106
N. W. MOORE
eating food directly contaminated by the pesticide. The extent to
which the pesticide has a direct effect on the species depends first on
the intrinsic reaction of the species to the chemical-this
can be
determined by toxicological experiments; secondly it depends on the
extent to which a species comes in contact with the pesticide, and this
depends on the amount of pesticide used, its properties and manner of
application, and on the behavioural characteristics of the species.
Most of the recorded direct effects are of pesticides on pests. There
are very few systematic descriptions of the effects on non-pest species.
A pesticide may have a direct effect on the individual or on its
reproduction - in other words on the next generation. In many cases
it is not obvious which type of effect is the more important. In a
laboratory study on pheasants by Azevedo et al. (1965) the overall effect
of DDT on reproduction was probably not significant a t dietary levels
which could be tolerated by adults. In a field study of residues in
pheasants in California they found high levels of DDT in the fat
(0-2 930 ppm, mean 740 ppm). Any effects on the population in this
case could be attributed to direct poisoning rather than to an indirect
effect on reproduction.
c. TOXIC EFFECT ON A SPECIES - DELAYED EFFECTS
A species may be affected by eating plants or animals which contain
residues of a pesticide previously ingested. As shown above, organochlorine insecticides become stored in body fat and this enables an
animal to concentrate these substances. When one animal eats another
the effects depend on the amount of pesticide that is ingested from the
prey and the reaction of the animal to it. A number of situations may
occur and special terms have been coined to describe them (Rudd, 1964).
Secondary poisoning occurs whenever one animal dies from eating
another animal containing a pesticide. Rudd makes the distinction
between secondary poisoning which occurs shortly after consuming
poisoned prey and “delayed toxicity” when contamination results in
death a t a later date. When an animal eats contaminated prey it may
not die itself but it may kill another animal which eats it - Rudd calls
this “delayed expression”. This situation usually occurs when the
intermediate species has the characteristic of being able to concentrate
the pesticide to levels which would cause death in its predator. Such
species have been called “biological concentrators”. Earthworms appear
to be an example of biological concentrators (e.g. Barker, 1958; Davis,
1966). Stickel et al. (1965) show that normal field use of heptachlor is
likely to result in concentrations of heptachlor epoxide in earthworms
which are likely to affect American woodcock feeding upon them.
Secondary poisoning has been recorded among the organochlorine
N. W. MOORE
eating food directly contaminated by the pesticide. The extent to
which the pesticide has a direct effect on the species depends first on
the intrinsic reaction of the species to the chemical-this
can be
determined by toxicological experiments; secondly it depends on the
extent to which a species comes in contact with the pesticide, and this
depends on the amount of pesticide used, its properties and manner of
application, and on the behavioural characteristics of the species.
Most of the recorded direct effects are of pesticides on pests. There
are very few systematic descriptions of the effects on non-pest species.
A pesticide may have a direct effect on the individual or on its
reproduction - in other words on the next generation. In many cases
it is not obvious which type of effect is the more important. In a
laboratory study on pheasants by Azevedo et al. (1965) the overall effect
of DDT on reproduction was probably not significant a t dietary levels
which could be tolerated by adults. In a field study of residues in
pheasants in California they found high levels of DDT in the fat
(0-2 930 ppm, mean 740 ppm). Any effects on the population in this
case could be attributed to direct poisoning rather than to an indirect
effect on reproduction.
c. TOXIC EFFECT ON A SPECIES - DELAYED EFFECTS
A species may be affected by eating plants or animals which contain
residues of a pesticide previously ingested. As shown above, organochlorine insecticides become stored in body fat and this enables an
animal to concentrate these substances. When one animal eats another
the effects depend on the amount of pesticide that is ingested from the
prey and the reaction of the animal to it. A number of situations may
occur and special terms have been coined to describe them (Rudd, 1964).
Secondary poisoning occurs whenever one animal dies from eating
another animal containing a pesticide. Rudd makes the distinction
between secondary poisoning which occurs shortly after consuming
poisoned prey and “delayed toxicity” when contamination results in
death a t a later date. When an animal eats contaminated prey it may
not die itself but it may kill another animal which eats it - Rudd calls
this “delayed expression”. This situation usually occurs when the
intermediate species has the characteristic of being able to concentrate
the pesticide to levels which would cause death in its predator. Such
species have been called “biological concentrators”. Earthworms appear
to be an example of biological concentrators (e.g. Barker, 1958; Davis,
1966). Stickel et al. (1965) show that normal field use of heptachlor is
likely to result in concentrations of heptachlor epoxide in earthworms
which are likely to affect American woodcock feeding upon them.
Secondary poisoning has been recorded among the organochlorine