mutation has been associated with pyrethroid and DDT resistance in houseflies [135]
and was later identified in pyrethroid- resistant mosquitoes [136]. In addition to
recommending resistance screening with insecticide-impregnated papers, the WHO
also recommends screening for the L1014 resistance kdr mutations to aid in data
collection for documenting the extent of insecticide resistance in malaria vectors. As
a result, a majority of the studies that provide evidence of adaptive resistance to
pyrethroids (as we have defined it) cannot, with certainty, attribute that resistance
development to pyrethroid selective pressures alone, because these populations
harbor a resistance mutation that is common in DDT-resistant populations, and
would also result from selection pressures exerted by DDT in the environment.
Still, the evidence of adaptive pyrethroid resistance in larval mosquitoes receiving
nontarget insecticide input is discussed below.
Diabate and colleagues [56] collected An. gambiae s.l. as larvae from four
different types of field sites in Burkina Faso including near cotton-growing regions
where pyrethroids are common agrochemicals, near an urban area where pyrethroid
use is common, and reference sites where pyrethroid use is uncommon. The authors
kept the larvae in a laboratory setting until the emergence of adults, at which time
they were challenged with filter paper containing 1% permethrin, 0.05%
deltamethrin, or 4% DDT as recommended by WHO protocols and animals, was
also monitored for common kdr mutations. These collections and tests were
performed over 2 years (1999 and 2000) in both dry and rainy seasons to elucidate
temporal trends. The authors found an increase in resistance to permethrin KDT 50
(threefold to fourfold) in cotton-growing and urban areas compared with reference
sites. In addition to pyrethroid resistance, DDT resistance (4- to 40-fold) was also
noted in cotton-growing and urban areas compared with reference sites. These
resistance phenotypes were associated with a marked increase in the leucine-tophenylalanine (L1014F; M. domestica) kdr allele frequencies in the vgsc (cottongrowing ¼ 0.896, urban ¼ 0.956, control ¼ 0.18). Resistance in urban areas was
attributed to coil and bomb use, while the intensive agrochemical use in cotton areas
explained the resistance increase in cotton areas. Further, in the dry season when
fewer pesticides are used, An. gambiae populations from cotton-growing areas were
more sensitive than during the wet season, when selective pressures are greater.
In a study in Northern Benin, Yadouleton et al. [59] collected An. gambiae larvae
from cotton production areas with different pest control regimes: two that involved
pesticide use and a third that only involved biological control measures (e.g.,
Bacillus thuringiensis). Larvae were sampled and then reared to adulthood for
sensitivity screening with 0.75% permethrin, 0.05% deltamethrin, or 4% DDT
insecticide-impregnated papers. Animals from cotton-growing agricultural regions
that used insecticides had increased KDT 50 s (up to 3.2-fold) for permethrin compared to those from cotton-growing regions with only biological control and the
reference laboratory population. A similar trend was noted with DDT (up to 2.5-fold
resistance), with elevated KDT 50 s from animals in sites with agricultural insecticide
use compared with biological control sites and control laboratory reference
populations. L1014F mutation frequencies were the highest among populations
from conventional pesticide use areas (0.51–0.78) and lowest (0.32–0.35) in
The Ecological and Evolutionary Implications of Pyrethroid Exposure: A New. . .
125
and was later identified in pyrethroid- resistant mosquitoes [136]. In addition to
recommending resistance screening with insecticide-impregnated papers, the WHO
also recommends screening for the L1014 resistance kdr mutations to aid in data
collection for documenting the extent of insecticide resistance in malaria vectors. As
a result, a majority of the studies that provide evidence of adaptive resistance to
pyrethroids (as we have defined it) cannot, with certainty, attribute that resistance
development to pyrethroid selective pressures alone, because these populations
harbor a resistance mutation that is common in DDT-resistant populations, and
would also result from selection pressures exerted by DDT in the environment.
Still, the evidence of adaptive pyrethroid resistance in larval mosquitoes receiving
nontarget insecticide input is discussed below.
Diabate and colleagues [56] collected An. gambiae s.l. as larvae from four
different types of field sites in Burkina Faso including near cotton-growing regions
where pyrethroids are common agrochemicals, near an urban area where pyrethroid
use is common, and reference sites where pyrethroid use is uncommon. The authors
kept the larvae in a laboratory setting until the emergence of adults, at which time
they were challenged with filter paper containing 1% permethrin, 0.05%
deltamethrin, or 4% DDT as recommended by WHO protocols and animals, was
also monitored for common kdr mutations. These collections and tests were
performed over 2 years (1999 and 2000) in both dry and rainy seasons to elucidate
temporal trends. The authors found an increase in resistance to permethrin KDT 50
(threefold to fourfold) in cotton-growing and urban areas compared with reference
sites. In addition to pyrethroid resistance, DDT resistance (4- to 40-fold) was also
noted in cotton-growing and urban areas compared with reference sites. These
resistance phenotypes were associated with a marked increase in the leucine-tophenylalanine (L1014F; M. domestica) kdr allele frequencies in the vgsc (cottongrowing ¼ 0.896, urban ¼ 0.956, control ¼ 0.18). Resistance in urban areas was
attributed to coil and bomb use, while the intensive agrochemical use in cotton areas
explained the resistance increase in cotton areas. Further, in the dry season when
fewer pesticides are used, An. gambiae populations from cotton-growing areas were
more sensitive than during the wet season, when selective pressures are greater.
In a study in Northern Benin, Yadouleton et al. [59] collected An. gambiae larvae
from cotton production areas with different pest control regimes: two that involved
pesticide use and a third that only involved biological control measures (e.g.,
Bacillus thuringiensis). Larvae were sampled and then reared to adulthood for
sensitivity screening with 0.75% permethrin, 0.05% deltamethrin, or 4% DDT
insecticide-impregnated papers. Animals from cotton-growing agricultural regions
that used insecticides had increased KDT 50 s (up to 3.2-fold) for permethrin compared to those from cotton-growing regions with only biological control and the
reference laboratory population. A similar trend was noted with DDT (up to 2.5-fold
resistance), with elevated KDT 50 s from animals in sites with agricultural insecticide
use compared with biological control sites and control laboratory reference
populations. L1014F mutation frequencies were the highest among populations
from conventional pesticide use areas (0.51–0.78) and lowest (0.32–0.35) in
The Ecological and Evolutionary Implications of Pyrethroid Exposure: A New. . .
125