The Organochlorine Insecticides - Organic Pollutants: An Ecotoxicological Perspective - page 116

in resistant strains, which is insensitive to DDT (Salgado 1999). As explained earlier
(Section 5.2.4), axonal Na + channels represent the normal target for p,p ′-DDT (and, inci-
dentally, for pyrethroid insecticides). Interestingly, insects developing kdr or super kdr
to DDT usually show cross-resistance to pyrethroids. In both cases, the resistant insects
have insensitive forms of the target site. Knockdown resistance has also been reported
in a number of other species exposed to DDT or pyrethroids or both, including Heliothis
virescens, Plutella xylostella, Blatella germanica , Anopheles gambiae , and Myzus per-
sicae . The appearance of resistant strains such as these can give valuable retrospective
evidence of the environmental impact of pollutants. Assays for resistance mechanisms
and the genes that operate them are valuable tools in ecotoxicology.
5.3 tHe cycLodIene InSectIcIdeS
Insecticides belonging to this group are derivatives of hexachlorocyclopentadiene, syn-
thesized by the Diels-Alder reaction (see Brooks 1974). They did not come into use
until the early 1950s and not to any important extent in Europe or North America before
the mid-1950s. Thus, they did not begin to produce environmental side effects until
at least 6 years after the onset of DDE-induced eggshell thinning in sparrow hawks,
peregrines, and bald eagles, as described in Section 5.2.5.1. The cyclodienes include
dieldrin, aldrin, heptachlor, endrin, chlordane, endosulfan, telodrin, chlordecone, and
mirex. Some of them have only been used to a limited extent, and the following account
will be restricted to dieldrin, aldrin, heptachlor, endrin, telodrin, and chlordane, com-
pounds that have caused most concern about environmental side effects.
5.3.1 c h e m i c a l p r o p e r T i e s
The cyclodienes are stable solids having low water solubility and marked lipophilic-
ity (Table 5.1), and their active ingredients have cage structures (Figure 5.5). The
technical insecticides aldrin and dieldrin contain, respectively, the molecules HHDN
and HEOD as their active ingredients. HHDN and HEOD are abbreviations of their
formal chemical names (structures given in Figure 5.5). Here, the term “aldrin” will
be used synonymously with HHDN, and the term “dieldrin” will be used synony-
mously with HEOD, unless otherwise indicated, thus following the common practice
in the literature. Similarly, the common names of the other cyclodienes (e.g., hep-
tachlor and endrin) will be used to refer to the chemical structures of the principal
insecticidal ingredients of the technical products.
Of the examples given in Table 5.1, aldrin and heptachlor have the lowest water
solubilities and the highest vapor pressures. They are readily oxidized, both chemi-
cally and biochemically, to their epoxides—dieldrin and heptachlor epoxide,
respectively (Figure 5.5). (It should be noted that dieldrin has been marketed as an
insecticide in its own right, whereas heptachlor epoxide has not.) The two epoxides
have greater polarity, and consequently greater water solubility and lower volatil-
ity, than their precursors. Endrin is also an epoxide—in fact, it is a stereoisomer of
dieldrin—and has greater water solubility and lower vapor pressure than aldrin or
heptachlor. These relationships illustrate the importance of the electron-withdrawing
power of oxygen atoms in determining the properties of organic compounds.