flies is likely to involve adaptive resistance, given the lines of evidence listed
previously. Additional work would contribute more evidence toward resolving the
tolerance/resistance classification, including target site genotyping, testing with
multiple generations of laboratory reared animals, and environmental media measurements (to relate pyrethroid concentrations to tolerance).
4.2.3 Amphipods
In the Central Valley of California, pyrethroid resistance has been documented in the
nontarget amphipod H. azteca. Unlike mosquitoes and black flies, members of the H.
azteca species complex have no history as pests and instead act as important
indicators of water quality in bioassessments as well as model laboratory organisms
in ecotoxicological studies. H. azteca have also been documented as a food source
for fish [143] and birds [144] in North America, confirming their role in aquatic food
webs. This species complex has been documented as one of the most sensitive
arthropods to pyrethroid pesticides [68], with LC 50 s consistently under 5 ng L
À1
in cyfluthrin 96 h water only exposures [57, 58]. By exposing field-collected and
laboratory populations of H. azteca to the pyrethroid pesticides cyfluthrin and
bifenthrin in 96 h acute toxicity tests, the authors found up to 550-fold resistance
in some populations of H. azteca from waterways surrounded by agricultural and
urban land use. Although the populations screened for pyrethroid sensitivity spanned
six different species groups, laboratory-reared populations and wild populations in
waterways without pyrethroid pesticide inputs remained similarly sensitive to pyrethroids, indicating that pyrethroid pre-exposure from nearby land use was responsible for the changes in sensitivity, rather than species group composition. Analysis of
sediment samples for commonly used pyrethroids (bifenthrin, cyfluthrin,
cyhalothrin, cypermethrin, deltamethrin, esfenvalerate, permethrin, and tefluthrin)
consistently showed levels of pyrethroids in agricultural and urban sites that were
sufficient to be acutely toxic to sensitive H. azteca during 10 d acute exposures,
while reference sites without predicted pyrethroid use did not have sediments with
acutely toxic levels of pyrethroids [57, 58]. Point mutations leading to single amino
acid substitutions (L925I or L925V and M918L, M. domestica nomenclature) in the
Vgsc were identified only in resistant (by tenfold or greater) populations, at high
frequencies (>0.8), and sometimes appearing to be fixed within the population
[58]. These mutations have been previously associated with resistance in target
pest species [145–147]. Further, given that multiple species of H. azteca harbored
resistance alleles, the phylogenetic structure of the species complex revealed that
pyrethroid-resistant alleles in H. azteca evolved independently a minimum of six
separate times, suggesting that pyrethroid selective pressures in urban and agricultural waterways are sufficient enough to repeatedly lead to genetic convergent
evolution in impacted H. azteca spp. [58]. Interestingly, some pyrethroid-resistant
H. azteca also harbored a nonsynonymous base pair substitution at the same Vgsc
I936 locus as pyrethroid-resistant sea lice, although in H. azteca, the mutation was
documented as a change from isoleucine to phenylalanine (I936F instead of I936V).
The Ecological and Evolutionary Implications of Pyrethroid Exposure: A New. . .
129
previously. Additional work would contribute more evidence toward resolving the
tolerance/resistance classification, including target site genotyping, testing with
multiple generations of laboratory reared animals, and environmental media measurements (to relate pyrethroid concentrations to tolerance).
4.2.3 Amphipods
In the Central Valley of California, pyrethroid resistance has been documented in the
nontarget amphipod H. azteca. Unlike mosquitoes and black flies, members of the H.
azteca species complex have no history as pests and instead act as important
indicators of water quality in bioassessments as well as model laboratory organisms
in ecotoxicological studies. H. azteca have also been documented as a food source
for fish [143] and birds [144] in North America, confirming their role in aquatic food
webs. This species complex has been documented as one of the most sensitive
arthropods to pyrethroid pesticides [68], with LC 50 s consistently under 5 ng L
À1
in cyfluthrin 96 h water only exposures [57, 58]. By exposing field-collected and
laboratory populations of H. azteca to the pyrethroid pesticides cyfluthrin and
bifenthrin in 96 h acute toxicity tests, the authors found up to 550-fold resistance
in some populations of H. azteca from waterways surrounded by agricultural and
urban land use. Although the populations screened for pyrethroid sensitivity spanned
six different species groups, laboratory-reared populations and wild populations in
waterways without pyrethroid pesticide inputs remained similarly sensitive to pyrethroids, indicating that pyrethroid pre-exposure from nearby land use was responsible for the changes in sensitivity, rather than species group composition. Analysis of
sediment samples for commonly used pyrethroids (bifenthrin, cyfluthrin,
cyhalothrin, cypermethrin, deltamethrin, esfenvalerate, permethrin, and tefluthrin)
consistently showed levels of pyrethroids in agricultural and urban sites that were
sufficient to be acutely toxic to sensitive H. azteca during 10 d acute exposures,
while reference sites without predicted pyrethroid use did not have sediments with
acutely toxic levels of pyrethroids [57, 58]. Point mutations leading to single amino
acid substitutions (L925I or L925V and M918L, M. domestica nomenclature) in the
Vgsc were identified only in resistant (by tenfold or greater) populations, at high
frequencies (>0.8), and sometimes appearing to be fixed within the population
[58]. These mutations have been previously associated with resistance in target
pest species [145–147]. Further, given that multiple species of H. azteca harbored
resistance alleles, the phylogenetic structure of the species complex revealed that
pyrethroid-resistant alleles in H. azteca evolved independently a minimum of six
separate times, suggesting that pyrethroid selective pressures in urban and agricultural waterways are sufficient enough to repeatedly lead to genetic convergent
evolution in impacted H. azteca spp. [58]. Interestingly, some pyrethroid-resistant
H. azteca also harbored a nonsynonymous base pair substitution at the same Vgsc
I936 locus as pyrethroid-resistant sea lice, although in H. azteca, the mutation was
documented as a change from isoleucine to phenylalanine (I936F instead of I936V).
The Ecological and Evolutionary Implications of Pyrethroid Exposure: A New. . .
129