Agriculture Reference
In-Depth Information
Acute toxicity studies are designed to express the potency of the toxicant in terms of a median
lethal dose LD
, causing the death of 50% of the universal population of the species exposed,
under the defined conditions of the test. When indirect administration of a toxicant is done, the
potency is expressed as the median lethal concentration LC
50
, which is more often the case in
earthworm studies. The Workshops on Earthworm Ecotoxicology held in the Netherlands and
Denmark concluded that acute earthworm toxicity testing has been well established with standard-
ized end points, but there is still room for improvement (C. Kula 1998). Various researchers,
including Van Gestel (1992) and Spurgeon and Weeks (1998), have pointed out the limitations of
this test for purposes of extrapolation to the field. These tests
50
do not provide sufficient
information to predict the effect of a chemical applied in field situations.
These acute artificial soil tests on earthworms are now conducted with well-established proto-
cols (Reinecke 1992; Van Straalen and Van Gestel 1993), have preliminary screening value, and
provide useful information on relative toxicity of chemicals. Edwards and Coulson (1992) recom-
mended that initial screening, using the Organization for Economic Cooperation and Development
(OECD) Guideline 207, based on the pioneering reports of Edwards (1983, 1984) on ring tests
undertaken by 35 laboratories, remains the standard laboratory practice. Van Gestel (1992) sup-
ported this view based on the results of Van Gestel and Ma (1990) and Van Gestel et al. (1991)
and after studying the influence of soil characteristics on the toxicity of chemicals. This approach
was also supported by subsequent international workshops (Bembridge 1998). Edwards and Bohlen
(1992) reviewed all published work on the toxicity of chemicals to earthworms involving the effects
of more than 200 chemicals and ranked the chemicals as nontoxic, slightly toxic, moderately toxic,
very toxic, and extremely toxic.
To a large extent, the evaluation of the environmental risk of chemicals has until now relied
on acute earthworm toxicity tests, although no clear foundation for the validity of such an approach
existed. These laboratory tests have been conducted over a wide range of chemicals and earthworms
(Goats and Edwards 1982; Neuhauser et al. 1983, 1986; Roberts and Dorough 1983, Vermeulen et
al. 2001), but the application of these test results to environmental risk analysis is lacking. Callahan
et al. (1994) followed a new approach toward comparing earthworm species toxicity by integrating
an extensive database on the acute toxicity of chemicals to earthworms based on the Weibull function
(Weibull 1951) used by Shirazi and Lowrie (1988) for fish. This technique generalized the rela-
tionship between the chemical and the organism in terms of two parameters: toxicity (scale factor
per se
k
). Their analysis provided an assessment of the relative tolerance of
four earthworm species to each of 62 chemicals and in relationship to each other using two test
protocols, the contact test and the artificial soil test. Their results also suggested that
) and tolerance (form factor
a
E. fetida
may
be representative of a group of species:
Allolobophora tuberculata,
Eudrilus eugeniae,
and
Perionyx
excavatus.
Although laboratory tests cannot fully simulate the structure of soil or the behavior of earth-
worms in the field, certain end points other than mortality can be obtained from these tests. The
use of the same test protocol to study changes in body weight and reproduction can provide useful
data when integrated with results of field tests, but does not preclude the need for the latter, when
required. Acute toxicity tests in artificial soil can probably be extrapolated to natural soils, but data
are still scarce (Spurgeon and Hopkin 1995). Acute toxicity tests are designed to identify very toxic
chemicals that have immediate effects and not to determine acceptable environmental concentra-
tions. They can, however, serve as a qualitative screen for detection of other toxicological effects
and give a first estimate of the NOEC (no observed effect concentration) level for continuous
exposure. These tests do not clear any chemical from further testing but do assist in practice in
setting priorities for further testing of sublethal effects. The bottom line is that the OECD standard
artificial soil test was not necessarily developed to enable the direct extrapolation of toxicity results
to the field situation (Spurgeon and Weeks 1998).
