deltamethrin, or fenvalerate), or an organochlorine (DDT). Larvae from agricultural
areas were significantly more tolerant to fenvalerate (88.2-fold), deltamethrin (90.0fold), cypermethrin (22.9-fold), and DDT (59.2-fold) compared with reference site
larvae. Given the high levels of DDT and pyrethroid tolerance in larvae from
agricultural sites, the authors concluded that a kdr- type mutation was likely. Further,
the lack of tolerance to organophosphates and carbamates indicated a limited
contribution of detoxifying enzymes toward resistance phenotypes. The authors
concluded that tolerance was likely to be driven by pyrethroid exposure given that
pyrethroids were heavily used in that agricultural region at the time of the study,
while DDT had not been utilized for two decades in the same region [62]. In a
subsequent study, Montagna et al. [61] showed that the basis for increased Simulium
spp. tolerance to DDT and fenvalerate in some populations from an agricultural area
was likely to be more complex than a kdr mutation alone could explain. In larval
toxicity challenges with fenvalerate or DDT in the presence of synergists PBO
(which inhibits monooxygenases) and tribufos (which inhibits esterases), the authors
found reduced tolerance to both DDT and fenvalerate with pre-treatment with PBO,
indicating that tolerance likely involved monooxygenase activity. Pre-treatment with
tribufos only marginally reduced the resistance phenotype to fenvalerate, but esterase activity in the tolerant population was nearly threefold higher than in the
sensitive population, indicating that esterase activity also played a role in the tolerant
phenotype. Despite the implication of metabolic enzymes in the tolerant phenotype,
a kdr-type mutation was still presumed to confer a portion of the tolerance, although
that mutation remained uncharacterized [61]. A third study on Simulium spp.
documented both pyrethroid (deltamethrin, 130–250-fold) and organophosphate
(azinphos methyl, 1.7–4.6-fold) tolerance in an agricultural population. Given that
pyrethroids had recently been replaced by organophosphates after nearly two
decades of consistent agricultural use, the authors highlighted the role of increased
esterases in the tolerant population as a mechanism of metabolic resistance that
confers resistance to both pyrethroids and organophosphates [60]. While the mechanism of increased tolerance to pyrethroids and DDT in black flies appears complex,
the high magnitude of resistance between agricultural and reference animals and the
inability of the metabolic enzymes to fully explain that tolerance suggests a kdr
mutation may be responsible for the partial loss in sensitivity. Further, the primary
use of pyrethroids in the agricultural region that harbors tolerant animals suggests
that pyrethroids have been responsible for driving that tolerance in some black fly
populations given that DDT had not been used in that region for two decades at the
time that tolerance was first documented. This suggests that DDT would have played
a minimal role in selecting for and then maintaining resistance in black flies. Given
that larval animals were taken directly from the field and challenged with toxicants,
their increased tolerance phenotypes could reflect a mixture of physiological acclimation, maternal effects, and adaptive resistance, which is supported by the complex
metabolic and potential kdr mutation tolerance mechanisms proposed to play a role
in resistance phenotypes [60–62]. While this marked decreased in Simulium spp.
sensitivity as a result of agricultural pesticide use cannot technically be termed
“resistance” by our strict definition, we conclude that increased tolerance in black
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K. M. Major and S. M. Brander
areas were significantly more tolerant to fenvalerate (88.2-fold), deltamethrin (90.0fold), cypermethrin (22.9-fold), and DDT (59.2-fold) compared with reference site
larvae. Given the high levels of DDT and pyrethroid tolerance in larvae from
agricultural sites, the authors concluded that a kdr- type mutation was likely. Further,
the lack of tolerance to organophosphates and carbamates indicated a limited
contribution of detoxifying enzymes toward resistance phenotypes. The authors
concluded that tolerance was likely to be driven by pyrethroid exposure given that
pyrethroids were heavily used in that agricultural region at the time of the study,
while DDT had not been utilized for two decades in the same region [62]. In a
subsequent study, Montagna et al. [61] showed that the basis for increased Simulium
spp. tolerance to DDT and fenvalerate in some populations from an agricultural area
was likely to be more complex than a kdr mutation alone could explain. In larval
toxicity challenges with fenvalerate or DDT in the presence of synergists PBO
(which inhibits monooxygenases) and tribufos (which inhibits esterases), the authors
found reduced tolerance to both DDT and fenvalerate with pre-treatment with PBO,
indicating that tolerance likely involved monooxygenase activity. Pre-treatment with
tribufos only marginally reduced the resistance phenotype to fenvalerate, but esterase activity in the tolerant population was nearly threefold higher than in the
sensitive population, indicating that esterase activity also played a role in the tolerant
phenotype. Despite the implication of metabolic enzymes in the tolerant phenotype,
a kdr-type mutation was still presumed to confer a portion of the tolerance, although
that mutation remained uncharacterized [61]. A third study on Simulium spp.
documented both pyrethroid (deltamethrin, 130–250-fold) and organophosphate
(azinphos methyl, 1.7–4.6-fold) tolerance in an agricultural population. Given that
pyrethroids had recently been replaced by organophosphates after nearly two
decades of consistent agricultural use, the authors highlighted the role of increased
esterases in the tolerant population as a mechanism of metabolic resistance that
confers resistance to both pyrethroids and organophosphates [60]. While the mechanism of increased tolerance to pyrethroids and DDT in black flies appears complex,
the high magnitude of resistance between agricultural and reference animals and the
inability of the metabolic enzymes to fully explain that tolerance suggests a kdr
mutation may be responsible for the partial loss in sensitivity. Further, the primary
use of pyrethroids in the agricultural region that harbors tolerant animals suggests
that pyrethroids have been responsible for driving that tolerance in some black fly
populations given that DDT had not been used in that region for two decades at the
time that tolerance was first documented. This suggests that DDT would have played
a minimal role in selecting for and then maintaining resistance in black flies. Given
that larval animals were taken directly from the field and challenged with toxicants,
their increased tolerance phenotypes could reflect a mixture of physiological acclimation, maternal effects, and adaptive resistance, which is supported by the complex
metabolic and potential kdr mutation tolerance mechanisms proposed to play a role
in resistance phenotypes [60–62]. While this marked decreased in Simulium spp.
sensitivity as a result of agricultural pesticide use cannot technically be termed
“resistance” by our strict definition, we conclude that increased tolerance in black
128
K. M. Major and S. M. Brander