4. Discussion
The 20 pesticides encountered in this study include at
least six RUPs (according to EPA classification) and
contain 18 different active ingredients, nine of which are
‘highly hazardous’ or ‘moderately hazardous’ (according
to WHO classification). Yet all pesticides are freely
available in stores or markets and applied by smallholder
farmers, particularly those with a higher income and
good (road) access to local and regional markets. Farmers
apply pesticides without adequate personal protection
and in an unsafe manner, moreover, even if they have
sufficient income to take protective measures, being a
matter of lower priority compared to expenses for child
education for example. In addition, poor labourers also
are hired for pesticide application without providing
them with the necessary protective clothing. Shirt and
trousers, for example, are frequently worn for extensive
periods of time after being contaminated that seems
almost inconceivable as most farmers are aware of the
negative effects of pesticides on their health. One reason
for not wearing protective clothing, even if it were
available, is that the tropical climate makes the wearing
of full protective gear impractical because of the
potential for body heat stress, as reported by Ohayo-
Mitoko (1997). Farmers are generally less aware of the
potentially adverse impacts of pesticide deposition on the any such impacts.
The rates at which insecticides are applied to both rice
and corn fields are lower than for herbicides, even when
taking into consideration that, in contrast to herbicides, the
insecticides are generally applied more than once. This
tendency to apply less insecticides and more herbicides is in
line with the situation at the national level (with
insecticides accounting for 29 percent of total pesticide
use in the Philippines; IBON, 2000), as well as globally, but
is counter to the assumed trend of higher insecticide usage
by most developing countries, where insects usually cause
the greatest problems (Ecobichon, 2001). This comment is
not to deny the observation that insecticides are more toxic
to humans and their environment than herbicides. Pesticide
usage is also related to other factors, such as manpower
availability and level of mechanization. For example,
insecticides are generally used far less widely than
herbicides in countries where there has been a shift from
manual labour to machine-intensive agricultural practices,
as in Europe and North America (Ecobichon, 2001). In
countries like Vietnam, on the other hand, where there is
an abundance of relatively cheap labour, most weeding is
still done by hand and herbicide usage is consequently
limited, with 80% of volume pesticide consumption
accounted for by insecticides (Tenenbaum, 1996). It is
therefore surprising that the low-income smallholders in
the present study focus more than expected on herbicides,
even though they work under conditions more similar to
those in Vietnam than in Europe or North America. In the
case of rice, the high expenditure on herbicides can in
theory be explained in terms of a shift in methods of rice
cultivation, viz., from transplanting to direct seeding,
which magnifies the weed problem, as reported by Karim
et al. (2004). Johnson (1996) cites yield losses due to
uncontrolled weed growth ranging from 45 to 75 percent
for direct-seeded lowland rice and about 50 percent for
transplanted lowland rice. The smallholder lowland farmers
in this study, who are highly dependent on herbicides,
still use the transplanting method. A more likely explanation
may be a shortage of labour, as the population, and
youth in particular, are increasingly moving out of
agriculture and turning to the industry and service sectors
for employment (NEDA, 2006). One farmer mentioned
that it was cheaper to buy a bottle of herbicide to treat his
field (one bottle of butachlor, tradename Machete:
US$8.82) than to hire people to weed it (cost: 10 workers
for more than 1 day at US$1.37 per person per day). More
research is needed to confirm these trends.
The spray simulation experiments showed that at 0.5m
from the boundary of the spayed plot pesticide drift is
about 12% on average, falling to a minimum of about 2%
of the within-field dosage between 1.5 and 2.0m from
the sprayed field. These levels are much higher than the
assumed levels of drift along sprayed fields found in the
literature. For example, emission measurements with
knapsack sprayers (field margin spraying) in the Netherlands
using water-sensitive paper strips have shown that at
low wind speeds (2.5–3.5ms1) approximately 2.7% of the
dosage per unit area drifts to a distance of 0.6m (ditch
bank) and less than 0.01% into the ditch (about 1.5–2m
from the field margin) (de Snoo and de Wit, 1993; de Snoo,
1999). In England, it is assumed that there is no longer any
significant drift beyond 1m from fields treated with a
knapsack sprayer (EPPO, 1993a, b). In our experiments,
the relatively high temperatures might have caused the
much greater range of drift found (up to 2 m). Higher
temperatures can cause mid-air evaporation, thus reducing
droplet size. Small droplets stay suspended in the air for
longer than larger ones and can therefore travel further
away from the target area. During the majority of the
experiments, moreover, the wind speed was zero, while it is
recommended to spray pesticides at wind speeds of
between 0.5 and 1.8ms1 (1.2–4 miles h1; Pesticides
Safety Directorate, 2006), when the potential for drift is
lowest. Conditions of temperature inversion generally
occur under light winds of 0.5ms1 or less, because of
which winds are unpredictable and variable in direction,
thus making it harder to control the drift path. The
variability of the wind may have caused some of the
variation among measurements. At the same time, though,
the farmers’ mode of spraying is also a contributing factor.
Farmers were found to spray in different ways, some
moving the spraying lance horizontally in front of them in
a semi-circular motion and others trying to make a full
3601 circular movement around them, all the while walking
forward in a straight line. Several farmers spray with the
spraying lance pointed downwards, whereas others spray
upwards.
The results with regard to deposition on farmers’ bodies
demonstrate that the legs are the most seriously exposed
during spraying activities. The high exposure of the legs is
most likely the result of a combination of factors, including
a leaking knapsack sprayer, direct unintentional deposition
from the sprayer, spray drift and secondary pick-up from
treated crops, the latter being more pronounced for legs
than for hands and arms (measurements affected by dew or
raindrops on crops were excluded from the analysis of this
study). Another possible explanation is that farmers may
accidentally spray directly onto their legs, as it is difficult to
coordinate the circular movement made with the spraying
rod with the forward motion of the legs.
Considering the exposure risk per insecticide application,
all the farmers in all four villages are likely to be at risk.
The application rates for lambda cyhalothrin and cypermethrin
(with 12 and 13 g a.i. ha1; Table 5) result in MOE
values clearly below 100. Taking into account the amount
of pesticide concentrate sprayed per year, moreover, the
levels to which farmers in the lowland village Magassi are
exposed within one year are at least five times higher than
those for farmers in the other upland villages. Within this
context, it is also important to distinguish between the
application methods for rice and corn. Although insecticide
application rates for rice are considerably lower than those for corn, suggesting a lower risk of exposure, rice is sprayed
more times per cropping season, particularly in the
lowlands. The period elapsing between two consecutive
applications is crucial: the shorter this period, the higher the
risk of active ingredients and residue compounds not being
removed from the farmer’s body and the environment
before the next application takes place. Hence, sufficient
time should be allowed (at least a week or more, depending
on the type of insecticide) before applying the next dosage.
In the case of corn, a lower application rate is also
advisable. The same is true for herbicide application rates:
although the active ingredients are less toxic than lambda
cyhalothrin and cypermethrin and are only applied once per
cropping season, the rates at which they are applied are
high and, in the case of 2,4-D ester and niclosamide (and
perhaps butachlor and pretilachlor, for which key data are
still lacking), easily exceed the no adverse effect levels for
humans (i.e., are below target MOE levels).
Moreover, current pesticide application rates are harmful
to the environment.Aquatic species in watercourses
along corn and rice fields are adversely affected by drift of
lambda cyhalothrin and cypermethrin up to at least 2m
from field borders. For bees, in contrast, no adverse effects
are expected at current pesticide application rates, as
hazard quotients are far below the critical level of 50.
Although the effects of pesticide usage on livestock (and
other mammals) are presumably far more limited, based on
present application rates, various farmers reported that
their carabao (domesticated subspecies of water buffalo,
Bubalus bubalis) suffered from diarrhoea after feeding on
pesticide-sprayed grasses and had peelings coming off their
hooves after passing through a field treated with the
isopropylamine salt of glyphosate and glyphosate acid
(tradename: power).
4. Discussion
The 20 pesticides encountered in this study include at
least six RUPs (according to EPA classification) and
contain 18 different active ingredients, nine of which are
‘highly hazardous’ or ‘moderately hazardous’ (according
to WHO classification). Yet all pesticides are freely
available in stores or markets and applied by smallholder
farmers, particularly those with a higher income and
good (road) access to local and regional markets. Farmers
apply pesticides without adequate personal protection
and in an unsafe manner, moreover, even if they have
sufficient income to take protective measures, being a
matter of lower priority compared to expenses for child
education for example. In addition, poor labourers also
are hired for pesticide application without providing
them with the necessary protective clothing. Shirt and
trousers, for example, are frequently worn for extensive
periods of time after being contaminated that seems
almost inconceivable as most farmers are aware of the
negative effects of pesticides on their health. One reason
for not wearing protective clothing, even if it were
available, is that the tropical climate makes the wearing
of full protective gear impractical because of the
potential for body heat stress, as reported by Ohayo-
Mitoko (1997). Farmers are generally less aware of the
potentially adverse impacts of pesticide deposition on the any such impacts.
The rates at which insecticides are applied to both rice
and corn fields are lower than for herbicides, even when
taking into consideration that, in contrast to herbicides, the
insecticides are generally applied more than once. This
tendency to apply less insecticides and more herbicides is in
line with the situation at the national level (with
insecticides accounting for 29 percent of total pesticide
use in the Philippines; IBON, 2000), as well as globally, but
is counter to the assumed trend of higher insecticide usage
by most developing countries, where insects usually cause
the greatest problems (Ecobichon, 2001). This comment is
not to deny the observation that insecticides are more toxic
to humans and their environment than herbicides. Pesticide
usage is also related to other factors, such as manpower
availability and level of mechanization. For example,
insecticides are generally used far less widely than
herbicides in countries where there has been a shift from
manual labour to machine-intensive agricultural practices,
as in Europe and North America (Ecobichon, 2001). In
countries like Vietnam, on the other hand, where there is
an abundance of relatively cheap labour, most weeding is
still done by hand and herbicide usage is consequently
limited, with 80% of volume pesticide consumption
accounted for by insecticides (Tenenbaum, 1996). It is
therefore surprising that the low-income smallholders in
the present study focus more than expected on herbicides,
even though they work under conditions more similar to
those in Vietnam than in Europe or North America. In the
case of rice, the high expenditure on herbicides can in
theory be explained in terms of a shift in methods of rice
cultivation, viz., from transplanting to direct seeding,
which magnifies the weed problem, as reported by Karim
et al. (2004). Johnson (1996) cites yield losses due to
uncontrolled weed growth ranging from 45 to 75 percent
for direct-seeded lowland rice and about 50 percent for
transplanted lowland rice. The smallholder lowland farmers
in this study, who are highly dependent on herbicides,
still use the transplanting method. A more likely explanation
may be a shortage of labour, as the population, and
youth in particular, are increasingly moving out of
agriculture and turning to the industry and service sectors
for employment (NEDA, 2006). One farmer mentioned
that it was cheaper to buy a bottle of herbicide to treat his
field (one bottle of butachlor, tradename Machete:
US$8.82) than to hire people to weed it (cost: 10 workers
for more than 1 day at US$1.37 per person per day). More
research is needed to confirm these trends.
The spray simulation experiments showed that at 0.5m
from the boundary of the spayed plot pesticide drift is
about 12% on average, falling to a minimum of about 2%
of the within-field dosage between 1.5 and 2.0m from
the sprayed field. These levels are much higher than the
assumed levels of drift along sprayed fields found in the
literature. For example, emission measurements with
knapsack sprayers (field margin spraying) in the Netherlands
using water-sensitive paper strips have shown that at
low wind speeds (2.5–3.5ms1) approximately 2.7% of the
dosage per unit area drifts to a distance of 0.6m (ditch
bank) and less than 0.01% into the ditch (about 1.5–2m
from the field margin) (de Snoo and de Wit, 1993; de Snoo,
1999). In England, it is assumed that there is no longer any
significant drift beyond 1m from fields treated with a
knapsack sprayer (EPPO, 1993a, b). In our experiments,
the relatively high temperatures might have caused the
much greater range of drift found (up to 2 m). Higher
temperatures can cause mid-air evaporation, thus reducing
droplet size. Small droplets stay suspended in the air for
longer than larger ones and can therefore travel further
away from the target area. During the majority of the
experiments, moreover, the wind speed was zero, while it is
recommended to spray pesticides at wind speeds of
between 0.5 and 1.8ms1 (1.2–4 miles h1; Pesticides
Safety Directorate, 2006), when the potential for drift is
lowest. Conditions of temperature inversion generally
occur under light winds of 0.5ms1 or less, because of
which winds are unpredictable and variable in direction,
thus making it harder to control the drift path. The
variability of the wind may have caused some of the
variation among measurements. At the same time, though,
the farmers’ mode of spraying is also a contributing factor.
Farmers were found to spray in different ways, some
moving the spraying lance horizontally in front of them in
a semi-circular motion and others trying to make a full
3601 circular movement around them, all the while walking
forward in a straight line. Several farmers spray with the
spraying lance pointed downwards, whereas others spray
upwards.
The results with regard to deposition on farmers’ bodies
demonstrate that the legs are the most seriously exposed
during spraying activities. The high exposure of the legs is
most likely the result of a combination of factors, including
a leaking knapsack sprayer, direct unintentional deposition
from the sprayer, spray drift and secondary pick-up from
treated crops, the latter being more pronounced for legs
than for hands and arms (measurements affected by dew or
raindrops on crops were excluded from the analysis of this
study). Another possible explanation is that farmers may
accidentally spray directly onto their legs, as it is difficult to
coordinate the circular movement made with the spraying
rod with the forward motion of the legs.
Considering the exposure risk per insecticide application,
all the farmers in all four villages are likely to be at risk.
The application rates for lambda cyhalothrin and cypermethrin
(with 12 and 13 g a.i. ha1; Table 5) result in MOE
values clearly below 100. Taking into account the amount
of pesticide concentrate sprayed per year, moreover, the
levels to which farmers in the lowland village Magassi are
exposed within one year are at least five times higher than
those for farmers in the other upland villages. Within this
context, it is also important to distinguish between the
application methods for rice and corn. Although insecticide
application rates for rice are considerably lower than those for corn, suggesting a lower risk of exposure, rice is sprayed
more times per cropping season, particularly in the
lowlands. The period elapsing between two consecutive
applications is crucial: the shorter this period, the higher the
risk of active ingredients and residue compounds not being
removed from the farmer’s body and the environment
before the next application takes place. Hence, sufficient
time should be allowed (at least a week or more, depending
on the type of insecticide) before applying the next dosage.
In the case of corn, a lower application rate is also
advisable. The same is true for herbicide application rates:
although the active ingredients are less toxic than lambda
cyhalothrin and cypermethrin and are only applied once per
cropping season, the rates at which they are applied are
high and, in the case of 2,4-D ester and niclosamide (and
perhaps butachlor and pretilachlor, for which key data are
still lacking), easily exceed the no adverse effect levels for
humans (i.e., are below target MOE levels).
Moreover, current pesticide application rates are harmful
to the environment.Aquatic species in watercourses
along corn and rice fields are adversely affected by drift of
lambda cyhalothrin and cypermethrin up to at least 2m
from field borders. For bees, in contrast, no adverse effects
are expected at current pesticide application rates, as
hazard quotients are far below the critical level of 50.
Although the effects of pesticide usage on livestock (and
other mammals) are presumably far more limited, based on
present application rates, various farmers reported that
their carabao (domesticated subspecies of water buffalo,
Bubalus bubalis) suffered from diarrhoea after feeding on
pesticide-sprayed grasses and had peelings coming off their
hooves after passing through a field treated with the
isopropylamine salt of glyphosate and glyphosate acid
(tradename: power).
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