(1) Human health effects aren't a focus in this course, but there are about 20,000 - 67,000 pesticide poisoning cases per year in the US (the number depends very much on who you read). Many of these are occupational, and many also involve careless homeowners and children; only about 27 per year are fatal. These cases of poisoning are relatively easy to track to pesticdes as causal -- these are considered "acute" cases, involving (usually) short term exposure to high doses.

More controversial are health effects resulting from chronic exposure to lower levels of pesticides. Chronic exposure is caused by use of contaminated drinking water, pesticide residues on crops, and so forth -- these are long-term and faily low-level explsures, in general. Chronic exposure may cause afflictions including cancers; damage to immune, endocrine, and reproductive systems; and damage to the nervous system. In cases of chronic exposure, it is more difficult to establish clear cause and effect. However, about 6000 cases of cancers per year are suspected to be pesticide related, according to the EPA's worst case estimate. Increasing amounts of data suggest links between pesticide exposure and lowered IQ's in children whohad prenatal exposure to certain pesticides (organophosphates and others) [see seires of papers in Environmental Health Perspectives from 2011], between pesticide exposure and Parkinson's disease, and other health problems, although researchers urge caution in leaping to conclusions about these relationships. In some cases, exposures are occupational, but in others arise in areas adjacent to sites of pesticide use, from the air or soil or via contaminated drinking water.  For example, the CA Department of Pesticide Regulation tested water from 136 wells for pesticide contamination, and found pesticides or pesticide residues in 103 of these (Sierra Jan/Feb 2012).  As is so often the case, however, the onus is on those who think that pesticides are causing problems to prove that is the case, rather than the onus being on the pesticide manufacturers to proved that the pesticides are not harmful. (Recall our discussion of the "precuationary principle" earlier in this unit on pesticides?) 

A 1995 study of the Midwestern US found herbicides in tap water in 28 of 29 cities tested, and in more than half of them, herbicide levels exceeded governmental safety standards. The US Geological Survey found two or more pesticides in groundwater at about half of the sites sampled as part of the National Water Quality Assessment between 1993 and 1995, and over 2/3 of the samples from the Central Columbia Plateau aquifer (in WA and ID) contained multiple pesticides. We have very little understanding of the possible consequences when pesticides (or their metabolites) mix, nor do we know much about potential consequences when this water is pumped up to be used for irrigation.

Coming closer to home in western OR, we see that the "accusing finger" should be pointed at urban and suburban landowners as well as at agricultural operations. A recent USGS (US Geological Survey) study found unhealthy levels of commonly used home and yard pesticides in Beaverton Creek, Pringle Creek (Salem, OR), the Clackamas River and siveral tributaried of that river. Concentrations were high enough that they were considered unhealthy for fish and other aquatic life. Similar data may well be uncovered for other urban areas in OR, but surveys haven't been completed. (Oregon Env'l Council, 1/05).

Foods that we eat are probably a main pathway for pesticide exposure [Environmental Health Perspecties Jan '08]. Surveys detected metabolites of malathion (an insecticide) and other organophosphorus pesticides in the urine of greater-Seattle area children, for example, which are suspected to have come from pesticide treated produce. Epidemiological studies suggest that such exposures may be associated with elevated rishks of various cancers, and to noncancer illnesses of the nervous, renal, respiratory, reproductive and endocrine systems.

Some pesticides are suspected of being "hormone mimics" or endocrine disruptors. For example, some seem to have estrogen-like effects on human male sperm. Pesticide metabolites were analyzed in the urine of 400 midwestern men, only two of which had occupational exposure to pesticides, in conjunction with assessment of these men's sperm quality. The study found a strong correlation between exposure to atrazine, diazinon or alachlor and poor sperm quality. Drinking water was suspected as the source of these pesticides [World Watch Nov/Dec '03]. Similar associations between atrazine exposure and reproductive anomalies in frogs have been suggested, but not always supported. (See further treatment of this topic lower on this page.)

For many years, the USDA's National Agricultural Statistics Service supported a program (the Agricultural Chemical Useage Program) that not only tracked use of particular pesticide compounds (and fertilizers and use of alternative pest control methods) by crop and state each year, but that also tested crops -- fruits and vegetables -- for pesticide levels. Data were used by EPA to carry out risk assessments and set safe levels for pesticides. Unfortunately, however, UDSA announced in 2008 that it was discontinuing this program, claiming that it was too expensive (~ $ 8 million per year). While a private company does test pesticides in agricultural crops, its results are proprietary and very costly to obtain (for anyone who wants to use them) and there are also concerns about the accuracy and precision of this company's findings (Union of Concerned Scientists). SO, we are left with no readily accessible or reliable tracking of use of pesticides at the Federal level, nor of their levels in our agricultural crops!! (Note that residue testing on a fraction of imported crops is still carried out -- this is conducted by the USDA Agricultural Marketing Service through their Pesticide Data Program, which was not affected by the cut in 2008 [M.Miller, USDA, pers. comm.])

Globally, WHO (the World Health Organization) estimated that, as of 1992, pesticides caused health problems in about 1 million people per year, with 4,000-19,000 deaths. Other estimates range from 3 to 25 million persons being affected: good numbers in this area are nearly impossible to come by!

The hazards of pesticides to human health are not equitably distributed around the world. For example, Latin American farm workers are 13 times more likely to suffer pesticide poisoning than US farm workers, as they often handle unlabeled substances, or substances bearing labels written in a language that they cannot read, while wearing little or no protective equipment. In general, the per capita rate of human injury associated with pesticides is far greater in developing countries than in the US.

(2) Pesticides are energy intensive in their manufacturing and application, hence there is reason to be concerned about their use from the perspective of energy conservation and global CO2 emissions.

(3) Pesticides are expensive for the growers and thus to the consumers (but so are the alternatives, as we'll see when we discuss alternative agriculture.)


Pesticides differ from one another in their toxicity and in their persistence in the environment. Of most concern are those that are:

BI390000.gif most toxic to the widest variety of organisms -- the most broad spectrum – as these have the most potential for damage to nontarget organisms, and

BI390000.gif those that are most persistent , as they linger longer in the environment and hence have more potential to do harm. In fact, in 2000, OR Governor Kitzhaber signed an executive order directing OR's Department of Environmental Quality to reach zero discharge of all persistent poisons by 2020 (not just pesticides). Some persistent compounds (and their residues) including some pesticides are known to disrupt hormone systems, even at low levels. Sometimes these are referred to as "gender benders" as they are hormone mimics, and can disrupt various aspects of sexuality. For example, studies of wildlife in the field and aboratory animals show that prolonged, low-level exposure to certain persistent compounds (or their residues) can result in problems such as deformed genitals, aberrant mating behavior, etc. These kinds of problems were found in male otters in the lower Columbia River, for example, and are believed to be attributable to chronic exposure to persistent toxins.

Concerns about ecological effects of pesticides come about in part because much of the pesticide applied does not land in the targeted agroecosystem. For example, about 65% of all pesticides applied in the US are applied via aircraft and only an average of 25 - 50% of the pesticide aerially applied lands in the target area; that is, as much as 50% drifts out of the target area. (Helicopters are more efficient than this; these figures are for airplane applications.)

Beyond these inefficiencies in reaching the target area, there are also inefficiencies in reaching the target organisms; we saw previously that only about 1% of pesticide applied actually hits the target creatures.

In addition, persistent pesticides can move out of the treated areas via soil, water, organisms, or the atmosphere, and can then influence nontarget organisms in adjoining ecosystems -- or far away, as DDT residues in Eskimos and seals has illustrated.

Salmon and steelhead in the Pacific Northwest are considered to be put at risk by 37 different pesticides. This is, at least in part, because many pesticides that make their way into water are toxic to the stream invertebrates on which these fish feed. In response, OR state officials are attempting to rapidly set benchmarks for maximum concentrations for seven of the priority compounds. While EPA is charged with setting water quality standards as part of its pesticide registration process, in practice there is often a big time lag (even 20 - 30 years!) between the time a compound is released onto the market and the setting of final in-stream standards. For example, the final in-stream standard for the insecticide diazinon wasn't set until it had already been banned for household use because of risks to humans, birds, and fish! So, OR officials are hoping to move faster than EPA...

Thus, the ecosystem problems related to pesticide use that we'll discuss are not limited to the targeted agroecosystem because of pesticide drift and movement of persistent material. Effects occur in both the target and nontarget ecosystems, such as the streams, lakes and other ecosystems into which spray drifts or into which it runs with runoff.

Some troubles with amphibians may be related to pesticides

You've probably heard about the mysterious and widespread decline in amphbians that is taking place in various regions of the world? When we talked about fertilizers, we mentioned that some amphibians seem to be adversely affected by nitrogenous fertilizers in the water, for example, and, when we talk about depletion of stratospheric ozone later this term, we'll see a suggestion that some amphibians are sensitive to increased levels of uv-b radiation.

Well, there are undoubtedly many causes of the various declines, and, in 2002, it was reported (Science, Vol 296) that some frog species are very sensitive to Atrazine, the most heavily used herbicide in the US. This compound has been banned in many European countries, and is being reviewed by the EPA here in the US. EPS's safe drinking water standard for this compound is 3 ppb (parts per billion), yet, in lab studies, frogs exposed to concentrations as much as 30 times lower than that (i.e., 0.1 ppb) show disruptions in their hormone systems. Male tadpoes, for example, develop extra gonads and become hermaphrodites. Concentrations equal to those used in the laboratory studies are observed in waters in the wild; it is common in the central US for waters to have concentrations of Atrazine between 1 and 10 ppb, with peaks of 100 - 200 ppb being reported. In addition, air borne atrazine, entrapped in rain, can cause concentrations in rain to be > 1 ppb. This chemical is usually applied in the spring, and, with spring runoff, ponds, ditches and streams near fields, where the frogs breed and the tadpoles develop, rceive Atrazine. Field collections in areas of high Atrazine use show a higher number of frogs with endocrine damage than do collections from areas devoid of the compound. It isn't clear yet that these problems affect reproduction and the future of populations, but it is hard to imagine that they won't! However, more recent reports cast the Atrazine link into question; effects occur where Atrazine isn't detected, and some lakes where Atrazine concentrations are elevated do not host damagted amphibians. It now seems that at least some of the cases of amphibian deformity in the western US are linked instead to increasing populations of a snail that harbors a trematode parasite that goes on to infect frogs and cause damages (and a fungus is also increasingly implicated...).

The story continues, however: It appears that the commonly used herbicide "Roundup," when applied according to the label instructions, can be very lethal to tadpoles and juvenile terrestrial frogs and toads, based on experiments in which ponds were sprayed using various concentrations of this herbicide. (Ecol. Appl. 2005). Some suggest that it may not be the herbicide itself that is so problematic, but rather the surfactant (compound mixed with the active ingredient, and that allows it to spread and make contact with surfaces more readily) that causes problems for amphibians. There is also the possibility that observed effects were not direct (as in causing toxicity) but were indirect, e.g., via causing decreases in algae (food).

And, the story continues further! Recall the trematode parasite mentioned just above? A study tested effects of four commonly used pesticides (including Atrazine and glyphosate ["Roundup"] applied at ecologically-relevant concentrations on both the trematode and on green frog tadpoles. The chemicals had little to no effect on the trematodes, but sublethal exposures of tadpoles increased their susceptibility to infection by the trematodes -- so there appears to be a link there (Ecol. App. 2008 1743-1753).

In some cases, amphibians may be affected indirectly by pesticides in yet another way -- application of the commonly used insecticide, Malathion, to a wetland mesocosm (mini ecosystem, basically) found that application at realistic doses decreased populations of zooplankton. With their populations diminished, populations of phytoplankton (floating microscopic algae that are eaten by zooplankton) boomed and caused a decrease in populations of periphyton (algae growing on rocks). Leopard frog tadpoles graze on periphyton, and, with less food available, they grew and developed more slowly, and hence were more vulnerable to death as the environment dried over the season. (Wood frogs, which develop more quickly, were not affected.) That is, the insecticide set off a "trophic cascade" whose effects rippled through the food chain and led, indirectly, to increased mortality of leopard frogs (Ecol. App. 2008 1728-1742).

Thus, pesticides can affect species diversity (the number of species in an area), food chains (the pathways by which energy and nutrients move from plants to other organisms in the ecosystem), and, potentially many other aspects of ecosystems. Organisms in ecosystems exist in complex interdependent associations and so losses of species (reductions in diversity) because of pesticides (or other causes!) can have far reaching and unpredictable effects.

Some examples of effects follow:

(1) Effects on diversity acting through food chains. (See also example of a trophic cascade a couple of paragraphs above.)

In a generic sense, such effects would work like this: Insecticides kill insects on which populations of grassland mice and voles depend, so that the mice and vole populations decrease; in turn, populations of organisms that depend on mice and voles (such as hawks, foxes) decrease.

A recent dramatic example involves Swainson's hawks and the Argentine pampas. Biologists in the US and Canada had been noticing mysterious and dramatic declines in Swainson's hawk populations on their breeding grounds in the Canadian prairie provinces and in several western US states, but didn't know why the declines were occurring. Then in the winter of 1995, having followed some radio-collared birds on their southward migration for the winter, several scientists found 4,000 dead birds at just four sites about 280 miles west of Buenos Aires, with a conservative estimate that 20,000 birds – about 5% of the world's Swainson's hawk population – perished in that single season. Forensic, anecdotal and physical evidence pointed to the pesticide monocrotophos as the primary killer.

This pesticide is used in Argentina, and in some other countries as well, to control grasshoppers and other sucking, chewing and boring insects. (It is no longer used in the US since DuPont [who used to manufacture it] voluntarily withdrew it from the US market because of its acute toxicity – both direct and indirect – to wildlife.) While no longer sold in the US, it is widely available from an international company with subsidiaries in the US and Argentina. After a long battle, and the death of untold thousands of birds, the Argentine government has now prohibited use of this pesticide on alfalfa and sunflowers. (Enforcement of the ban is left up to the provincial governments, which are strapped for resources, and there's a lot of already-purchased monocrotophos already sitting on the shelves...) Biologists who have worked on the problem urge, however, that the farmers themselves not be blamed: the problem results from an international marketing system that supplies farmers with inappropriate chemicals and with insufficient information about them to allow them to make the best choices.

Recent studies suggest that salmon in rivers of the Pacific Northwest may be injured by pesticides not onlythrough direct toxicity, but also through indirect, "bottomup" food chain effects; certain pesticides are toxic to aquatic macroinvertebrates or to the aquatic primary producers (plants and algae) that are food for some of these macroinvertebrates; macroinvertebrates, in turn, are important food sources ofr salmon.

(2) Effects involving pollinators . Some pollinators, such as honeybees, are very sensitive to pesticides and are important pollinators of both crops and native plants. Reduced seed and fruit production in plants that depend on these pollinators can result, with consequences for both native and agricultural plant production.

(3) Effects on nutrient cycling in ecosystems -- agroecosystems or others. A large proportion of pesticide used ultimately reaches the soil, where, as we saw before , soil building processes and the cycling of nutrients back into living plants is accomplished. Pesticides can affect the soil organisms involved in these processes directly (earthworms, for example, are sensitive to many pesticides) or indirectly (e.g., by reducing the organisms on which they feed or changing the chemistry of the leaf litter on which they depend).

Thus, rates of decomposition and mineralization can be reduced, resulting in reductions in soil fertility. In agroecosystems, this can aggrevate the need to apply inorganic chemical fertilizers!

(4) Effects on erosion, soil structure and fertility. Herbicides can reduce vegetative cover of the ground, promoting soil erosion via runoff and wind. Agricultural runoff of sediments (and associated fertilizers or pesticides) is a major problem in many surface waters , contributing to eutrophication and contamination of drinking water. Because soil loss involves loss of soil nutrients, enhanced erosion aggravates the need to apply fertilizers. In addition, frequent use of herbicides can reduce soil organic matter content because fewer plants grow in treated areas, lessening organic matter inputs.

Decreases in organic matter and increases in runoff and erosion, coupled with effects of pesticides on soil organisms can degrade soil structure with adverse effects on soil aeration, nutrient status, and water holding capacity or water percolation.

Finally, a study in the September 2001 issue of the journal "Nature" reports that certain pesticides disrupt the process of nitrogen-fixation in legumes. Pesticides demonstrated to have this effect, to date, are thoe classified as "oestrogenic compounds;" compounds that disrupt the endocrine system of animals by mimicing natural hormones. (DDT and methyl parathion are examples.) Apparently, the chemicals alter the signaling that occurs between the host plant and the symbiotic bacteria in its roots, decreasing nitrogen fixation by as much as 90%....

(5) Effects on water quality. Pesticides also enter water, either from the air or by runoff or percolation to groundwater, where they can have harmful effects on human health, and can cause disruptions of the aquatic ecosystems, including loss of diversity, disruption of stabilizing species interactions, disruptions of food chains, and so forth

IN SUMMARY , pesticides can change almost all aspects of both natural and managed ecosystems, such as agroecosystems:

The next section (">>") examines briefly the question of whether pesticides address root causes of pest outbreaks, or rather are "bandaids" on a symptom of imbalanced systems.

Page maintained by Patricia Muir at Oregon State University; last updated Oct. 24, 2012.

contents.gif prev1.gif next1.gif