The Organochlorine Insecticides - Organic Pollutants: An Ecotoxicological Perspective - page 131

include resistance of pine mice ( Microtus pinetorum) to endrin (Webb et al. 1972)
and the resistance of mosquito fish ( Gambusia affinis) ) in the Mississippi River to
aldrin (Wells et al. 1973). In both cases, resistance was attributed to enhanced meta-
bolic detoxication by the resistant strain.
5.4 HexacHLorocycLoHexaneS
Hexachlorocyclohexanes (HCHs) have not caused the problems of bioconcentra-
tion or bioaccumulation associated with DDT or the cyclodienes, and they have not
been implicated in large-scale environmental problems. They will only be discussed
briefly here, drawing attention to certain differences from the foregoing groups.
The first commercially available HCH insecticide sometimes misleadingly called
benzene hexachloride (BHC) was a mixture of isomers, principally alpha HCH (65-
70%), beta HCH (7-10%), and gamma HCH (14-15%). Most of the insecticidal activ-
ity was due to the gamma isomer (Figure 5.1), a purified preparation of which (>99%
pure) was marketed as lindane. In Western countries, technical HCH was quickly
replaced by lindane, but in some other countries (e.g., China) the technical product,
which is cheaper and easier to produce, has continued to be used. HCH has been used
as a seed dressing, a crop spray, and a dip to control ectoparasites of farm animals. It
has also been used to treat timber against wood-boring insects.
Gamma HCH (Figure 5.1) is more polar and more water soluble than most other
OCs (Table 5.1), and is metabolized relatively rapidly to water-soluble products. The
metabolism of gamma HCH is complex, and involves both dehydrochlorination
reactions mediated by reduced glutathione (GSH) and hydroxylations mediated by
cytochrome P450. The main excreted metabolites are various chlorophenols in free
or conjugated forms, prominent among them are trichlorophenols (Environmental
Health Criteria 124). Gamma HCH appears to be rapidly eliminated by vertebrate
species, and residues in free-living vertebrates and invertebrates were found to be
low when the compound was widely used in agriculture (e.g., levels in livers of birds
of prey were 0.01-0.1 ppm).
Gamma HCH acts upon GABA receptors in a similar fashion to cyclodienes
(Eldefrawi and Eldefrawi 1990), and cross-resistance of insects between the two
types of insecticide is sometimes due to a mutant form of the receptor, as discussed
in Section 5.3.5.2. Acute oral LD 50 s of gamma HCH to rodents range from 55-250
mg/kg. Thus, it is of a similar order of toxicity as DDT but, on the whole, less toxic
than aldrin, dieldrin, or endrin. It has not been implicated in field mortalities of
birds, in contrast to cyclodienes. On the other hand, bats can be lethally poisoned
when exposed to lindane-treated wood (Boyd et al. 1988). This poses a risk to bats
roosting in lofts containing treated timber. Gamma HCH is highly toxic to fish, LC 50
values for fish falling into the range 0.02-0.09 mg/L. When used as a sheep dip,
there was a hazard to freshwater fish when sheep farmers discharged sheep dips into
neighboring water courses.
Of the other HCH isomers, the alpha and beta forms are less toxic than the gamma
form. However, the beta form is more persistent than the gamma form, and unaccept-
ably high residues have sometimes been reported in foods originating from countries
where technical HCH is still used (Environmental Health Criteria 123).