Agriculture Reference
In-Depth Information
To date, earthworm behavior has not often been used as a criterion in ecotoxicological studies
(Reinecke et al. 2002), and its value is not fully understood. Field studies showed that earthworms
may migrate over large distances in response to environmental changes, and other studies demon-
strated avoidance behavior when exposed to highly contaminated soils (Yeardley et al. 1996; Hund-
Rinke and Wiechering 2001). These aspects of earthworm behavior may be utilized as ecotoxico-
logical end points. The Denmark earthworm ecotoxicology workshop (2001) recommended that
more attention be given to understanding earthworm behavior, interpreting avoidance responses
and surface migrations. This holds both for pesticide exposure testing and the risk assessment of
contaminated land.
EARTHWORM TOXICITY TESTING FOR REGULATORY PURPOSES
Over the years since the earlier workshops on earthworm ecotoxicology, laboratory toxicity
testing has become further standardized in the guidelines ISO 11268-1 (acute earthworm toxicity
test), ISO 11268-2 (earthworm reproduction toxicity test), and ISO 11268-3 (earthworm field
toxicity test). Moreover, several other international bodies are currently developing their own
testing guidelines (e.g., OECD and Environment Canada), basically applying principles similar
to the ones laid down in the ISO guidelines. The earthworm ecotoxicology workshop held in
Denmark in 2001 considered the possibilities of improving existing methods and the aspects
needed for proper use of earthworm ecotoxicology data in risk assessment. In addition to the
use of data to test the toxicity of new and existing chemicals and for the registration and
admission of pesticides, there was a strong emphasis on the value of using earthworm toxicity
methods for the assessment of the status of contaminated land. The Denmark workshop rec-
ommended that observations of performance (e.g., earthworm growth), burrowing activity, and
other sublethal end points in the existing toxicity tests should be considered as potential end
points in earthworm bioassays on contaminated soils. In the original test guideline of acute
earthworm toxicity testing (OECD 1984), a filter paper contact test was also described. Exposure
conditions using this method are vastly different from any exposure scenario in soil, bearing
little relevance to soil toxicity (see, e.g.,
Heimbach
1984;
Van Gestel and Van Dis
1988).
Participants at the workshop in Denmark (2001), however, argued that the filter paper test has
a role in the risk assessment procedure because it is useful to narrow down the range for
toxicants prior to soil testing (see, e.g.,
Roberts and Dorough
1983). In addition, it still is a
useful research tool when comparing different routes of exposure
.
In the artificial soil acute toxicity test on earthworms, weight changes of the earthworms used
are usually recorded. Because adult earthworms are used in this test, weight loss or gain cannot
and should not be interpreted as effects on growth. However, weight loss in acute toxicity tests on
earthworms can serve as a check of the validity of the exposure test. Weight loss (when there is
no mortality) can be used as an indication of a need for further testing. The Denmark workshop
recommended a compilation of existing data on earthworm weight changes in acute toxicity tests
to establish a Ñcritical weight lossÒ that might trigger further testing.
Attempts to demonstrate the development of resistance to chemicals such as heavy metals
among earthworm populations in highly polluted areas experimentally have been only partly
successful (Bengtsson et al. 1992; Spurgeon and Hopkin 1999, 2000; Reinecke et al. 1999; Morgan
personal communication). Therefore, it remains unclear whether earthworms can develop resistance
to long-term metal exposure. However, it remains an important issue if earthworms are, for
regulatory purposes, to be used in standardized toxicity tests or even as bioindicators because the
occurrence of resistant genotypes in a test population may bias results.
