Agriculture Reference
In-Depth Information
A minimum of four replicates, also for the control
Plot size at least between 5 and 10 m
2
Defined cropping
Timing of the experiment when earthworms are most active, with a duration of at least 6
months, but preferably longer for persistent chemicals
Use of toxic standards such as benomyl or carbaryl
Determination of residues in earthworms and soil to assess bioconcentration factor
The persistence of the chemical and the frequency of application are important considerations in
designing field tests.
Because the accurate assessment of earthworm populations is extremely difficult (Edwards
1991), a Ñworst-case approachÒ has been suggested (Lofs 1992) to determine a biologically sig-
nificant effect. However, the necessity for valid controls is obvious and was discussed fully by
Edwards (1998). For sampling, a combination of hand sorting and dilute formalin extraction could
be used. Earthworms in the top 5 to 10 cm of soil are hand sorted, after which the quadrat is treated
with dilute formalin. This method recovers both the species that live close to the soil surface
(endogeic) as well as those that penetrate deep into the soil (anecic).
Until more data can be obtained from well-designed and standardized field experiments, direct
comparison of field data with data from standardized laboratory experiments must be done with
care. To obtain a balanced judgment on the potential earthworm toxicity and environmental hazard
of a chemical, a suitable databank of results from both laboratory and field experiments is needed.
Heimbach (1992) has obtained promising results in such a comparative study. He obtained a good
correlation (
values from the artificial soil test and a standardized field test
based on a test design by Edwards and Brown (1982). There is a growing body of evidence that
LC
r
= 0.86) between LC
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tests in the laboratory could still be used, in spite of their known shortcomings, to predict
field results by using a compensation factor. However, further evidence is needed before the validity
of such an approach can be generally accepted.
Because of their demands on labor and costs, complete detailed field tests are rare and therefore
remain difficult to interpret. The time for recovery of earthworm populations after exposure is an
important aspect of toxicity in field experiments (Sheppard et al. 1998). What constitutes a signif-
icant percentage negative change in populations that could affect their functioning? This question
still remains unanswered and requires further study and discussion. It is generally accepted that
TMEs (Morgan and Knacker 1994) and microcosms (Burrows and Edwards 2002) may be useful
tools to bridge the gap between single-species laboratory tests and the field. Further research is
needed to explore the potential usefulness of TMEs on the risk assessment of chemicals for
earthworms.
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RESIDUES IN EARTHWORMS AND THEIR ROLE AS
BIOMONITORS AND BIOINDICATORS
The use of earthworms as biomonitors and bioindicators may be accomplished in different ways. The
concentration of a chemical in earthworms is sometimes a useful indicator of whether the chemical
is near toxic levels in the environment. It is very important to analyze both the soil and the earthworms
during field experiments to determine bioconcentration factors. The proximity of earthworms to the
soil contaminants makes them useful monitoring organisms for the soil environment.
A classic example of the use of earthworms as biological monitors was presented by Ma (1987);
he showed that amounts of heavy metals in moles were correlated closely with concentrations in
earthworms but not with the concentrations in soils. A study by Reinecke et al. (2000) obtained
similar results for the transference of lead in the food chain from soil to earthworms to shrews.
The presence of toxic amounts of a chemical in earthworms poses a serious risk of secondary
