Environmental Engineering Reference
In-Depth Information
which means that when it is applied it enters into a plant, is transported by the sap, and when insects
or other pests feed on other parts of the plant, they become poisoned.
The chemical structure of carbofuran is shown in Figure 1.1. As a group, carbamates can be
classifi ed into N-methyl carbamates of phenols (e.g., carbofuran, carbaryl (Figure 1.5) and pro-
poxur) and the N-methyl carbamates of oximes (e.g., aldicarb and methomyl). These carbamates
can be synthesised from the reaction of methyl isocyanate with the hydroxyl group of phenols and
oximes. The biological activity of these carbamates comes from their ability to essentially liberate
methyl isocyanate (MIC) inside the organism. Methyl isocyanate is quite reactive (i.e., toxic) and
binds to enzymes that have reactive sulfhydro (RSH) and hydroxy (OH) groups. Since the activity
of enzymes often relies on such groups repeatedly making and breaking bonds many thousand of
times a second, the enzymes become inactive (inhibited). MIC is the industrial compound that was
released into the air in 1984 in Bhopal (India) and caused the death of between 3 000 and 15 000
people and injured over half a million people (see also Chapter 4).
Other important pesticide groups include the organophosphorus pesticides (e.g., monocrotophos
(Figure 1.6), dimethoate, diazinon and phosalone), and the organochlorines (e.g., DDT (Figure 1.7),
aldrin, its metabolite dieldrin (Figure 1.8), and endrin), often abbreviated as 'OP/OPCs' or 'OCs',
respectively. Carbamates (often abbreviated as 'CMs' or 'CBs') and organophosphorus compounds
both have a non-discriminate (or broad-spectrum) mode of action, i.e., one that inhibits cholinesterase
enzyme activity in insects, mammals and birds. For this reason they are sometimes referred to as 'anti-
cholinesterases'. Involved in virtually all physiological responses and mechanisms, no other enzyme is
thought to perform such a complex or extensive set of functions within the animal kingdom. The mecha-
nism by which cholinesterase inhibition occurs and its clinical impact on avian and mammalian wildlife,
are further detailed in Chapter 2, which also discusses relevant diagnostic and rehabilitation measures.
It is this broad spectrum of activity that also makes carbofuran an ideal insecticide, acaricide
(against ticks and mites) and nematicide (against nematodes). Plant protection products contain-
ing carbofuran as the active ingredient (often denoted as 'ai' or 'AI') have been used worldwide
to control pests in sugarcane, sugar beet, maize, coffee and rice crops. Carbofuran is available in
liquid, silica-based granular and corncob formulations (further discussed in Chapter 8). Sand, clay
or granulated dried corncob formulations are intended to enable the active ingredient to be released
more slowly into the rhizosphere, the zone immediately surrounding the roots of a developing plant.
As such, carbofuran is particularly effective in controlling rice pests such as green leafhoppers
(
Nephotettix virescens
), brown plant hoppers (
Nilaparvata lugens
) and more generally, stem borers
and whorl maggots. This is because leaf hoppers and plant hoppers are piercing-sucking phloem
feeders, and carbofuran is phloem systemic and therefore available in the phloem sap. Other pests,
even if resistant to organophosphorus insecticides (e.g., white fl ies, leaf miners, ants, scale insects,
cockroaches, wasps and aphids), can be effectively controlled by carbofuran. Although both organo-
phosphates and carbamates have the same mode of action, different organisms can be resistant to one
class of compound but not necessarily resistant to the other.
Unfortunately, for reasons which remain unclear, birds in particular are simply not equipped to
detoxify (or effectively metabolise) either carbamate or organophosphorus compounds before suc-
cumbing to their toxic effect (Mineau 2009). Consequently, such 'general biocides' are now increas-
ingly viewed as 'old-fashioned' and, as such, are very slowly being phased out and replaced by
new compounds that
do
discriminate between target insects and non-target organisms or wildlife.
Such compounds are therefore inherently less 'ecotoxic', but may still have signifi cant unintended
impacts on benefi cial non-target insects, among others. For example, imidacloprid, a nicotinic sys-
temic insecticide, was introduced as a 'less toxic' replacement. While not very toxic to animals in
general (see www.pesticidemanual.com/ and http://www.beekeeping.com/articles/us/imidacloprid_
bayer.htm), studies have indicated that exposure to sublethal levels slows mobility and communica-
tion capacity in honeybees (e.g., Medrzycki, Montanari, Bortolotti et al. 2003).
Search WWH ::
Custom Search