As neurotoxicants, pyrethroids elicit their primary toxic mode of action in insects
by acting on the voltage-gated sodium channel (Vgsc). In the central and peripheral
nervous system, pyrethroids prevent the Vgsc from closing, causing repeated firing
of the neurons, leading eventually to paralysis, known as the “knockdown” phenotype, and death. To a lesser extent, pyrethroids also interact with a variety of other
sites including voltage-sensitive calcium and chloride channels [9–13]. Based on
chemical structure and mammalian (rat, mouse) toxicity phenotypes, pyrethroids are
broadly classified into two types: Type I or Type II [14]. Type II pyrethroids
(deltamethrin, cismethrin, esfenvalerate, λ-cyhalothrin, cyfluthrin, fenpropathrin)
have an α-cyano-3-phenoxybenzyl moiety, while Type I pyrethroids (S-bioallethrin,
cypermethrin permethrin, tefluthrin, bifenthrin) do not [15]. In general, Type I
pyrethroids tend to be reserved for urban use, while Type II pyrethroids are used
in agriculture [2]. Type II pyrethroids also produce a distinctive convulsive phenotype in invertebrates [16] and cause prolonged channel opening compared to Type I
pyrethroids [17].
In soils, most pyrethroids have half-lives ranging between 30 and 100 days, and
their hydrolysis in the aquatic compartment occurs on the order of days to weeks
[18]. Pyrethroids have high n-octanol-water partition coefficients (K ow ), with
log K ow values ranging from roughly 4 to 7.54, indicating that these chemicals are
much more likely to partition into the sediment and sorb to particulate organic matter
than to remain in the water column [19]. Despite being highly lipophilic, pyrethroids
may remain in the water column for days to weeks after introduction [20, 21] and are
soluble enough to produce biological and toxic effects at low dissolved concentrations [11, 22]. Because they are lipophilic, pyrethroids bioaccumulate in both fishes
and marine mammals. A recent study conducted in Spain found pyrethroids in 100%
of tissue samples collected from riverine fish [23, 24]. These insecticides also adsorb
to and persist in sediments [25] and associate with other environmental compartments such as algae [26].
Although they are still detected less frequently in the environment worldwide
than organochlorine- and organophosphate-based products [27], pyrethroids are
prevalent in aquatic ecosystems and are often found at levels sufficient to cause
toxicity to aquatic invertebrates [2, 6, 25, 28–31]. Pyrethroids are used ubiquitously
in agricultural and residential areas, primarily entering as runoff into the aquatic
compartment, but also through spray drift as well. Pyrethroids are also ubiquitous in
treated wastewater effluent, mostly due to high urban use for pest control in homes
[32, 33]. Historically, some pyrethroids were added to water directly as mosquito
and black fly larvicides [34–36], but their toxicity, hydrophobicity, and sediment
persistence have since been restricted their direct use in aquatic environments.
However, in aquaculture, pyrethroids are still added directly to the water as
chemotherapeutants to remove parasites from farmed fish [37] and shrimp [38].
While the relatively low mammalian toxicity of pyrethroids has fueled their
popularity and increased usage over the past few decades, pyrethroids are highly
toxic to fish and aquatic invertebrates at low part per billion or parts per trillion
concentrations. Toxicity to aquatic organisms is particularly problematic following
storm events, which transport residentially applied pyrethroids into local streams and
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K. M. Major and S. M. Brander
by acting on the voltage-gated sodium channel (Vgsc). In the central and peripheral
nervous system, pyrethroids prevent the Vgsc from closing, causing repeated firing
of the neurons, leading eventually to paralysis, known as the “knockdown” phenotype, and death. To a lesser extent, pyrethroids also interact with a variety of other
sites including voltage-sensitive calcium and chloride channels [9–13]. Based on
chemical structure and mammalian (rat, mouse) toxicity phenotypes, pyrethroids are
broadly classified into two types: Type I or Type II [14]. Type II pyrethroids
(deltamethrin, cismethrin, esfenvalerate, λ-cyhalothrin, cyfluthrin, fenpropathrin)
have an α-cyano-3-phenoxybenzyl moiety, while Type I pyrethroids (S-bioallethrin,
cypermethrin permethrin, tefluthrin, bifenthrin) do not [15]. In general, Type I
pyrethroids tend to be reserved for urban use, while Type II pyrethroids are used
in agriculture [2]. Type II pyrethroids also produce a distinctive convulsive phenotype in invertebrates [16] and cause prolonged channel opening compared to Type I
pyrethroids [17].
In soils, most pyrethroids have half-lives ranging between 30 and 100 days, and
their hydrolysis in the aquatic compartment occurs on the order of days to weeks
[18]. Pyrethroids have high n-octanol-water partition coefficients (K ow ), with
log K ow values ranging from roughly 4 to 7.54, indicating that these chemicals are
much more likely to partition into the sediment and sorb to particulate organic matter
than to remain in the water column [19]. Despite being highly lipophilic, pyrethroids
may remain in the water column for days to weeks after introduction [20, 21] and are
soluble enough to produce biological and toxic effects at low dissolved concentrations [11, 22]. Because they are lipophilic, pyrethroids bioaccumulate in both fishes
and marine mammals. A recent study conducted in Spain found pyrethroids in 100%
of tissue samples collected from riverine fish [23, 24]. These insecticides also adsorb
to and persist in sediments [25] and associate with other environmental compartments such as algae [26].
Although they are still detected less frequently in the environment worldwide
than organochlorine- and organophosphate-based products [27], pyrethroids are
prevalent in aquatic ecosystems and are often found at levels sufficient to cause
toxicity to aquatic invertebrates [2, 6, 25, 28–31]. Pyrethroids are used ubiquitously
in agricultural and residential areas, primarily entering as runoff into the aquatic
compartment, but also through spray drift as well. Pyrethroids are also ubiquitous in
treated wastewater effluent, mostly due to high urban use for pest control in homes
[32, 33]. Historically, some pyrethroids were added to water directly as mosquito
and black fly larvicides [34–36], but their toxicity, hydrophobicity, and sediment
persistence have since been restricted their direct use in aquatic environments.
However, in aquaculture, pyrethroids are still added directly to the water as
chemotherapeutants to remove parasites from farmed fish [37] and shrimp [38].
While the relatively low mammalian toxicity of pyrethroids has fueled their
popularity and increased usage over the past few decades, pyrethroids are highly
toxic to fish and aquatic invertebrates at low part per billion or parts per trillion
concentrations. Toxicity to aquatic organisms is particularly problematic following
storm events, which transport residentially applied pyrethroids into local streams and
112
K. M. Major and S. M. Brander