SA Graan December 2018

at very high concentrations (50 000 mg/kg)

. A third group found

that lethal doses were dependent on the commercial formulation

used

. The Domínguez research group conceded that, based on re-

sults already available on glyphosate and different earthworm spe-

cies, it is unlikely that there will be major effects of acute toxicity

Herbicide impact on earthworm

reproduction and weight

International research findings suggest sub-lethal parameters, such

as reproduction or weight of the earthworms to be generally more

sensitive than mortality when assessing the effect of herbicides.

Although the earthworms accordingly do not necessarily die from

exposure, they can be affected, for example, with regard to their

capability to reproduce. As a result, various studies report on the

effect (or lack of effect) of herbicides on the number of earthworm

juveniles or cocoons.

Atrazine drastically reduced growth and cocoon production in

E. fetida

when applied at 100 parts per million (ppm) and completely

stopped growth and reproduction at 200 ppm. Paraquat also slowed

down weight gain and cocoon production at 200 ppm

. Similar to

the contradictory results obtained by mortality studies, the effect of

glyphosate application on earthworm reproduction varies between

no significant effect

, a decrease in number of juveniles and/or co-

coons

and increase in cocoon production

Several international studies similarly report concentration-depend-

ent weight loss in various earthworm species because of glypho-

sate intoxication

14,18

. Aminomethylphosphonic acid (AMPA) is one of

the main metabolites of glyphosate and is classified as being per-

sistent in soils. The Domínguez research group

found that the

biomass loss experienced by the earthworms at various AMPA dos-

ages appeared to be related to the higher reproduction rate in these

treatments.

Results indicated that although a higher number of cocoons were

produced, the cocoons had lower levels of fertility

or that the juve-

niles and cocoons produced had a decreased biomass. These stud-

ies concluded that the physiological ability of earthworms growing

in soils contaminated with high doses of AMPA decreases in terms of

development, growth, reproduction and their ability to accomplish

key ecosystem functions

Herbicide impact on locomotor

activity of earthworms

The skin of oligochaete worms is richly supplied with chemorecep-

tors and many species exhibit a capacity for considerable locomotor

activity (i.e. movements or methods that earthworms use to move

from one place to another). It was speculated that earthworms likely

have the capability to both detect contaminated soil and move away

from it. In a laboratory study, researchers observed that the earth-

worm

E. fetida

migrates away from soil contaminated by the fungi-

cide Mancozeb

During 2004 a Washington State University research group

made

similar observations when glyphosate contaminated soil was in-

corporated in their trial. They also suggested that exposure to rec-

ommended glyphosate concentrations results in low to negligible

acute toxicity in

E. fetida

. It was, however, also demonstrated that

glyphosate appeared to alter the locomotor activity of these earth-

worms. The exposure of the earthworms to glyphosate stimulated

avoidance activity, with the worms appearing to migrate to uncon-

taminated soils.

Based on these results researchers speculated that under natural

field conditions, these earthworms would most likely migrate onto

the surface of contaminated soil where they will be exposed to

predation, UV radiation and/or desiccation. Therefore, although

the glyphosate might not directly kill the earthworms, the applica-

tion thereof might alter their locomotor activity in such a way that it

could potentially compromise their survival

Points to ponder

Critique expressed by some researchers in earthworm related re-

search is that most studies make use of compost earthworms (

Ei-

senia

species) that commonly do not inhabit agroecosystems and

that more studies should be done on the species that are more com-

mon to agroecosystems

. A study conducted during 2016 further

concluded that chemical uptake by earthworms cannot be general-

ised between species because the influence of species’ traits can

vary depending on the nature of the studied chemicals

Studies report that some earthworm species such as

Aporrectodea

caliginosa

and

Allolobaphora chlorotica

demonstrate the ability to

adapt to residual glyphosate and atrazine contamination in agri-

cultural soils by activating detoxification and antioxidant enzymes

faster

23,24

. It is therefore entirely possible that species shifts could

(or have) occurred over time to favour certain species of earthworm

as a result of herbicide application. The dynamics associated with

such a shift warrant further investigation.

There is accordingly much we don’t know as to what is happening

with the earthworms. One reason might be that current research

does not cast the net wide enough to include more earthworm spe-

cies. A second reason may be that the studies undertaken fail to

investigate a more comprehensive scenario that would accurately

reflect a true on-farm scenario. Such a comprehensive study would

typically have to include different soil types, soil pH, organic matter,

various agricultural tillage practices, crop rotation effects and the

whole range of agrochemicals that are annually applied.

For instance, no-till practices regulate soil temperature to a greater

extent than tilled soils. Recent studies suggested that exposure to

the same pesticide concentration at different temperatures lead to

different toxicity responses in earthworms, with increase in temper-

ature exposure generally resulting in an increase in toxicity while a

decrease in temperature leads to a decrease in toxicity

Although various studies showed that herbicides as well as fun-

gicides have a potential detrimental impact on earthworms under

laboratory conditions, it remains unclear exactly what takes place

under field conditions. The question also remains whether certain

soil characteristics or cultivation practices provide a buffer effect

that minimises the potential detrimental effects of agrochemicals

on earthworms.

Readers are welcome to contact the authors at ARC-

Grain Crops with any enquires they might have at

018 299 6100.

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December 2018