Pyrethroid Insecticides - Organic Pollutants: An Ecotoxicological Perspective - page 238

a need here for combining ecological profiling with chemical analysis, to facilitate
the detection of chemicals that cause changes in community structure in natural
waters.
12.6.2 p o p u l a T i o n g e n e T i c s
The continuing use of pyrethroids in agriculture has led to the emergence of resistant
strains of pests. One of the best-studied examples is the tobacco budworm ( Heliothis
virescens ), a very serious pest of cotton in the southern United States (McCaffery
1998). Indeed, the resistance problem has sometimes been severe enough to threaten
a loss of control over the pest. A study of a number of resistant strains from the field
has revealed two major types of resistance mechanism. Some individuals possess
aberrant forms of the target site, the Na + channel. At least two forms are known
that confer either “kdr” (<100-fold) or “super kdr” (>100-fold) resistance, which
is the consequence of the presence of insensitive forms of the Na + channel protein
(McCaffery 1998, and Chapter 4, Section 4.4 of this topic).
This type of resistance has been found in a number of species of insects, includ-
ing Musca domestica, Heliothis virescens, Plutella xylostella, Blatella germanica,
Anopheles gambiae, and Myzus persicae. Kdr has been attributed to three different
changes of single amino acids of the voltage-dependent sodium channel, and super
kdr to changes in pairs of amino acids, also located in the sodium channel (Salgado
1999). Interestingly, it appears that earlier selective pressure by dichlorodiphenyl
trichloroethene (DDT) raised the frequency of kdr genes in the population before
pyrethroids came to be used. Thus, some “pyrethroid resistance” already existed
before these insecticides were applied in the field.
The other major mechanism of pyrethroid resistance found in some field strains of
Heliothis virescens was enhanced detoxication due to a high rate of oxidative detoxi-
cation, mediated by a form of cytochrome P450 (McCaffery 1998). Some strains,
such as PEG 87, which was subjected to a high level of field and laboratory selec-
tion, possessed both mechanisms. Other example of pyrethroid resistance due to
enhanced detoxication may be found in the literature on pesticides.
12.7 Summary
Pyrethroid insecticides were modeled upon naturally occurring pyrethrins, which
were once quite widely used as insecticides but had the disadvantages of being photo-
chemically unstable and susceptible to rapid metabolic detoxication. Pyrethroids are
more stable than pyrethrins and, like DDT, act upon the voltage-dependent sodium
channel of the nerve axon. They are lipophilic but are readily biodegradable by most
organisms of higher trophic levels. Although they can undergo bioconcentration in
the lower trophic levels of aquatic food chains, unlike OC insecticides, they are not
prone to biomagnification in the upper trophic levels of either aquatic or terrestrial
food chains. They are, however, strongly adsorbed in soils and sediments where they
can be persistent.
Pyrethroids are much more toxic to invertebrates than to most vertebrates. They can
have serious effects upon aquatic invertebrates, at least in the short term. They can be