Agriculture Reference
In-Depth Information
CHRONIC TOXICITY TESTS AND SUBLETHAL EFFECTS
Living earthworms in the soil are exposed, more frequently over much longer periods of time, to
chemical agents at levels lower than those that are fatal. To simulate such exposure, short-term
(subchronic) and long-term (chronic) studies are required. The former are usually conducted over 14
days or longer, whereas the latter are conducted over 1 or 2 years. Sublethal earthworm toxicity testing
has now been accepted as part of the routine regulatory testing scheme in Europe and is designed to
detect subtle effects, such as disturbances in earthworm behavior, retarded development, lowered
fertility, teratogenic effects, and the like that may cause population changes without necessarily leading
to mortality. Data referring to the impairment of vital functions of organisms often have a greater
ecological relevance than those obtained from measurements of acute toxicity (Ma 1984).
Standardization of a chronic toxicity test for earthworms has not been achieved fully because
procedures vary and standardization is limited. The ecological significance of the results is still
uncertain, and reproducibility is limited. A variety of tests has been suggested. The pesticide test for
sublethal effects (Kokta 1992), which was subjected to a ring test in Germany, and the artificial soil
test proposed by Van Gestel et al. (1989) have been used with minor adjustments by various research-
ers. International Organization for Standardization (ISO) draft guideline DIS 11268-2 (Bembridge
1998) for an earthworm reproduction study provides an adequate method for a second-tier assessment,
and previous workshops on earthworm ecotoxicology have considered it sufficient.
Useful end points that have emerged from various studies and are mentioned in the recommen-
dations of the 1997 earthworm ecotoxicology workshop (Bembridge 1998) include the following:
¤ Growth of juveniles
¤ Changes in body weight of adults
¤ Reproduction
Rate of cocoon production
Number of hatchlings
Hatching success
Incubation time
Sperm parameters
¤ Behavior
¤ Morphological effects
¤P ysiological effects
¤
Decomposition in the soil as an indirect measure of activity
Chronic toxicity tests are labor intensive and time consuming but seem to be the next logical
step, within certain toxicity limits, after the acute toxicity level of a chemical has been established
in screening tests (Kokta and Rothert 1992) if the estimated exposure concentration was high.
However, it must be kept in mind that environmental behavior and other factors associated with a
chemical may still require that chronic toxicity testing be done although a chemical has ÑpassedÒ
the acute toxicity test and has been rated as Ñprobably a low risk.Ò
The relevance of life history parameters in earthworm ecotoxicology was demonstrated
clearly by Cluzeau et al. (1992) and Reinecke and Reinecke (1996) and also stressed by Forbes
(personal communication). Cluzeau et al. showed that disturbances in nontarget earthworm
population dynamics could affect the quality of cultivated soils. It is especially relevant in the
context of population recovery after a disturbance (Edwards and Brown 1982). Helling et al.
(2000) showed that juvenile earthworm growth and cocoon production can respond sensitively
to very low levels of copper-containing fungicides, although the chemical is considered relatively
harmless to earthworms.
