A SYNOPSIS O F THE PESTICIDE I’ELOBI,EM
91
colonize an environment that was subject to spraying. As a result it
would only be affected above a certain level - that at which the sprayed
environment was colonized. This could happen on a large scale, for
example when a bird species living in an uncultivated habitat colonizes
agricultural land; or on a small scale, as for example when members of
an insect species which normally lives in parts of plants untouched by
sprays are forced by population pressure to live on the exposed parts
of the plant. Despite these exceptions pesticides are essentially a densityindependent factor. Therefore they do not necessarily “control” pests
in the biological sense of the word. They may only kill a proportion
of the surplus which would bc eliminated in any case by a densitydependent factor (e.g. predators) (Nicholson, 1939; Stern et al., 1959).
An important difference between the medical and agricultural use of
pesticides should be noted. In many programmes done to control
disease-carrying organisms the aim is to exterminate the vector: in agricultural programmes the aim is to reduce its numbers so that they
are of negligible economic importance.
B. TOXICITY
The only generalization that can be made about pesticide properties
is that there is great variation. Toxicity and persistence are especially
important ecologically and are discussed below; the range of effect is
very great, for both. Pesticides may kill, have sublethal effects, or none
at all; and whereas some remain active after several years, others are
broken down or inactivated almost as soon as they are applied.
1. Acute Toxicity
Pesticides affect organisms in many different ways. Despite a formidable amount of research the exact mechanisms are rarely understood, The phenoxyacetic acid group of herbicides, which make up the
bulk of those used today, kill plants by interferences with growth
processes (see, for example, Wain, 1958). The primary effect of most
insecticides (organochlorine, organophosphorus and carbamate substances) is on the nervous system. The organophosphorus and carbamate insecticides inhibit the activity of cholinesterase. Many pesticides
have more than one effect, for example DDT affects the central nervous
system, met,abolic processes and reproduct,ion. In this case the effect on
the nervous system appears to be primary but 111 others it is often
difficult to define the most important effect, especially since systems
interact.
Pesticides of the same chemical group have the same general effect
and yet vary greatly in their toxicity. For example, for rats there is
nearly a 700-fold difference in acute oral toxicity between the two
91
colonize an environment that was subject to spraying. As a result it
would only be affected above a certain level - that at which the sprayed
environment was colonized. This could happen on a large scale, for
example when a bird species living in an uncultivated habitat colonizes
agricultural land; or on a small scale, as for example when members of
an insect species which normally lives in parts of plants untouched by
sprays are forced by population pressure to live on the exposed parts
of the plant. Despite these exceptions pesticides are essentially a densityindependent factor. Therefore they do not necessarily “control” pests
in the biological sense of the word. They may only kill a proportion
of the surplus which would bc eliminated in any case by a densitydependent factor (e.g. predators) (Nicholson, 1939; Stern et al., 1959).
An important difference between the medical and agricultural use of
pesticides should be noted. In many programmes done to control
disease-carrying organisms the aim is to exterminate the vector: in agricultural programmes the aim is to reduce its numbers so that they
are of negligible economic importance.
B. TOXICITY
The only generalization that can be made about pesticide properties
is that there is great variation. Toxicity and persistence are especially
important ecologically and are discussed below; the range of effect is
very great, for both. Pesticides may kill, have sublethal effects, or none
at all; and whereas some remain active after several years, others are
broken down or inactivated almost as soon as they are applied.
1. Acute Toxicity
Pesticides affect organisms in many different ways. Despite a formidable amount of research the exact mechanisms are rarely understood, The phenoxyacetic acid group of herbicides, which make up the
bulk of those used today, kill plants by interferences with growth
processes (see, for example, Wain, 1958). The primary effect of most
insecticides (organochlorine, organophosphorus and carbamate substances) is on the nervous system. The organophosphorus and carbamate insecticides inhibit the activity of cholinesterase. Many pesticides
have more than one effect, for example DDT affects the central nervous
system, met,abolic processes and reproduct,ion. In this case the effect on
the nervous system appears to be primary but 111 others it is often
difficult to define the most important effect, especially since systems
interact.
Pesticides of the same chemical group have the same general effect
and yet vary greatly in their toxicity. For example, for rats there is
nearly a 700-fold difference in acute oral toxicity between the two