Chemistry Reference
In-Depth Information
(
Milvus milvus
) and the golden eagle (
Aquila chrysaetos
), is illegal, and gamekeep-
ers following this practice have been prosecuted and fined.
Although OPs have mainly been used for pest or vector control on land, there
has been limited use of them in the aquatic environment, for example, to control
parasites of salmon farmed in the marine environment (Grant 2002). Dichlorvos
and azamethiphos have been used for this purpose, although this practice has been
restricted by legislation to protect the environment in certain countries. OPs of
relatively low mammalian toxicity (e.g., malathion) have sometimes been released
into surface waters to control insect pests, for example, in water cress beds. Apart
from the very small direct application of OPs to surface waters, there is continuing
concern about unintentional contamination. Overspraying of surface waters, runoff
from land, and movement of insecticides through fissures in agricultural soil and so
into water courses are all potential sources of contamination with OPs, as indeed
they are for agricultural pesticides more generally.
OPs are often applied as sprays. Commonly, the formulations used for spraying
are emulsifiable concentrates, where the OP is dissolved in an organic liquid that acts
as a carrier. OPs are also used as seed dressings and as components of dips used to
protect livestock against ectoparasites. Some highly toxic OPs have been incorpo-
rated into granular formulations for application to soil or to certain crops.
Some OPs, such as chlorfenvinphos, are more persistent than most, having greater
chemical stability and lower vapor pressures than is usual. Such compounds have
been used where some persistence in the soil is desirable, as in the case of insecti-
cidal seed dressings. Also, some OPs have been formulated in a way that increases
their persistence. Thus, the highly toxic compounds disyston and phorate are formu-
lated as granules for application to soil or directly to certain crops. The insecticides
are incorporated within a granular matrix from which they are only slowly released,
to become exposed to the usual processes of chemical and biochemical degrada-
tion. Insecticidal action may thereby be prolonged for a period of 2-3 months, much
longer than would occur if they were formulated in other ways (e.g., as emulsifiable
concentrates), where release into the environment is more rapid.
Notwithstanding the limited persistence of OPs generally, and the fact that they
do not tend to biomagnify in the higher trophic levels, they have sometimes been
implicated in the poisoning of predatory birds (for examples from the United States,
United Kingdom, and Canada, see Mineau et al. 1999). Most reported cases have
involved OPs of very high acute toxicity. Cases of poisoning as the result of approved
use of insecticides have been explained on the grounds of a few predisposing causes.
These have included direct contact of predators with spray residues and consump-
tion of prey carrying sufficiently high pesticide burdens to poison the predators. The
latter may be the consequence of prey (e.g., large insects or earthworms), immedi-
ately after OP spraying, carrying quantities of insecticide externally which are far
in excess of the levels needed to poison them. If predation occurs very soon after
exposure of prey to OP, tissue levels of insecticide in prey may sometimes be high
enough to cause poisoning because there has been insufficient time for effective
detoxication. Even though insects generally are poor vectors of insecticides because
of their sensitivity to them, some strains have acquired resistance to OPs as they have
insensitive forms of ChE (see Section 10.2.4) so are able to tolerate relatively high
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