4.3.4 Risk Assessment Implications
If the pyrethroid presence is strong enough, some populations of sensitive taxa may
evolve. However, relying on populations to have the genetic background population
size to evolve to resist pyrethroids is not a sufficient protective strategy for aquatic
ecosystems. Even when evolution is possible, it may not happen quickly enough in
wild populations and can come with fitness trade-offs [99]. In itself, the measurement of genetic, adaptive resistance is an indicator that pyrethroids’ selective
pressures have removed sensitive individuals from the population. It may also signal
acute and/or sublethal toxicity for other members of the aquatic ecosystem, potentially leading to the loss of other sensitive taxa. Regarding sea lice treatment with
pyrethroids, risk assessments should be undertaken in a fish farm site-specific
manner to prevent undue harm from pesticide treatments on nontarget life
[180]. The evolution of resistance in aquatic invertebrate nontargets in urban and
agricultural environments on a global scale suggests that pyrethroids are not being
adequately regulated to prevent undue harm in the aquatic environment. In the
United States, agricultural pyrethroid use is monitored, but urban use is not
[58]. In African countries, such as Ghana, pyrethroid use in general is poorly
regulated [139]. Without closer regulation of pyrethroid use, disease vectors and
other invertebrates will continue to experience strong selective pressures from
pyrethroids, perpetuating the human health, evolutionary, and ecological effects
described above.
Adequate protection for wild populations may not be achieved by utilizing
adapted populations to make risk assessment decisions [86], largely because sensitive organisms have been removed from resistant populations. However, the genetic
adaptive pyrethroid resistance in some wild populations of H. azteca presents a
unique model system to incorporate the field of evolutionary toxicology directly into
risk assessment decisions. H. azteca are both sensitive to pyrethroids and amenable
to laboratory culture. As such, they are ideal candidates for bioassessment and
biomonitoring programs and an ideal laboratory surrogate for determining thresholds for the protection of aquatic life. The stable pyrethroid resistance mutations in
one population of H. azteca have already been used as a type of biological toxicity
identification evaluation (TIE) tool to identify the source of toxicity in environmental samples [151]. The repeated, convergent evolution of the same resistance mutations across different species groups within the H. azteca species complex suggests
that screening new populations for genetic changes in the target site (vgsc) may
provide evidence of pyrethroid impairment in new locations. Further, given that
other crustaceans are often similarly sensitive to pyrethroids [69], phenotype assays
and genetic screening for pyrethroid resistance could be developed for taxa that are
often used in regulatory decisions outside of the United States (e.g., Gammarus).
These methods may be able to refine which areas or regions are at the greatest risk for
impairment from pyrethroids.
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If the pyrethroid presence is strong enough, some populations of sensitive taxa may
evolve. However, relying on populations to have the genetic background population
size to evolve to resist pyrethroids is not a sufficient protective strategy for aquatic
ecosystems. Even when evolution is possible, it may not happen quickly enough in
wild populations and can come with fitness trade-offs [99]. In itself, the measurement of genetic, adaptive resistance is an indicator that pyrethroids’ selective
pressures have removed sensitive individuals from the population. It may also signal
acute and/or sublethal toxicity for other members of the aquatic ecosystem, potentially leading to the loss of other sensitive taxa. Regarding sea lice treatment with
pyrethroids, risk assessments should be undertaken in a fish farm site-specific
manner to prevent undue harm from pesticide treatments on nontarget life
[180]. The evolution of resistance in aquatic invertebrate nontargets in urban and
agricultural environments on a global scale suggests that pyrethroids are not being
adequately regulated to prevent undue harm in the aquatic environment. In the
United States, agricultural pyrethroid use is monitored, but urban use is not
[58]. In African countries, such as Ghana, pyrethroid use in general is poorly
regulated [139]. Without closer regulation of pyrethroid use, disease vectors and
other invertebrates will continue to experience strong selective pressures from
pyrethroids, perpetuating the human health, evolutionary, and ecological effects
described above.
Adequate protection for wild populations may not be achieved by utilizing
adapted populations to make risk assessment decisions [86], largely because sensitive organisms have been removed from resistant populations. However, the genetic
adaptive pyrethroid resistance in some wild populations of H. azteca presents a
unique model system to incorporate the field of evolutionary toxicology directly into
risk assessment decisions. H. azteca are both sensitive to pyrethroids and amenable
to laboratory culture. As such, they are ideal candidates for bioassessment and
biomonitoring programs and an ideal laboratory surrogate for determining thresholds for the protection of aquatic life. The stable pyrethroid resistance mutations in
one population of H. azteca have already been used as a type of biological toxicity
identification evaluation (TIE) tool to identify the source of toxicity in environmental samples [151]. The repeated, convergent evolution of the same resistance mutations across different species groups within the H. azteca species complex suggests
that screening new populations for genetic changes in the target site (vgsc) may
provide evidence of pyrethroid impairment in new locations. Further, given that
other crustaceans are often similarly sensitive to pyrethroids [69], phenotype assays
and genetic screening for pyrethroid resistance could be developed for taxa that are
often used in regulatory decisions outside of the United States (e.g., Gammarus).
These methods may be able to refine which areas or regions are at the greatest risk for
impairment from pyrethroids.
136
K. M. Major and S. M. Brander