populations from biological control cotton-growing sites. While DDT cannot be
ruled out as a selective pressure, the authors suggested that pyrethroids are likely to
be causing selection for pyrethroid and DDT-resistant An. gambiae populations
given that pyrethroids, not DDT, were the recommended insecticides for cotton
farming in West Africa. A recent structured survey of farmers in North-East Benin
confirmed that the most reported insecticides used were pyrethroids and
organophosphates [137].
Two studies have provided more evidence of pyrethroids as likely drivers of
resistance in larval mosquitoes by collecting and analyzing environmental media
(water, sediments) for pesticides in addition to tracking pyrethroid-resistant phenotypes and kdr mutation frequencies in Anopheles mosquitoes. Hien et al. [63]
collected water and soil samples in pesticide-intensive cotton-growing agricultural
sites and biological control (or organic) cotton-growing sites in Burkina Faso. They
also collected larval mosquitoes from the same sites and subjected them to control
(spring water), biological cotton, or conventional cotton water samples to document
mortality at the larval stage. Larval mortality was the highest in conventional cotton
site waters (66.5%) and biological site waters (49.75%) and low in spring water
control (3%), indicating that agricultural site waters were toxic to larval mosquitoes.
Treatment with insecticide-impregnated filter papers (0.05% deltamethrin) for 1 h
followed by a 24 h recovery period showed that emergent adults were nominally
more resistant to deltamethrin at conventional cotton sites compared with biological
cotton sites (52.04% and 75.96% mortality, respectively), although that result was
not statistically significant. Importantly, the authors also documented that allele
frequencies of the L1014 kdr mutations were high (F ¼ 0.95, S ¼ 0.4) in resistant
populations. The L1014S mutation confers DDT and permethrin (Type I) resistance
based on voltage clamp analysis with modified Drosophila para Vgsc expressed in
Xenopus oocytes [138]. Soil samples taken at sites before seasonal pesticide treatments revealed trace amounts of compounds including diuron, benzoyprop-ethyl,
and fungicides chloroneb, pyridate, allethrin, and bromacil, mostly at low concentrations. Water samples taken after pesticide application but before harvest at
conventional cotton sites revealed deltamethrin and lambda cyhalothrin at high
levels (0.0147 μg L
À1 and 1.49 μg L
À1 , respectively), documenting a direct link
between agricultural pyrethroid use and selective pressure on larvae [63]. Notably,
the authors did not detect DDT in soil or water samples, suggesting that pyrethroids
are the primary drivers of resistance in these populations.
In a second study of larval mosquitoes, resistance mutations, and environmental
media, Kudom et al. [139] surveyed urban residential mosquito breeding sites in
Ghana and collected larval mosquitoes and water samples. Larval mosquitoes were
reared to adulthood and then challenged with pyrethroid-impregnated filter papers
containing either 0.05% deltamethrin, 0.75% permethrin, 0.15% cyfluthrin, or 0.5%
etofenprox for 1 h and allowed to recover for 24 h after which time mortality was
scored to determine resistance phenotype. Most mosquitoes were classified as
Anopheles coluzzii, with a minority being An. gambiae, and all resistant animals
were genotyped for L1014 resistance mutations. While water samples revealed that
pyrethroids, organochlorines, and organophosphates were present in most samples,
126
K. M. Major and S. M. Brander
ruled out as a selective pressure, the authors suggested that pyrethroids are likely to
be causing selection for pyrethroid and DDT-resistant An. gambiae populations
given that pyrethroids, not DDT, were the recommended insecticides for cotton
farming in West Africa. A recent structured survey of farmers in North-East Benin
confirmed that the most reported insecticides used were pyrethroids and
organophosphates [137].
Two studies have provided more evidence of pyrethroids as likely drivers of
resistance in larval mosquitoes by collecting and analyzing environmental media
(water, sediments) for pesticides in addition to tracking pyrethroid-resistant phenotypes and kdr mutation frequencies in Anopheles mosquitoes. Hien et al. [63]
collected water and soil samples in pesticide-intensive cotton-growing agricultural
sites and biological control (or organic) cotton-growing sites in Burkina Faso. They
also collected larval mosquitoes from the same sites and subjected them to control
(spring water), biological cotton, or conventional cotton water samples to document
mortality at the larval stage. Larval mortality was the highest in conventional cotton
site waters (66.5%) and biological site waters (49.75%) and low in spring water
control (3%), indicating that agricultural site waters were toxic to larval mosquitoes.
Treatment with insecticide-impregnated filter papers (0.05% deltamethrin) for 1 h
followed by a 24 h recovery period showed that emergent adults were nominally
more resistant to deltamethrin at conventional cotton sites compared with biological
cotton sites (52.04% and 75.96% mortality, respectively), although that result was
not statistically significant. Importantly, the authors also documented that allele
frequencies of the L1014 kdr mutations were high (F ¼ 0.95, S ¼ 0.4) in resistant
populations. The L1014S mutation confers DDT and permethrin (Type I) resistance
based on voltage clamp analysis with modified Drosophila para Vgsc expressed in
Xenopus oocytes [138]. Soil samples taken at sites before seasonal pesticide treatments revealed trace amounts of compounds including diuron, benzoyprop-ethyl,
and fungicides chloroneb, pyridate, allethrin, and bromacil, mostly at low concentrations. Water samples taken after pesticide application but before harvest at
conventional cotton sites revealed deltamethrin and lambda cyhalothrin at high
levels (0.0147 μg L
À1 and 1.49 μg L
À1 , respectively), documenting a direct link
between agricultural pyrethroid use and selective pressure on larvae [63]. Notably,
the authors did not detect DDT in soil or water samples, suggesting that pyrethroids
are the primary drivers of resistance in these populations.
In a second study of larval mosquitoes, resistance mutations, and environmental
media, Kudom et al. [139] surveyed urban residential mosquito breeding sites in
Ghana and collected larval mosquitoes and water samples. Larval mosquitoes were
reared to adulthood and then challenged with pyrethroid-impregnated filter papers
containing either 0.05% deltamethrin, 0.75% permethrin, 0.15% cyfluthrin, or 0.5%
etofenprox for 1 h and allowed to recover for 24 h after which time mortality was
scored to determine resistance phenotype. Most mosquitoes were classified as
Anopheles coluzzii, with a minority being An. gambiae, and all resistant animals
were genotyped for L1014 resistance mutations. While water samples revealed that
pyrethroids, organochlorines, and organophosphates were present in most samples,
126
K. M. Major and S. M. Brander