Muller [107] and references therein). In the peach potato aphid (Myzus persicae),
resistance to organophosphates and carbamates is mediated via a genetically based
increase in copy number of carboxylesterases. DNA methylation controls whether or
not esterase copies are expressed, thereby providing a heritable epigenetic mechanism that can silence esterase production in the absence of insecticide in the
environment [108, 109], potentially ameliorating fitness costs associated with
energy-intensive esterase production [110]. Altered global DNA methylation patterns have been correlated with insecticide sensitivity in mosquitoes through the F2
generation [111]. For pyrethroids specifically, decreased global methylation was
apparent in pyrethroid-resistant mites, suggesting that epigenetic control mechanisms may play a part in pyrethroid resistance [112], although the extent to which
those methylation changes are heritable has not been addressed. The evidence of
adaptive (genetic) and potentially adaptive (transgenerational epigenetic) features
associated with pesticide resistance is both abundant and rapidly expanding.
4.1.2 Sea Lice
In the aquatic environment, salmon fisheries have been employing pyrethroids, specifically deltamethrin and cypermethrin, as delousing agents (chemotherapeutants) for
commercially raised fish since the 1990s [113, 114]. Sea lice are copepod ectoparasites
in the family Caligidae that feed on the mucous, blood, and tissue of host fish [115] to
the detriment of the fish, causing outcomes including decreased size/weight,
suppressed immune function, and increased morbidity and/or mortality
[116]. Lepeophtheirus salmonis is the most frequently reported parasite for salmonids,
while those in the genus Caligus are more generalist sea lice, with C. elongatus as one
of the most frequently cited pests in the Northern Hemisphere [117]. In the Southern
Hemisphere, C. rogercresseyi is the primary species that infects salmonids in Chile
[118]. The most common method for delousing fish is a bath treatment that involves
enclosing submerged fish cages with a tarpaulin, applying the pesticide at a
recommended dose for a specific duration of time (on the order of 30 min to 1 h) and
then removing the tarpaulin, allowing the pesticide to disperse in the surrounding water
[119]. An appropriate dose is high enough to be toxic to the sea lice without eliciting
toxicity to the fish. Pyrethroids have been administered as bath treatments to kill sea lice
in Canada, Chile, the Faroe Islands, Ireland, Norway, and Scotland with treatment
failures reported beginning in the early 2000s [120]. Resistance of sea lice to pyrethroid
(and other chemical) treatments has negatively impacted the aquaculture industry and
has generally required increased pyrethroid use over time [121], which in turn may
negatively impact host fish. To overcome treatment failures from pyrethroid resistance,
pyrethroids are even sometimes combined with other classes of pesticides such as
avermectins (added to fish feed), organophosphates, and/or hydrogen peroxide (both as
bath treatments).
By the early 2000s, it was clear that pyrethroid treatments were becoming
less effective among some sea louse populations from regions where bath
treatments were common. Decreased pyrethroid sensitivity has been documented
120
K. M. Major and S. M. Brander
resistance to organophosphates and carbamates is mediated via a genetically based
increase in copy number of carboxylesterases. DNA methylation controls whether or
not esterase copies are expressed, thereby providing a heritable epigenetic mechanism that can silence esterase production in the absence of insecticide in the
environment [108, 109], potentially ameliorating fitness costs associated with
energy-intensive esterase production [110]. Altered global DNA methylation patterns have been correlated with insecticide sensitivity in mosquitoes through the F2
generation [111]. For pyrethroids specifically, decreased global methylation was
apparent in pyrethroid-resistant mites, suggesting that epigenetic control mechanisms may play a part in pyrethroid resistance [112], although the extent to which
those methylation changes are heritable has not been addressed. The evidence of
adaptive (genetic) and potentially adaptive (transgenerational epigenetic) features
associated with pesticide resistance is both abundant and rapidly expanding.
4.1.2 Sea Lice
In the aquatic environment, salmon fisheries have been employing pyrethroids, specifically deltamethrin and cypermethrin, as delousing agents (chemotherapeutants) for
commercially raised fish since the 1990s [113, 114]. Sea lice are copepod ectoparasites
in the family Caligidae that feed on the mucous, blood, and tissue of host fish [115] to
the detriment of the fish, causing outcomes including decreased size/weight,
suppressed immune function, and increased morbidity and/or mortality
[116]. Lepeophtheirus salmonis is the most frequently reported parasite for salmonids,
while those in the genus Caligus are more generalist sea lice, with C. elongatus as one
of the most frequently cited pests in the Northern Hemisphere [117]. In the Southern
Hemisphere, C. rogercresseyi is the primary species that infects salmonids in Chile
[118]. The most common method for delousing fish is a bath treatment that involves
enclosing submerged fish cages with a tarpaulin, applying the pesticide at a
recommended dose for a specific duration of time (on the order of 30 min to 1 h) and
then removing the tarpaulin, allowing the pesticide to disperse in the surrounding water
[119]. An appropriate dose is high enough to be toxic to the sea lice without eliciting
toxicity to the fish. Pyrethroids have been administered as bath treatments to kill sea lice
in Canada, Chile, the Faroe Islands, Ireland, Norway, and Scotland with treatment
failures reported beginning in the early 2000s [120]. Resistance of sea lice to pyrethroid
(and other chemical) treatments has negatively impacted the aquaculture industry and
has generally required increased pyrethroid use over time [121], which in turn may
negatively impact host fish. To overcome treatment failures from pyrethroid resistance,
pyrethroids are even sometimes combined with other classes of pesticides such as
avermectins (added to fish feed), organophosphates, and/or hydrogen peroxide (both as
bath treatments).
By the early 2000s, it was clear that pyrethroid treatments were becoming
less effective among some sea louse populations from regions where bath
treatments were common. Decreased pyrethroid sensitivity has been documented
120
K. M. Major and S. M. Brander