1 Pyrethroid Insecticides
Chapter “Introduction to Pyrethroid Insecticides: Chemical Structures, Properties,
Mode of Action and Use” summarizes information related to the chemical structures,
properties, mode of action, and use of pyrethroid insecticides. They have been used
worldwide since the 1980s because of their high level of effectiveness and low
toxicity compared to other insecticides, such as organochlorine, organophosphorus,
and carbamic ester compounds. Pyrethroids are the most widely used insecticides
worldwide, accounting for about 25% of the pesticide use, and they are applied
in households, in commercial products, and in medicine. Several desirable
characteristics contribute to the commercial success of pyrethroids, including
their efficacy against a broad range of insect pests and mites, low mammalian
and avian toxicities, low potential to contaminate ground water, and relatively low
application rates. They were believed to be the ideal pesticides, since they are
not persistent and were thought to be metabolized and not bioaccumulated.
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 [1].
The routes of degradation may be abiotic, hydrolysis, photolysis and oxidation,
or mediated by bacteria and fungi. Therefore, they do not meet the requirements to
be considered persistent organic pollutants (POPs). However, the continuous use of
these insecticides in the different applications for which they are described makes
them ubiquitous in the different environmental compartments. That because they are
considered pseudo-POPs. In addition, different studies in both aquatic and terrestrial
biota have shown the presence of pyrethroids in different tissues and at not negligible
levels of concentration. Therefore, it is necessary to determine the relationship
between pyrethroid metabolization and bioaccumulation. Something similar has
been observed for humans. Finally, we must not forget the studies indicating the
diverse toxicity of these compounds. All these data suggests reconsidering
the theory that pyrethroids are the ideal insecticides (Fig. 1).
It is important to note that when we use the term pyrethroid insecticides, we
are encompassing a large number of different compounds that, despite having a
similar chemical structure, they have different physicochemical properties and, more
important, different toxicological effects. As can be seen throughout the different
chapters of this book, most studies focus on the same pyrethroids, such as bifenthrin,
cyfluthrin, cyhalothrin, cypermethrin, deltamethrin, esfenvalerate, or permethrin.
However, other pyrethroids such as allethrin, fluvalinate, imiprothrin, prallethrin,
or resmethrin have been less studied, and it would be convenient to have more
information also for these compounds.
The study of the different behavior of isomers and enantiomers of each
pyrethroid is also crucial, as reflected throughout chapter “Stereoselectivity and
Environmental Behaviour of Pyrethroids”. A stereochemical approach is required
to better understand the impacts of pyrethroids on the environment and on human
health. Upon entering the environment, the chiral pyrethroids undergo selective
enantiomeric bioaccumulation and degradation. And, as different toxicity has been
306
E. Eljarrat
Chapter “Introduction to Pyrethroid Insecticides: Chemical Structures, Properties,
Mode of Action and Use” summarizes information related to the chemical structures,
properties, mode of action, and use of pyrethroid insecticides. They have been used
worldwide since the 1980s because of their high level of effectiveness and low
toxicity compared to other insecticides, such as organochlorine, organophosphorus,
and carbamic ester compounds. Pyrethroids are the most widely used insecticides
worldwide, accounting for about 25% of the pesticide use, and they are applied
in households, in commercial products, and in medicine. Several desirable
characteristics contribute to the commercial success of pyrethroids, including
their efficacy against a broad range of insect pests and mites, low mammalian
and avian toxicities, low potential to contaminate ground water, and relatively low
application rates. They were believed to be the ideal pesticides, since they are
not persistent and were thought to be metabolized and not bioaccumulated.
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 [1].
The routes of degradation may be abiotic, hydrolysis, photolysis and oxidation,
or mediated by bacteria and fungi. Therefore, they do not meet the requirements to
be considered persistent organic pollutants (POPs). However, the continuous use of
these insecticides in the different applications for which they are described makes
them ubiquitous in the different environmental compartments. That because they are
considered pseudo-POPs. In addition, different studies in both aquatic and terrestrial
biota have shown the presence of pyrethroids in different tissues and at not negligible
levels of concentration. Therefore, it is necessary to determine the relationship
between pyrethroid metabolization and bioaccumulation. Something similar has
been observed for humans. Finally, we must not forget the studies indicating the
diverse toxicity of these compounds. All these data suggests reconsidering
the theory that pyrethroids are the ideal insecticides (Fig. 1).
It is important to note that when we use the term pyrethroid insecticides, we
are encompassing a large number of different compounds that, despite having a
similar chemical structure, they have different physicochemical properties and, more
important, different toxicological effects. As can be seen throughout the different
chapters of this book, most studies focus on the same pyrethroids, such as bifenthrin,
cyfluthrin, cyhalothrin, cypermethrin, deltamethrin, esfenvalerate, or permethrin.
However, other pyrethroids such as allethrin, fluvalinate, imiprothrin, prallethrin,
or resmethrin have been less studied, and it would be convenient to have more
information also for these compounds.
The study of the different behavior of isomers and enantiomers of each
pyrethroid is also crucial, as reflected throughout chapter “Stereoselectivity and
Environmental Behaviour of Pyrethroids”. A stereochemical approach is required
to better understand the impacts of pyrethroids on the environment and on human
health. Upon entering the environment, the chiral pyrethroids undergo selective
enantiomeric bioaccumulation and degradation. And, as different toxicity has been
306
E. Eljarrat