mutations [40, 151, 172]. Salmon lice from Norway harbor organophosphate and
carbamate resistance alleles [173, 174]. Other populations of sea lice show a marked
reduction in sensitivity to emamectin benzoate, an avermectin [123]. An. gambiae
have demonstrated adaptive resistance to pyrethroids and organophosphates
[175]. Some populations of black flies (Simulium spp.) that are resistant to pyrethroids are less sensitive to organophosphates [60]. These examples of evolution to
multiple classes of pesticides serve as evidence that these populations are under
potentially strong selective pressures from multiple chemicals. Concurrent, strong
selective pressures may leave populations even more vulnerable to extinction or
losses in genetic diversity. While concerns regarding the genetic diversity of pest
species (e.g., sea lice, mosquitoes, black flies, and sea lice) are rarely expressed,
these concerns are markedly more prominent when considering nontarget species
(like H. azteca) that are not disease vectors or pests. Evidence of decreased genetic
diversity caused by insecticide applications are suggested in the literature for insects.
Allelic richness was negatively correlated with deltamethrin resistance in mosquitoes (Aedes aegypti) harboring a Vgsc kdr mutation, potentially due to founder
effects from genetic bottlenecks caused by insecticide selective pressures
[176]. However, if gene flow is high, losses in genetic diversity are not always
apparent in pyrethroid-resistant insect populations [177]. H. azteca is a poor disperser relative to the flying insects [178]. Thus, if selection for pyrethroid resistance
kdr alleles is capable of driving genetic bottlenecks in H. azteca, and if gene flow is
not sufficient to compensate for decreases in genetic diversity, resistant H. azteca
populations with kdr mutations at high frequencies may be particularly prone to
having low genetic diversity or being at a greater risk for genetic drift. The functional
fixation of resistance alleles at the L925 locus in six different populations of H.
azteca also suggests that genetic diversity may be reduced in those populations. As
mentioned previously, one L925I-fixed population of pyrethroid-resistant H. azteca
have already demonstrated a reduced tolerance to other stressors and increased
fitness costs compared to sensitive populations [150], potentially due to fitness
costs associated with the resistance mutation, or possibly from a loss in genetic
diversity associated with a past founder effect. Explicit studies of genetic diversity in
resistant populations of H. azteca have yet to be performed, but are essential to
building our understanding of the way that selection for kdr mutations is affecting
populations and their resilience to other environmental changes and stressors. These
studies may also serve to move the field of evolutionary toxicology forward as we
gain a better understanding of the evolutionary impact of strong selective pressures
on nontargets.
4.3.3 Ecological Implications
Adaptive pyrethroid resistance from target (sea lice) and nontarget (mosquitoes,
black flies, and amphipods) pyrethroid exposures may signal ecosystem-level pesticide stress. If pyrethroids are present at levels sufficient to drive selection of target
site mutations of large effect in these populations, then they are likely causing acute
The Ecological and Evolutionary Implications of Pyrethroid Exposure: A New. . .
133
carbamate resistance alleles [173, 174]. Other populations of sea lice show a marked
reduction in sensitivity to emamectin benzoate, an avermectin [123]. An. gambiae
have demonstrated adaptive resistance to pyrethroids and organophosphates
[175]. Some populations of black flies (Simulium spp.) that are resistant to pyrethroids are less sensitive to organophosphates [60]. These examples of evolution to
multiple classes of pesticides serve as evidence that these populations are under
potentially strong selective pressures from multiple chemicals. Concurrent, strong
selective pressures may leave populations even more vulnerable to extinction or
losses in genetic diversity. While concerns regarding the genetic diversity of pest
species (e.g., sea lice, mosquitoes, black flies, and sea lice) are rarely expressed,
these concerns are markedly more prominent when considering nontarget species
(like H. azteca) that are not disease vectors or pests. Evidence of decreased genetic
diversity caused by insecticide applications are suggested in the literature for insects.
Allelic richness was negatively correlated with deltamethrin resistance in mosquitoes (Aedes aegypti) harboring a Vgsc kdr mutation, potentially due to founder
effects from genetic bottlenecks caused by insecticide selective pressures
[176]. However, if gene flow is high, losses in genetic diversity are not always
apparent in pyrethroid-resistant insect populations [177]. H. azteca is a poor disperser relative to the flying insects [178]. Thus, if selection for pyrethroid resistance
kdr alleles is capable of driving genetic bottlenecks in H. azteca, and if gene flow is
not sufficient to compensate for decreases in genetic diversity, resistant H. azteca
populations with kdr mutations at high frequencies may be particularly prone to
having low genetic diversity or being at a greater risk for genetic drift. The functional
fixation of resistance alleles at the L925 locus in six different populations of H.
azteca also suggests that genetic diversity may be reduced in those populations. As
mentioned previously, one L925I-fixed population of pyrethroid-resistant H. azteca
have already demonstrated a reduced tolerance to other stressors and increased
fitness costs compared to sensitive populations [150], potentially due to fitness
costs associated with the resistance mutation, or possibly from a loss in genetic
diversity associated with a past founder effect. Explicit studies of genetic diversity in
resistant populations of H. azteca have yet to be performed, but are essential to
building our understanding of the way that selection for kdr mutations is affecting
populations and their resilience to other environmental changes and stressors. These
studies may also serve to move the field of evolutionary toxicology forward as we
gain a better understanding of the evolutionary impact of strong selective pressures
on nontargets.
4.3.3 Ecological Implications
Adaptive pyrethroid resistance from target (sea lice) and nontarget (mosquitoes,
black flies, and amphipods) pyrethroid exposures may signal ecosystem-level pesticide stress. If pyrethroids are present at levels sufficient to drive selection of target
site mutations of large effect in these populations, then they are likely causing acute
The Ecological and Evolutionary Implications of Pyrethroid Exposure: A New. . .
133