils, most of thecationexchange sites are occupied byprotons.
Once pesticides are in the soil there are several processes that can take place
Leach further into the soil– Themore soluble the pesticide the faster it will leach from the soil. A heavy rain after application can cause excessive leaching, leading to nonpoint pollution as well as reducing the effectiveness of the pesticide.
Once pesticides are in the soil there are several processes that can take place
Volatilization to the atmosphere–Warm, moist soil is more likely to lose pesticides through volatilization than cool, dry soils. The soil type alsoaffectsthe volatilization of pesticides. Soils high in clay and organic matter tend to lose less pesticides from volatilization due to their ability to bind tightly to the pesticide.
Once pesticides are in the soil there are several processes that can take place
Taken in by plants–Whenplants absorb pesticides, they can either be degraded by the plant or removed once the plant is harvested.
Once pesticides are in the soil there are several processes that can take place
Carried away by runoff or soilerosion-Asrunoff water moves over the soil it can cause desorption of pesticides bound to the soil and carry the pesticide away from the target area. Large amounts of pesticides can be lost when soil particles that have pesticides bound to them are carried by wind or water.
Once pesticides are in the soil there are several processes that can take place
Degraded by microbes, chemicals or the sun–Theprimary organisms that play a role in degradation are algae, fungi,actinomycetesand bacteria. When an organic pesticide is added to the soil these organisms will immediately try to use it as an energy source. If they are able to use the pesticide as an energy source their numbers will increase until the pesticide has been degraded. Chemical reactions in the soil can also degrade pesticides. Pesticides can undergo chemical hydrolysis, or be degraded by high or low pH depending on the pesticide. Pesticides that are not incorporated into the soil are susceptible tophotodegradation.
Once pesticide residues are present in the soil they can have several effects.
They may be toxic to plant or animal life in the soilThey may lead to populations of resistant plant speciesThey may change metabolic or reproductive activityThey may be taken in by organisms and passed up the food chain, leading tobiomagnification.
biomagnification
Organochlorineinsecticides have been the most thoroughly studied compound related tobiomagnification. DDT is an example that is able to accumulate in cell membranes and fat stores. Benthic feeders that live in waters contaminated with DDT can store the chemical in fat. When these organisms are fed upon by the next trophic level, the levels of DDT can accumulate even further. By the time it reaches the top of the food chain DDT levels can be up to 100,000 times greater than the amount found in contaminated water supplies. The effects of contamination can even be detrimental to the next generation. The bookSilent SpringbyRachel Carson brought this problem to the general public’s attention. Although the situation is not as grim as she lead people to believe, the accumulation of pesticides moving up the food chain is still of great concern to environmental biologist.
Pesticides in water sources
Water soluble pesticides are transferred to water sources through leaching or from precipitation from the atmosphere.According to the composition of the chemical it can rapidly decompose, volatize, or be incorporated into the bottom sediment. Hydrophobic chemicals are the most likely to accumulate in organisms. They also can accumulate at the surface of water leading to volatilization and degradation from the sun.Micro-organisms also play a role in chemical degradation in water sources.Runoff from agricultural land, containing high levels of nitrate and phosphates, can lead to the cultural eutrophication of bodies of water.
Sources of water pollution
Soils from agriculture, forests or parks that have had application of pesticidesPesticide storage locationsWaste water from pesticide manufacturing plantsWater bodies that have been treated with pesticides to combat insects, trash fish or aquatic weedsWaste water from agricultural and public lands, such as irrigation runoff and snow melt
ONCE PESTICIDES ARE IN AQUATIC ENVIRONMENTS THERE ARE SEVERAL PROCESSES THAT TAKE PLACE
Dilution and abiotic movement– Dilution is the primary way pesticide levels are reduced in bodies of standing water. In moving water, turbulence causesresuspensionof sediment that is contaminated with pesticides, which can cause loss due to volatilization.Volatilization– This is dependent on vapor pressure, temperature, water solubility and adsorption characteristics.Adsorption– Pesticides can adsorb to both biotic and abiotic substances. They can be adsorbed by sediments or aquatic organisms such as plankton, invertebrates, fish or plant life.Absorption by biota– Pesticides can be absorbed by most aquatic microbes as well as higher plants and animals.
ONCE PESTICIDES ARE IN AQUATIC ENVIRONMENTS THERE ARE SEVERAL PROCESSES THAT TAKE PLACE
Biotic movement–Oncethe pesticide has moved into a living organism several things can happen. The pesticide can move within the organism, move betweenorganismsor be moved long distances by an organism.Biomagnificationin food chains–Aspesticides move up the food chain they can reach higher concentrations at each higher trophiclevel.Degradation–Thechemical reactiveness of water aids in the degradation of pesticides. Water can aid in the hydrolysis, reduction, oxidation, decarboxylation, isomerization and elimination of pesticides in the water. Living organisms in the water can also detoxify and degrade pesticides in the water.Photodegradationin water depends on several factors:light quality and quantity, water turbidity, pH, adsorbing surfaces and chemical reactants in the water.
Indirect effects of pesticides in water sources
The use of herbicides to control aquatic vegetation, for example, can affect species composition. Aquatic plants can provide protection from predators, serve as a food source, as well as produce oxygen.
Pesticides in the atmosphere
Pesticides are usually not present in high concentrations in the atmosphere; however they can be transported long distances on wind. The largest contributor is the agriculture industry. Crop dusting, fumigation, manufacturing plants and burning waste containing residues are other sources of pesticides entry into atmosphere.
Pesticide entry into the atmosphere
Entry as particles– Small particles can be transported into the upper troposphere by air currents.Pesticidaldusts, aqueous sprays, fog and smoke formulations can contribute toatmosphereicaccumulation of pesticides.Entry as vapor– Most pesticides that are used lose part of the application as vapor to the atmosphere.Entry as volatilization– Loss of pesticides through volatilization from soil, water, plants or from biotic or abiotic material is responsible for a large percent of the residues in the atmosphere.Entry by wind erosion and water spray– Pesticides that have adsorbed to small soil particles can be blown away in dry conditions, especially after cultivation of the soil. Plant material such as pollen or spores can also adsorb pesticides and be transported into the atmosphere. Water spray from wind and waves can rise into the air and evaporate leaving small particles of pesticide suspended in the air that can be carried into the atmosphere.
resistance
This can be a problem if pesticides with the same mode of action are used over an extended period of time. Although pesticides themselves do not cause genetic changes in organisms, certain organisms in a population may have a genetic mutation that allows them to survive while the rest of the species is wiped out. Without competition for resources the organism that survived can develop into a population that is resistant to the pesticide.
conclusion
Although there is concern about the use of pesticides, they are a valuable tool in combating the growing food needs of the world’s population.There has been a recent movement to try and control the use of pesticides in the environment. Methods such as conservation tillage and low input agriculture reduce the input of residues from soil erosion and the use of sustainable agriculture includes monitoring economic levels of crop pests and avoiding treatment when unnecessary .Integrated pest management (IPM) is another relatively new concept that uses all available control methods, such as cultural and mechanical methods, not just chemicals.
Literature Cited
Carson, R. 1962.Silent Spring. Houghton Mifflin. Boston, MA.Cockerham, L. & Shane, B. 1994.Basic Environmental Toxicology. CRC press. Boca Raton, FL.Duffus, J. 1980.Environmental Toxicology. John Wiley & Sons Inc. New York, NY.Gould, R. 1966.Organic Pesticides in the Environment. American Chemical Society. Washington D.C.Gould, R. 1972.Fate of Organic Pesticides in the Aquatic Environment. American Chemical Society. Washington D.C.Graham, J. & Wiener, J. 1997.Risk versus Risk. Harvard University Press. Cambridge, MA.Hill, I. & Wright, S. 1978.Pesticide Microbiology.Academic Press Inc. New York, NY.Koritz, G.,Ruckebusch, Y. &Toutain, P. 1983.Veterinary Pharmacology and Toxicology. AVI Publishing Company Inc. Westport, CT.McEwen, F. & Stephenson, G. 1979.The Use and Significance of Pesticides in the Environment. John Wiley & Sons Inc. New York, NY.Schnoor, J. 1992.Fate of Pesticides & chemicals in the Environment. John Wiley & Sons Inc. New York, NY.Ware, G &Whitacre, D. 2004.The Pesticide Book.MeisterProInformation Resources. Willoughby, OH.
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