Report from the Sub-comittee on the environment and health

Effects of insecticides on
fauna
Effects of herbicides on
fauna
82
A large number of studies show that pesticides can affect <strong>the</strong> fauna in
freshwater ecosystems, both directly and indirectly. Generally speaking,
insecticides, in particular, have an adverse effect on <strong>the</strong> fauna, whereas
direct effects <strong>from</strong> herbicides are often seen only at relatively high
concentrations. Indirect effects are seen particularly in <strong>the</strong> highest trophic
levels (fish, amphibians and birds) owing to reduced quantities of prey or
in <strong>the</strong> form of changes in <strong>the</strong> function of <strong>the</strong> entire ecosystem. For
example, Wallace et al. (1991) found that <strong>the</strong> decomposition of leaves
into fine organic matter fell considerably in a forest watercourse after
treatment with <strong>the</strong> insecticide methoxychlor owing to elimination of <strong>the</strong>
insects that comminute <strong>the</strong> leaves. Methoxychlor may no longer be used
in Denmark. If <strong>the</strong> possibility of recolonisation of a watercourse is poor,
e.g. owing to barriers or <strong>the</strong> geographical location, it can take a long time
(years) for <strong>the</strong> watercourse’s biological conditions to be reestablished,
even if <strong>the</strong> supply of pesticide ceases. In <strong>the</strong> case of stagnant freshwater
systems, <strong>the</strong> effects of pesticides seem to disappear within a similar
period of time. These ecosystems are generally affected by both
fertilisers and pesticides.
In <strong>the</strong> case of <strong>the</strong> herbicide atrazine, <strong>the</strong> concentrations found in
watercourses are generally so low that no effect on <strong>the</strong> fauna is likely.
For example, in a study by Grande et al. (1994), <strong>the</strong> mortality among
trout fry, which are more sensitive than adult fish, only increased at
atrazine concentrations of more than 50 microgrammes per litre.
However, Lampert et al. (1989) showed that concentrations right down to
0.1 microgramme per litre could affect daphnia in stagnant water, even
though, in a single-species test, EC50 was 2 microgrammes per litre. The
study shows that <strong>the</strong> sensitivity at community level is far greater because
both direct and indirect effects play a part. Atrazine concentrations of
more than 0.1 microgramme per litre have been found in a pond near
Køge, but <strong>the</strong>re are very few o<strong>the</strong>r measurements. As in <strong>the</strong> case of
atrazine, <strong>the</strong> concentrations of hexazine that have been found are so
small that no serious effect on <strong>the</strong> fauna is likely.
The fungicide propiconazole has been found in concentrations up to 0.8
microgramme per litre in a watercourse and 0.1 microgramme per litre in
a pond. These concentrations are below <strong>the</strong> concentration found to have
an effect on zooplankton, invertebrates and fish.
The insecticide dimethoate has been found to affect (increase) activity in
watercourse invertebrates at concentrations of more than 1 microgramme
per litre and to reduce <strong>the</strong> density of some species (Bækken, Aanes
1994). In <strong>the</strong> case of zooplankton, an LC50-value of around 20
microgramme per litre has been found in mesocosm experiments (Hessen
et al. 1994). The concentrations in Danish watercourses and lakes lie
below <strong>the</strong>se values but not significantly below <strong>the</strong> concentrations that
produce effects.
The pyrethroid insecticides are generally very toxic to largely all
freshwater fauna, i.e. to zooplankton, invertebrates, crustaceans, fish and
amphibians, in very small concentrations – normally less than 1
microgramme per litre. Besides that, <strong>the</strong> substances accumulate in <strong>the</strong>
organisms (e.g. Andersen 1982), but are eliminated when <strong>the</strong> exposure
ends. The pyrethroid fenvalerate has been found in ponds in a