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The effects of herbicides on soil life
W
hilst the use of effective herbicides may provide a
feasible option for enhancing weed control, it can be-
come a toxicological risk to invertebrates such as
earthworms. It is unavoidable that herbicides will affect
earthworms because they are an important component of the soil
system that occurs at different soil depths where they interact with
plants and other organisms.
Not only can herbicides cause immediate death in some instances,
but it can also influence the function, growth, reproduction and
overall health of earthworms.
Therefore, it is important to be
aware of the possible effects that
any chemical may have on earth-
worms before applying it to the
soil or on crops.
In the fourth part of our series of
articles, we focus on international
research findings regarding the
impact of herbicides on earth-
worms. Information presented
here represents only a fraction of
published results, however.
Why earthworms
matter
Earthworms are important com-
ponents in temperate ecosystems
where they influence nutrient
cycling and overall ecosystem
functioning. They are, further-
more, considered as ecosystem
engineers because they increase
overall soil fertility by shredding
and redistributing organic material in soil and increase soil penetra-
bility for plant roots.
Because of this role that they play in soil formation and maintenance
of soil structure and fertility, earthworms are accordingly viewed
as a very important group of soil biota in many agroecosystems
and natural environments.
Earthworms represent up to 80% of the total invertebrate
biomass, and because they can incorporate toxic substances
through their skin or by consumption of large amounts of soil, they
are the ideal organisms to be used as bio-indicators of soil pollution.
The close relationship between earthworms and the soil enables
them to serve as indicators and early warning systems in field
studies if soil quality deteriorates. In such field studies, variables
such as earthworm density, biomass changes, diversity and re-
production serve as soil quality parameters. As a result, numerous
studies were conducted to determine the impact of agrochemicals
on earthworm populations. Pesticides
1,2
, particularly fungicides
3
and herbicides
4
, prove to have negative effects on earthworms that
include death, stunted growth and/or poor reproduction.
Herbicide and earthworm mortality
Internationally, research findings published regarding the effect
of herbicides on earthworm mortality are contradictory. A 100%
mortality rate was reported in
Eisenia fetida
(the model earthworm
species used in studies of annelid toxicology) during a laboratory
study where soil was treated with 500 mg/kg and 1 000 mg/kg of
2,4-D, respectively
5
. In contrast to
the laboratory study, no measurable
effect was reported on earthworm
numbers in a similar field study
published during 1990
6
.
According to Wardle
7
who published
his article during 1995, such discrep-
ancies between laboratory and field
studies can partially be explained
by the unrealistically high herbi-
cide concentrations that are used in
many laboratory studies. Unfor-
tunately, the interpretation of the
relevance of findings is much more
complex. Factors such as soil char-
acteristics (pH, texture and organic
matter content) play a role in the
persistence of herbicides and have
an effect on soil biological activity.
All of these factors must be taken
into account during research stud-
ies and in the interpretation of their
findings.
A research study which focused on
the impact of acetochlor on earthworm populations during 2004 sug-
gested that the mortality of earthworms was dependent on the time
period of exposure to the herbicide. They stated that the mortality
of earthworms increased with increased exposure time to any given
concentration acetochlor
8
.
Therefore, the fatality of a specific herbicide will accordingly de-
pend on factors such as pH, texture and organic matter which play a
role in the persistence of a specific herbicide in the soil. Unfortu-
nately, the latter aspect is rarely taken into consideration during re-
search studies.
Numerous studies that focused on glyphosate also showed contra-
dictory findings. Generally, glyphosate is regarded as an environ-
mentally friendly herbicide due to its biodegradation and strong
adsorption to soil
9,10
. However, Domínguez and her research group
based in South America
11
indicated that the effects of glyphosate on
soil fauna remains far from conclusive.
Several studies consistently found very low mortality in certain
earthworm species tested
12
while others state significant mortality
ON FARM LEVEL
Toxicological risk / Invertebrates
Integrated pest control
DR MARYKE CRAVEN, CHARNÉ MYBURGH, OWEN RHODE
and
DR JEANETTA SAAYMAN-DU TOIT,
ARC-Grain Crops, Potchefstroom
Part 4: Earthworms
“
...because they can
incorporate toxic
substances through
their skin or by
consumption of large
amounts of soil, they
are the ideal organisms
to be used as
bio-indicators of
soil pollution.
“
Desember 2018
22