90
N. W. MOORE
changes in vegetation (Thomas, 1960) and some changes in animal
populations (Moore, 1956). Yet this disease was virtually restricted to
the rabbit. A specific pesticide which eliminated the grey squirrel, the
woodpigeon, the grey rat or bracken, for example, would be likely
to produce similar fundamental changes in Britain. These might be
complicated.
However, most pesticides are not specific: most of the pesticide
problems of agriculture, conservation and medicine arise from this
fact. The reasons for the rarity of specific pesticides are these. New
pesticides are largely discovered by trial and error - by subjecting test
organisms to a very wide range of chemical substances. The chances of
specific pesticides being found in this way are very slight. The systematic discovery of specific pesticides would depend on knowing more
about specific biochemical features of the pest species. In general such
basic information is lacking and the cost of the necessary fundamental
research would be considerable. The incentive to find specific pesticides
may be slight because the profits resulting from the sale of specific
pesticides are likely to be less than those from the sale of wide-spectrum
ones. To conclude, non-specific pesticides are likely to be used for inany
years to come and their effects are likely to be complicated.
There is a complete range of type of effect from total to specific
effect. A few pesticides kill virtually all living organisms in the area of
application (e.g. DNOC); others kill all plants (e.g. paraquat, simazine)
and hence have a severe secondary effect on animals. But most herbicides and most insecticides are more selective: they kill species of certain
large taxa and have little or no effect on the species of other taxa. For
example, dalapon kills most monocotyledonous plants but does not
harm dicotyledonous species, whereas phenoxyacetic acid herbicides in
general have an opposite effect. The insecticides in common use are far
more toxic to arthropods than they are to mammals and birds (see
Table 111). In the case of DDT this is because it is much more rapidly
absorbed through insect cuticle than through vertebrate skin. Some systemic organophosphorus insecticides (e.g. schradan) kill aphids and other
plant-sucking species but not their insect predators, while others (e.g.
demeton) kill both aphids and predators. Owing to differences in reactions to pesticides by different species (see p. 94 and Table 111),
pesticides have differential effects on populations.
3. Density-Independent Effects
Pesticides usually act independently of the density of the species
they affect. However, in some circumstances they could have densitydependent effects. For example, a species living in an environment that
was not normally sprayed might begin to increase and be forced to
N. W. MOORE
changes in vegetation (Thomas, 1960) and some changes in animal
populations (Moore, 1956). Yet this disease was virtually restricted to
the rabbit. A specific pesticide which eliminated the grey squirrel, the
woodpigeon, the grey rat or bracken, for example, would be likely
to produce similar fundamental changes in Britain. These might be
complicated.
However, most pesticides are not specific: most of the pesticide
problems of agriculture, conservation and medicine arise from this
fact. The reasons for the rarity of specific pesticides are these. New
pesticides are largely discovered by trial and error - by subjecting test
organisms to a very wide range of chemical substances. The chances of
specific pesticides being found in this way are very slight. The systematic discovery of specific pesticides would depend on knowing more
about specific biochemical features of the pest species. In general such
basic information is lacking and the cost of the necessary fundamental
research would be considerable. The incentive to find specific pesticides
may be slight because the profits resulting from the sale of specific
pesticides are likely to be less than those from the sale of wide-spectrum
ones. To conclude, non-specific pesticides are likely to be used for inany
years to come and their effects are likely to be complicated.
There is a complete range of type of effect from total to specific
effect. A few pesticides kill virtually all living organisms in the area of
application (e.g. DNOC); others kill all plants (e.g. paraquat, simazine)
and hence have a severe secondary effect on animals. But most herbicides and most insecticides are more selective: they kill species of certain
large taxa and have little or no effect on the species of other taxa. For
example, dalapon kills most monocotyledonous plants but does not
harm dicotyledonous species, whereas phenoxyacetic acid herbicides in
general have an opposite effect. The insecticides in common use are far
more toxic to arthropods than they are to mammals and birds (see
Table 111). In the case of DDT this is because it is much more rapidly
absorbed through insect cuticle than through vertebrate skin. Some systemic organophosphorus insecticides (e.g. schradan) kill aphids and other
plant-sucking species but not their insect predators, while others (e.g.
demeton) kill both aphids and predators. Owing to differences in reactions to pesticides by different species (see p. 94 and Table 111),
pesticides have differential effects on populations.
3. Density-Independent Effects
Pesticides usually act independently of the density of the species
they affect. However, in some circumstances they could have densitydependent effects. For example, a species living in an environment that
was not normally sprayed might begin to increase and be forced to