Abstract
Agricultural application of pyrethroid insecticides may result in unintended contamination of
nearby surface waters by facilitated transport through macropores and field drains. This is of
concern, as the pyrethroids are extremely toxic to aquatic invertebrates. With effect
concentrations (ECx) in the range of ng - μg L-1, the pyrethroids are believed to be the class of
pesticides with the greatest negative impact on species composition and dynamics for aquatic
invertebrates in freshwater ecosystems. Despite of this, most studies are based on standard
toxicity tests with standard test species, which likely do not reflect the typically pulsed
exposure events of the pyrethroids.
In addition, pyrethroids can interact synergistically with azole fungicides greatly increasing the
negative biological effect of the insecticide, although the extent of this synergy varies
significantly between different azole fungicides. The proposed mechanism for these synergistic
interactions is azole induced inhibition of the cytochrome P450 enzymes, which are important
for pyrethroid biotransformation resulting in slower elimination and thus higher internal
concentrations.
The objectives of the current PhD thesis were therefore; I) to investigate how seasonality
affects the sensitivity of stream living invertebrates like Gammarus pulex towards pyrethroids.
II) to test if toxicokinetic and toxicodynamic (TKTD) models could describe species specific
differences in sensitivity towards pyrethroids. III) to assess why some azole fungicides can
induce stronger synergistic interactions with pyrethroids than others. IV) to study synergistic
interactions in Chironomus riparius and whether the resulting change in survival over time can
be described by TKTD-models.
Seasonal variations in the sensitivity of G. pulex towards a short pulse of the pyrethroid
cypermethrin were observed across the 16 consecutive months of sampling (EC50: 0.21 ± 0.05 μg L-
1 to 6.60 ± 3.46 μg L-1) with the lowest sensitivity observed during fall (September and October).
However, this difference in sensitivity decreased from 30-fold after one day of recovery to seven
fold after seven days of recovery indicating that the seasonal variations primarily affects acute
sensitivity. These changes in sensitivity was explained by fitness level indicators such as total
protein content and in vitro cytochrome P450 activity of G. pulex, and was not correlated with the
changing water temperatures in the stream at the time of organism collection as initially expected.
The results of the species sensitivity study revealed very large species specific difference
towards a 24 hour pulse of cypermethrin as some species had EC50-values below 0.1 μg L-1 after
six days of recovery, while the two non-arthropod species were hardly affected at the highest
test concentration (7.6 μg L-1). Description of the surviving fractions of organisms over time
following the cypermethrin pulse by TKTD-models proved difficult due to a very fast
development of mortality during the pulse exposure for most species. The measured uptake
and elimination of cypermethrin in all species did not explain the observed differences in
sensitivity although they did prove large toxicokinetic differences between the test species.
Sensitivity of aquatic invertebrates towards pyrethroid insecticides is therefore likely a
consequence of toxicodynamic differences in form of the specific chemical structure of the
sodium channels in the nerve systems as these are the primary targets of the pyrethroids.
As the toxicokinetic differences between the two azole fungicides prochloraz and propiconazole in
Daphnia magna were well predicted by their physicochemical properties, their different
synergistic potentials towards pyrethroids are therefore caused by induced differences in
toxicodynamics in form of enzyme inhibition. The measured IC50-values for inhibition of in vivo
cytochrome P450 (ECOD) activity in D. magna was approximately 500-fold lower for prochloraz
compared to propiconazole. This explains why prochloraz is a stronger synergist, but the measured
differences in IC50-values did not reflect the only five-fold differences in the observed synergistic
potentials.
However, as no synergy was observe between propiconazole and α-cypermethrin in C. riparius the
synergistic potential of different azole fungicides does not only depend on enzyme inhibition, but
just as much on exposure duration, time of assessment and the species investigated.
The decreased levels of synergy observed in C. riparius between prochloraz and α-cypermethrin
compared to previous studies with D. magna were at least partly explained by a much higher
sensitivity towards the azoles themselves, although this could not be supported by the
toxicokinetics measurements. In contrast, the prochloraz induced synergy was well described by
the TKTD-models, when including a synergy parameter to account for the decreased
biotransformation of α-cypermethrin in the presence of prochloraz.
In conclusion, the current results show that aquatic invertebrates in general are highly sensitive
to pyrethroid insecticides at very low concentrations and that the sensitivity of field-collected
species may change depending on the season. Pyrethroid toxicity may be even further
increased by synergistic interactions with azole fungicides, but the degree of synergy is difficult
to predict between various test species due to toxicokinetic and toxicodynamic differences.