of pyrethroids in the environment. However, other less sensitive taxa may still be
under substantial selective pressures from pyrethroids. Environmentally relevant
measurements of pyrethroids in water and sediment often exceed regulatory recommendations [2]. A variety of other freshwater and marine crustaceans (Menippe
mercenaria, Gammarus lacustris, Crangonyx pseudogracilis, Gammarus
pseudolimnaeus, Americamysis bahia, Chaoborus sp.) have similar pyrethroid sensitivities (2.6- to 9.3-fold lower) to H. azteca [68]. Potential impairment for other
important prey for fish including caddisfly (Hydropsyche spp.) have been
documented at environmentally relevant levels of bifenthrin [182]. The abundance
of sensitive invertebrate taxa, % Ephemeroptera-Plecoptera-Trichoptera (EPT), and
some mayfly taxa has been negatively correlated with bifenthrin sediment concentrations [28]. Further, a mesocosm experiment with bifenthrin-laden sediments has
documented reduced larval macroinvertebrate abundance, richness, and biomass at
concentrations 2.5 times lower than the recorded 10 d sediment LC 50 for H. azteca
[31]. The same authors also predict altered emergence dynamics and trophic cascades in some stream scenarios. Another mesocosm experiment showed impairment
of the majority of examined macroinvertebrate and zooplankton taxa in response to a
tertiary mixture of environmentally relevant concentrations of two pyrethroids and
an organophosphate. H. azteca and D. magna showed acute toxic responses, while
snails (Radix sp.) and copepods displayed chronic, sublethal responses [183]. Thus,
it is possible that other taxa are under substantial acutely toxic selective pressures
from pyrethroids, and at minimum, they are experiencing sublethal fitness costs from
pyrethroid presence. Even sublethal fitness costs incurred by aquatic populations
under pyrethroid stress may drive resistance to pyrethroids in affected populations,
although that adaptive resistance would most likely occur through complex phenotypes caused by polygenic selection, which would be likely to carry with them their
own set of fitness costs [99]. In populations without sufficient standing genetic
variation on which evolution can act, or in taxa that have longer life cycles, evolution
may not be a feasible response to environmental stress. For example, H. azteca are
obligate aquatic invertebrates and have a generation time of 1 month under standardized laboratory conditions [184]. In contrast, some mayflies, for example,
remain nymphs for up to multiple years before emergence [185], and a longer
generation time may allow pyrethroids to impact population densities via acute or
sublethal toxicity to an extent that prevents evolved resistance to pyrethroids and
instead contributes to local extinctions.
Aside from the loss of sensitive taxa from ecosystems with toxic levels of
pyrethroids, potential fitness costs and decreased resilience to other environmental
stressors in some resistant H. azteca populations [150] may contribute to present or
future declines in densities, which could also impact the fish and other predators that
rely on them for food. In addition, pyrethroid-resistant H. azteca harbor higher levels
of these pesticides capable of causing sublethal toxicity to forage fishes and potentially increasing the risk of bioaccumulation in piscivores or birds which may reach
farther up the aquatic food web [186]. The mosquito and blackfly populations that
are resistant to pyrethroids may also pose a higher risk for bioaccumulation in
predators (birds, fish, frogs, and other insects) that rely on these larval and adult
insects as a food source, although those studies have yet to be performed.
The Ecological and Evolutionary Implications of Pyrethroid Exposure: A New. . .
135
under substantial selective pressures from pyrethroids. Environmentally relevant
measurements of pyrethroids in water and sediment often exceed regulatory recommendations [2]. A variety of other freshwater and marine crustaceans (Menippe
mercenaria, Gammarus lacustris, Crangonyx pseudogracilis, Gammarus
pseudolimnaeus, Americamysis bahia, Chaoborus sp.) have similar pyrethroid sensitivities (2.6- to 9.3-fold lower) to H. azteca [68]. Potential impairment for other
important prey for fish including caddisfly (Hydropsyche spp.) have been
documented at environmentally relevant levels of bifenthrin [182]. The abundance
of sensitive invertebrate taxa, % Ephemeroptera-Plecoptera-Trichoptera (EPT), and
some mayfly taxa has been negatively correlated with bifenthrin sediment concentrations [28]. Further, a mesocosm experiment with bifenthrin-laden sediments has
documented reduced larval macroinvertebrate abundance, richness, and biomass at
concentrations 2.5 times lower than the recorded 10 d sediment LC 50 for H. azteca
[31]. The same authors also predict altered emergence dynamics and trophic cascades in some stream scenarios. Another mesocosm experiment showed impairment
of the majority of examined macroinvertebrate and zooplankton taxa in response to a
tertiary mixture of environmentally relevant concentrations of two pyrethroids and
an organophosphate. H. azteca and D. magna showed acute toxic responses, while
snails (Radix sp.) and copepods displayed chronic, sublethal responses [183]. Thus,
it is possible that other taxa are under substantial acutely toxic selective pressures
from pyrethroids, and at minimum, they are experiencing sublethal fitness costs from
pyrethroid presence. Even sublethal fitness costs incurred by aquatic populations
under pyrethroid stress may drive resistance to pyrethroids in affected populations,
although that adaptive resistance would most likely occur through complex phenotypes caused by polygenic selection, which would be likely to carry with them their
own set of fitness costs [99]. In populations without sufficient standing genetic
variation on which evolution can act, or in taxa that have longer life cycles, evolution
may not be a feasible response to environmental stress. For example, H. azteca are
obligate aquatic invertebrates and have a generation time of 1 month under standardized laboratory conditions [184]. In contrast, some mayflies, for example,
remain nymphs for up to multiple years before emergence [185], and a longer
generation time may allow pyrethroids to impact population densities via acute or
sublethal toxicity to an extent that prevents evolved resistance to pyrethroids and
instead contributes to local extinctions.
Aside from the loss of sensitive taxa from ecosystems with toxic levels of
pyrethroids, potential fitness costs and decreased resilience to other environmental
stressors in some resistant H. azteca populations [150] may contribute to present or
future declines in densities, which could also impact the fish and other predators that
rely on them for food. In addition, pyrethroid-resistant H. azteca harbor higher levels
of these pesticides capable of causing sublethal toxicity to forage fishes and potentially increasing the risk of bioaccumulation in piscivores or birds which may reach
farther up the aquatic food web [186]. The mosquito and blackfly populations that
are resistant to pyrethroids may also pose a higher risk for bioaccumulation in
predators (birds, fish, frogs, and other insects) that rely on these larval and adult
insects as a food source, although those studies have yet to be performed.
The Ecological and Evolutionary Implications of Pyrethroid Exposure: A New. . .
135