Impact of - IDL-BNC @ IDRC - International Development Research ...

Progress in improving safety
Pesticides in use at the turn of the century typically had a broad spectrum of
biological activity and a low level of pesticidal activity. They were not particularly
toxic to humans, but had to be applied in large quantities (compared
with current application rates) to compensate for their low potency. The
resulting human health hazard could, therefore, become significant.
Research has focused primarily on improving selectivity, i.e., enhancing the
pesticidal activity on target species while leaving nontarget species, including
man, more or less untouched. In the process, a higher efficacy is usually also
obtained.
Qualitative selectivity is achieved when a pesticide interferes with a process
or processes that occur only in the target species. For example, inhibition (by
an acylurea pesticide) of chitin formation, which is essential for the formation
of the legs, shield, etc., of insects, does not affect mammals. Interfering with
photosynthesis in weeds does not harm birds and mammals as these organisms
do not photosynthesize. Herbicides may, therefore, have a favourable
toxicity profile (Table 2).
Quantitative selectivity, on the other hand, is based on differences between
the target and nontarget species that determine susceptibility, e.g., skinpenetration
rate, metabolism (toxification or detoxification), and excretion
rate. A drawback of quantitative selectivity is the possibility that the target
species becomes resistant. The properties that make nontarget species less
susceptible to the pesticide may, in a number of generations, be acquired by
the target species through genetic selection. Oppenoorth and Besemer
Table 2. Comparative toxicity of several herbicides, aspirin,
and caffeine.
Product
LD5o in rats
(mg/kg)
Caffeine 200
Alachlor 1 200
Cyanazine 1 200
Aspirin 1 750
Metribu2in 2 200
Terbutryme 2 380
Flamprop-isopropyl >3000
Glyphosate 4 320
Terbacil 5 000
Asulam > 5 000
Imazapyr > 5 000
Sulfometuron-methyl > 5 000
Chlortoluron
Source: Graham-Bryce (1990).
> 10 000
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