http://panna.igc.org/resources/pestis/PESTIS.1996.129.html
Glyphosate, Part 2: Human Exposure and Ecological Effects
by Caroline Cox
OVERVIEW
Residues of the commonly-used herbicide glyphosate have been found in a variety
of fruits and vegetables. Residues can be detected long after glyphosate
treatments have been made.
Lettuce, carrots, and barley planted a year after glyphosate treatment contained
residues at harvest.
In California, where reporting of pesticide-caused illnesses is more
comprehensive than in other states, glyphosate exposure was the third most
commonly-reported cause of pesticide illness among agricultural workers. For
landscape maintenance workers, glyphosate ranked highest.
Glyphosate can drift away from the site of its application. Maximum drift
distance of 400 to 800 meters (1300-2600 feet) have been measured.
Glyphosate residues in soil have persisted over a year.
Although not expected for an herbicide, glyphosate exposure damages or reduces
the population of many animals, including beneficial insects, fish, birds, and
earthworms. In some cases glyphosate is directly toxic; for example,
concentrations as low as 10 parts per million can kill fish and 1/20 of
typical application rates caused delayed development in earthworms. In other
cases, (small mammals and birds, for example) glyphosate reduces populations by
damaging the vegetation that provides food and shelter for the animals.
Glyphosate reduces the activity of nitrogen-fixing bacteria. These bacteria
transform nitrogen, an essential plant nutrient, into a form that plants can
use. Glyphosate reduces the growth of mycorrhizal fungi, beneficial fungi that
help plants absorb water and nutrients. Glyphosate also increases the
susceptibility of plants to diseases, including Rhizoctonia root rot, take-all
disease, and anthracnose.
-------------------------------------------------------
Glyphosate is a widely-used, broad-spectrum herbicide that is used to kill
unwanted plants in a wide variety of agricultural, lawn and garden, aquatic, and
forestry situations. It ranks among the top ten herbicides used in the U.S.,
both in agricultural and nonagricultural situations. Common brand names are
Roundup, Rodeo, Accord, and Vision. This is the second part of a summary of
glyphosate's hazards. Part 1 (JPR 15(3):14-20) discussed the toxicology of
glyphosate, its breakdown products, and the other ingredients in glyphosate-containing
products. This part discusses human exposure to glyphosate and its
ecological effects.
Human Exposure
The most important ways that people are exposed to glyphosate are through
workplace exposure (for people who use glyphosate products on the job), eating
of contaminated food, exposure caused by off-target movement following application (drift), contact with
contaminated soil, and drinking or bathing in contaminated water. The next five
sections of this factsheet summarize information about these five routes of
exposure. The third section, discussing drift, also covers impacts on plants.
Contamination of Food
Analysis of glyphosate residues is "in general laborious, complex, and
costly."1 For this reason, it is not included in
government monitoring of pesticide residues in food.1 The only
information available about contamination of food comes from research
situations. Such studies demonstrate several important points:
* First, glyphosate can be taken up by plants and moved to parts of the plant
that are used for food. For example, glyphosate has been found in strawberries,2
wild blueberries and raspberries,3 lettuce, carrots, barley,4
and fish5,6 following treatment.
* Second, pre-harvest use of glyphosate on wheat (to dry out the grain prior to
harvest) results in "significant residues in the grain,"1
according to the World Health Organization. Bran contains between 2 and 4 times
the amount on whole grains. Residues are not lost during baking.1
* Third, glyphosate residues can be found in food long after treatments have
been made. For example, lettuce, carrots, and barley contained glyphosate
residues at harvest when planted a year after treatment.4
Occupational Exposure
Workers in a variety of occupations are exposed to glyphosate. Researchers have
documented exposure for forestry workers in Finland7 and the
southeastern U.S., palm plantation workers in Malaysia1 and
conifer nursery workers in Mississippi and Oregon.8 All of these
studies generally found low, but consistent, exposure rates.
Physicians, however, paint a different picture. In California, the state with
the most comprehensive program for reporting of pesticide-caused illness,
glyphosate was the third most commonly reported cause of pesticide illness among
agricultural workers.9 Among landscape maintenance workers,
glyphosate was the most commonly reported cause.10 (Both these
statistics come from reviews of illness reports collected between 1984 and
1990.) Even when glyphosate's extensive use in California is considered, and the
illness statistics presented as "number of acute illnesses reported per
million pounds used in California," glyphosate ranked twelfth.9
Drift
In general, movement of a pesticide through unwanted drift is
"unavoidable."11 Drift of glyphosate is no
exception. Glyphosate drift, however, is a particularly significant problem. Its
wide use means that there is a correspondingly large potential for drift.12
When drift does occur, "damage is likely to be much more extensive and more
persistent than with many other herbicides."13 This is
because glyphosate translocates (moves) within plants readily so that even
unexposed parts of a plant can be damaged. Damage to perennial plants (when not
exposed to enough glyphosate to kill them) is persistent, with some symptoms
lasting several years.13 In addition, plant
susceptibility varies widely. Some wildflowers are almost a hundred times more
sensitive than others; small amounts of drift will damage these species.14
A fundamental question about drift is "How far can I expect glyphosate to
travel off-site?" Unfortunately, the question is difficult to answer, since
drift is "notoriously variable."15 Factors that
increase drift are aerial application techniques, high wind speeds (over 10
kilometers, or 6 miles, per hour), spray nozzles that produce a high proportion
of fine droplets, and calm conditions (without enough turbulence to drive the
glyphosate droplets onto plant foliage).15 Drift distances
that have been measured for the major application techniques include the
following:
* Ground Applications: Between 14 and 78 percent of glyphosate applied as ground
sprays moves off-site.15 Seedling mortality has been
demonstrated 20 meters (66 feet) downwind when using a tractor-mounted sprayer.
Sensitive species were killed at 40 meters (131 feet).16
Models indicate that even more sensitive species would be killed at distances
approaching 100 meters (328 feet).14 Glyphosate residues have
been measured 400 meters (1312 feet) downwind from ground applications.17
* Helicopter applications: Between 41 and 82 percent of glyphosate applied from
helicopters moves off the target site.15 Two studies done in
Canada18,19 measured glyphosate residues 200 meters (656 feet)
from target areas following helicopter applications to forest sites. In both
studies, 200 meters was the farthest distance at which samples were taken, so
the longest distance glyphosate travelled is not known.18,19 A
third study (from California) found glyphosate 800 meters (2624 feet) downwind
following a helicopter application. Again, this was the farthest distance at
which measurements were made. Plant injury was recorded 400 meters (1312 feet)
downwind.17
Fixed-wing aircraft: Long drift distances occur following applications of
glyphosate made from fixed-wing airplanes. Three studies on forested sites
conducted by Agriculture Canada (the Canadian agricultural ministry) showed that
glyphosate was consistently found at the farthest distance from the target areas
that measurements were made (200, 300, and 400 meters, or 656, 984, and
1312 feet).20-22 A California study found glyphosate 800
meters downwind of an
airplane application. Again, this was the farthest distance at which
measurements were made. Plant injury was observed at 100 meters (328 feet).
Unlike the first three studies, this study used a grass field as the test site.17
One of the Canadian studies22 calculated that buffer zones of
between 75 and 1200 meters (246 feet - 0.75 miles) would be required to protect
nontarget vegetation.
Soil Contamination
Persistence: Glyphosate's persistence in soil varies widely, so giving a simple
answer to the question "How long does glyphosate persist in soil?" is
not possible. Half-lives (the time required for half of the amount of glyphosate
applied to break down or move away) as low as 3 days and as long as 141 days
have been measured by glyphosate's manufacturer.4 Initial
degradation (breakdown) is faster than the subsequent degradation of what
remains, resulting in long persistence.23 Long persistence has
been measured in the following studies: 55 days on an Oregon Coast Range
forestry site24; 249 days on Finnish agricultural soils25;
between 259 and 296 days on eight Finnish forestry sites23; 335 days on an Ontario
(Canada) forestry site26; 360 days on 3 British Columbia
forestry sites27; and, from 1 to 3 years on eleven
Swedish forestry sites.28 These are minimum estimates because,
in all but two of these studies, glyphosate was detected on the last date
samples were analyzed.
Glyphosate is thought to be "readily bound to many soils and clay
minerals"1 and therefore "immobile or slightly
immobile in many soils."1 This means that the
glyphosate will be unlikely to move away from the application site and
contaminate water or soil elsewhere. However, a new study29
paints a different picture. The researchers found that glyphosate bound readily
to the four soils studied. However, desorption, when glyphosate unbinds from
soil particles, also occurred readily. In one soil, 80 percent of the added
glyphosate desorbed in a two hour period. The study concludes that "this
herbicide can be extensively mobile in the soil environment.."29
Water Contamination
Based on the prevailing view that glyphosate binds readily to soil particles, it
does not have the chemical characteristics of a pesticide that is likely to
leach into either ground or surface water.1 (If it readily
desorbs, as described above, this picture would change.) In either case,
glyphosate can move into surface water when the soil particles to which it is bound are washed
into streams or rivers.4 How often this happens is not known,
because routine monitoring for glyphosate in water is infrequent.1
However, glyphosate has been found in both ground and surface water. Examples
include two farm ponds in Ontario, Canada, contaminated by run-off from an
agricultural treatment (one pond) and a spill (the other pond)30;
the run-off from a watersheds treated with Roundup during production of no-till
corn and fescue31; contaminated surface water in the
Netherlands1; and seven U.S. wells (one in Texas, six in
Virginia) contaminated with glyphosate.32
Glyphosate's persistence in water is shorter than its persistence in soils. Two
Canadian studies found glyphosate persisted 12 to 60 days in pond water
following direct application.33,34 Glyphosate persists longer
in sediments. For example, a study of Accord applied to forest ponds found
glyphosate residues in sediment 400 days after application.1 The
half-life in pond sediments in a Missouri study was 120 days; persistence was
over a year in pond sediments in Michigan and Oregon.4
Ecological Effects
Glyphosate can impact many organisms not intended as targets of the
herbicide. The next two sections describe both direct mortality and indirect
effects, through destruction of food or shelter.
Effects on Nontarget Animals
Beneficial insects: Glyphosate-containing products pose hazards to insects that
are economically beneficial because they kill pest insects. The
International Organization for Biological Control found that exposure to freshly
dried Roundup killed over 50 percent of three species of beneficial insects: a parasitoid wasp, a lacewing, and a ladybug.35 Over
80 percent of a fourth species, a predatory beetle, was killed.
Similar impacts on beneficial insects have been shown in field studies. In North
Carolina winter wheat fields, populations of large carabid beetles declined
after treatment with a commercial glyphosate product and did not recover for 28
days.36 A study of Roundup treatment of pasture hedgerows in
the United Kingdom showed a similar decline in carabid beetles.37
Roundup treatment of a Maine clear-cut caused an 89 percent decline in the
number of herbivorous (plant-eating) insects. While these are not usually
considered beneficial insects, they serve as an important food resource for
birds and insect-eating small mammals.38
Aquatic insects can also be affected by glyphosate. Midge larvae (important food
for breeding waterfowl39) are killed by glyphosate in amounts
that vary widely. For example, one study found that 55 parts per million (ppm)
of glyphosate killed midge larvae6 while other studies found that 65040 -560039
ppm of Rodeo (containing glyphosate and water) were required to kill the larvae.
Part of the variability is related to water hardness.39
The U.S. Fish and Wildlife Service has identified one endangered species of
insect, a longhorn beetle, that would be jeopardized by use of glyphosate.41
Other arthropods: Glyphosate and glyphosate-containing products kill a variety
of other arthropods. For example, over 50 percent of test populations of a
predatory mite that is an important predator of pest mites was killed by
exposure to Roundup.35 In another laboratory study, Roundup
exposure caused a decrease in survival and a decrease in body weight of woodlice. These arthropods are important in humus production and soil
aeration.42 Roundup treatment of pasture hedgerows reduced the
number of spiders, probably by killing the plants they preferred for
web-spinning.37 The water flea Daphnia pulex is killed by
concentrations of Roundup between 3 and 25 ppm.6,43,44 Young
Daphnia are more susceptible than mature individuals, and suspended sediments in
the water increased the toxicity.43 The red swamp crawfish, a
commercial species, was killed by 47 ppm of Roundup.45
Fish: Both glyphosate and the commercial products that contain glyphosate are
acutely toxic to fish. In general, glyphosate alone is less toxic than the
common glyphosate product, Roundup, and other glyphosate products have
intermediate toxicity. Part of these differences in toxicity to fish can be
explained by the toxicity of the surfactant (detergent-like ingredient) in
Roundup. It is about 30 times more toxic to fish than glyphosate itself.44
Acute toxicities of glyphosate vary widely: median lethal concentrations (LC50s;
the concentrations killing 50 percent of a population of test animals) from 10
ppm to over 1000 ppm have been reported depending on the species of fish and
test conditions.1 In soft water there is little difference
between the toxicities of glyphosate and Roundup.
Acute toxicities of Roundup to fish range from an LC50 of 3.2 ppm to an LC50 of
52 ppm.1 Acute toxicities of Rodeo (used with the surfactant
X-77 per label recommendations) vary from 120 to 290 ppm.46
Factors important in determining the toxicity of glyphosate or glyphosate-containing
products to fish include the following:
* First, different species of fish have different susceptibilities. For example,
coho and chinook salmon are more tolerant of glyphosate than pink or chum
salmon.47
* Water quality is important: glyphosate in soft water was 20 times more toxic
to rainbow trout than was glyphosate in hard water. For Roundup, the reverse is
true: it is more toxic in hard water than in soft.47,48
* Age affects the susceptibility of fish because juveniles are often more
susceptible than adults. For example, Roundup was four times more toxic to
rainbow trout fry and fingerlings than it was to larger fish.6
* Nutrition also can determine toxicity. Hungry fish are more susceptible to
glyphosate than fed fish. For example, fed flagfish were 10 times more tolerant
of glyphosate than unfed fish.49
* Finally, glyphosate toxicity increases with increased water temperature. In
both rainbow trout and bluegills, toxicity about doubled between 7 and 17!C (45
and 63!F).6 Treatment of riparian areas with glyphosate causes
water temperatures to increase for several years following treatment50
because the herbicide kills shading vegetation. This means that repeated use of
glyphosate in a watershed could favor its increased toxicity to fish. In
addition, the temperature increase itself could be critical for fish, like
juvenile salmon, that are sensitive to water temperature.
Sublethal effects of glyphosate on fish are also significant and occur at low
concentrations. Studies of rainbow trout and Tilapia found that concentrations
of about 1/2 and 1/3 of the LC50 (respectively) caused erratic swimming.51,52
The trout also exhibited labored breathing.51 Behavioral
effects can increase the risk that the fish will be eaten, as well as affecting
feeding, migration, and reproduction.52
Birds: Glyphosate is acutely toxic to birds, but only in large amounts. The
LC50, the amount in food that kills 50 percent of a population of test animals,
is often above 4000 milligrams per kilogram of food.1
Glyphosate also has indirect impacts on birds. Because glyphosate kills plants,
its use creates a dramatic change in the structure of the plant community. This
affects bird populations, since the birds depend on the plants for food,
shelter, and nest support.
For example, a study of four glyphosate-treated clear-cuts (and an unsprayed
control plot) in Nova Scotia found that the densities of the two most common
species of birds (white-throated sparrow and common yellowthroat) decreased for
two years after glyphosate treatment. By the fourth year post-spray, densities
had returned to normal for these two species. However, the unsprayed plot had by
then been colonized by new species of birds (warblers, vireos, and a
hummingbird). These species did not appear on the sprayed plots.53
An earlier three year study of songbird abundance following glyphosate treatment
of clear-cuts in Maine forests showed similar results. Abundances of the total
number of birds (Figure 2) and three common species decreased. The decrease in
bird abundance was correlated with decrease in the diversity of the habitat.54
Black grouse avoided glyphosate-treated clear-cuts in Norway for several years
after treatment.55 Researchers recommended that the herbicide
not be used near grouse courtship areas.
Small mammals: In field studies, small mammals have also been indirectly
affected when glyphosate kills the vegetation they (or their prey) use for food
or shelter. This was first shown in studies of clear-cuts in Maine.38
Insect-eating shrews declined for three years post-treatment; plant-eating voles
declined for two. A second study in Maine56 found similar
results for voles, but not shrews. A British Columbia study found that deer mice
populations were dramatically (83 percent) lower following glyphosate treatment.57
While some other studies have found no affect on mice, this may have occurred
because treated areas were small.1 This suggests that effects
are more severe when large areas are treated.
In Norway, there was a "strong reduction" in use of sprayed clear-cuts
by mountain hare.58
Earthworms: A study of the most common earthworm found in agricultural soils in
New Zealand showed that glyphosate significantly affects growth and survival of
earthworms. Repeated biweekly applications of low rates of glyphosate (1/20 of
typical rates) caused a reduction in growth, an increase in the time to maturity, and an increase in mortality.59
Effects on Nontarget Plants
As a broad-spectrum herbicide, glyphosate has potent acutely toxic effects on
most plant species. However, there are other kinds of serious effects. These
include effects on endangered species, reduction in the ability to fix nitrogen,
increased susceptibility to plant diseases, and reduction in the activity of mycorrhizal fungi.
Endangered species: Because essentially all plants are susceptible to glyphosate-caused
damage or mortality, glyphosate can seriously impact endangered plant species.
The U.S. Fish and Wildlife Service has identified 74 endangered plant species
that it believes could be jeopardized by use of glyphosate. This list is based on the use of glyphosate on 9 crops, and does not
include over 50 other uses.41
Nitrogen fixation: Nitrogen is important because of its "near
omnipresence" in membranes, proteins, and genetic material of living
things. Most living things cannot use nitrogen in its common form and instead
use ammonia and nitrates, much rarer compounds. The processes by which ammonia
and nitrates are created are called nitrogen fixation and nitrification. They
are carried out by certain bacteria.60
A number of studies (from Iowa,61 Australia,62 eastern Canada,63
and Ontario (Canada)64,65) have shown that commercial
glyphosate products can reduce nitrogen-fixing or nitrification activity of
soils. The amount of glyphosate that produces inhibitory effects varies
from 262 to 200063 ppm. Effects can be persistent; the formation of nitrogen-fixing nodules on
clover roots was inhibited 120 days after treatment. 62
In addition, tests of cultured nitrogen-fixing bacteria have also shown that
glyphosate inhibits nitrogen-fixation. These studies included the
nitrogen-fixing species in roots of soybeans66 and clover.67-68
Given the importance of nitrogen-fixation to agriculture, more research is
crucial.
Mycorrhizal fungi: Mycorrhizal fungi are beneficial fungi that live in and
around plant roots. They help plants absorb nutrients and water and can
protect them from cold and drought.69 Glyphosate is toxic to
many species of mycorrhizal fungi. Effects, mostly growth inhibition, have been
observed at concentrations between 1 and 100 ppm.70-73
Plant diseases: Glyphosate treatment increases the susceptibility of crop plants
to a number of diseases. For example, glyphosate reduced the ability of bean
plants to defend themselves against the disease anthracnose.74
Glyphosate increased the growth of take-all disease in soil from a wheat field.
In addition, the proportion of soil fungi which was antagonistic to the take-all
fungus decreased.75 Bean seedlings also survived glyphosate
treatment when grown on sterile soil, but not when grown on normal (not
sterilized) soil.76 Spraying of Roundup prior to planting
barley increased the severity of Rhizoctonia root rot and decreased barley
yield.77 In addition, Roundup injection of lodgepole pine
inhibited the defensive response of the tree to blue stain fungus.78
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49. Holdway, D.A. and D.G. Dixon. 1988. Acute toxicity of permethrin or
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========================================================
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