pyrethroids (especially permethrin at 1.283 μg L
À1 and deltamethrin at
0.370 μg L
À1 ) were present at high levels above regulatory threshold levels for
surface waters [140]. Conversely, most organochlorine and organophosphates were
found at lower levels, considered to be within regulatory thresholds. This was true
for the organochlorines DDT and methoxychlor, which also target the Vgsc
[106]. Further, mosquitoes sampled from urban sites were highly resistant across
all four pyrethroids (0–16.7% mortality) and harbored high frequencies of the
L1014F kdr mutation (0.935). While these studies do not definitively demonstrate
that pyrethroid exposure alone is driving resistance in mosquitoes, they provide
evidence that pyrethroids are prevalent at toxic levels in urban and agricultural larval
breeding grounds, which suggests that pyrethroids play a role in driving adaptive
resistance.
Low frequencies of the L1014F kdr mutation of some less sensitive An. gambiae
mosquitoes collected adjacent to conventional agricultural activity as larvae in the
field and then screened for pyrethroid resistance suggest that metabolic resistance
also exists in some areas [141]. In fact, many of the studies discussed above fail to
test for other mechanisms that may be contributing to resistance or tolerance.
Further, most studies that have documented pyrethroid resistance in mosquitoes
near agricultural and urban areas rely on the collection and testing of mosquitoes
coming directly from the field, which means that differences in sensitivity between
resistant and sensitive populations may reflect mechanisms including physiological
acclimation, maternal affects, and/or adaptive resistance (Fig. 2). However, the
studies discussed herein also screen for the kdr target site mutation at locus L1014
in the Vgsc because it has been implicated in the conferral of pyrethroid resistance
elsewhere [135, 142]. It is important to realize that these studies provide key data that
nontarget pyrethroid exposure drives adaptive resistance in Anopheles. They document (1) a pyrethroid-tolerant phenotype, (2) evidence of increased frequency of
well-documented resistance mutations in these populations near agricultural and
urban areas, and (3) a pyrethroid presence in acutely toxic levels in associated
environmental media.
4.2.2 Black Flies
Black flies (Simulium spp.) are another human health and livestock disease vector
and pest worldwide that have demonstrated decreased pyrethroid sensitivity attributed to nontarget exposure to agricultural spray drift and runoff. Larvae from fruit
production agricultural irrigation channels in Northern Patagonia (Argentina) have
demonstrated up to 400-fold decreased sensitivity to deltamethrin and fenvalerate
relative to field-reference larvae during controlled laboratory exposures [60, 62]. The
source of that decreased sensitivity has been suggested to be target site insensitivity
in the form of a kdr resistance mutation [62, 63] and/or increased esterase and
monooxygenase activity [60, 61]. In the first study, larval black flies were taken
from agricultural and reference areas and then subjected to 24-h water-only toxicity
challenges with organophosphates, carbamates, pyrethroids (cypermethrin,
The Ecological and Evolutionary Implications of Pyrethroid Exposure: A New. . .
127
À1 and deltamethrin at
0.370 μg L
À1 ) were present at high levels above regulatory threshold levels for
surface waters [140]. Conversely, most organochlorine and organophosphates were
found at lower levels, considered to be within regulatory thresholds. This was true
for the organochlorines DDT and methoxychlor, which also target the Vgsc
[106]. Further, mosquitoes sampled from urban sites were highly resistant across
all four pyrethroids (0–16.7% mortality) and harbored high frequencies of the
L1014F kdr mutation (0.935). While these studies do not definitively demonstrate
that pyrethroid exposure alone is driving resistance in mosquitoes, they provide
evidence that pyrethroids are prevalent at toxic levels in urban and agricultural larval
breeding grounds, which suggests that pyrethroids play a role in driving adaptive
resistance.
Low frequencies of the L1014F kdr mutation of some less sensitive An. gambiae
mosquitoes collected adjacent to conventional agricultural activity as larvae in the
field and then screened for pyrethroid resistance suggest that metabolic resistance
also exists in some areas [141]. In fact, many of the studies discussed above fail to
test for other mechanisms that may be contributing to resistance or tolerance.
Further, most studies that have documented pyrethroid resistance in mosquitoes
near agricultural and urban areas rely on the collection and testing of mosquitoes
coming directly from the field, which means that differences in sensitivity between
resistant and sensitive populations may reflect mechanisms including physiological
acclimation, maternal affects, and/or adaptive resistance (Fig. 2). However, the
studies discussed herein also screen for the kdr target site mutation at locus L1014
in the Vgsc because it has been implicated in the conferral of pyrethroid resistance
elsewhere [135, 142]. It is important to realize that these studies provide key data that
nontarget pyrethroid exposure drives adaptive resistance in Anopheles. They document (1) a pyrethroid-tolerant phenotype, (2) evidence of increased frequency of
well-documented resistance mutations in these populations near agricultural and
urban areas, and (3) a pyrethroid presence in acutely toxic levels in associated
environmental media.
4.2.2 Black Flies
Black flies (Simulium spp.) are another human health and livestock disease vector
and pest worldwide that have demonstrated decreased pyrethroid sensitivity attributed to nontarget exposure to agricultural spray drift and runoff. Larvae from fruit
production agricultural irrigation channels in Northern Patagonia (Argentina) have
demonstrated up to 400-fold decreased sensitivity to deltamethrin and fenvalerate
relative to field-reference larvae during controlled laboratory exposures [60, 62]. The
source of that decreased sensitivity has been suggested to be target site insensitivity
in the form of a kdr resistance mutation [62, 63] and/or increased esterase and
monooxygenase activity [60, 61]. In the first study, larval black flies were taken
from agricultural and reference areas and then subjected to 24-h water-only toxicity
challenges with organophosphates, carbamates, pyrethroids (cypermethrin,
The Ecological and Evolutionary Implications of Pyrethroid Exposure: A New. . .
127