A SYNOPSIS O F THE I’KSTICIDH I’IIOBI~EM
95
measure can be obtained which both approaches natural conditions and
fulfils requirements of Probit Analysis (Heath and Stickel, 1965).
2. Sublethal Eflects
In the course of toxicity tests many individuals that survive high
doses exhibit pathological symptoms. I n general, it is safe to assume
that a population in which some individuals are killed by a pesticide
will also contain individuals which exhibit sublethal effects. I n the easy
conditions of laboratory life the consequences of sublethal poisoning are
unlikely to be serious, but in the field, a very slight loss in efficiency or an
alteration in behaviour may have important consequences. Sublethal
effects, therefore, are of great potential ecological importance. They are
notoriously difficult to study and assess, and there is little evidence to show
how important in fact they are in the field compared to the effects of acute
toxicity. I n many cases a hard division cannot be made between lethal and
sublethal effects. For example, a sublethal dose may make an animal more
susceptible to disease; thus both the pesticide and the disease are causes
of death. Cases where pesticides are one of several causes of death are a t
least as likely as those in which pesticides are the sole cause.
(i) Eflects on reproduction, development. A very wide range of sublethal
effects attributable to pesticides have been observed but few have
received systematic study. Most of the published work is on the effects
of insecticides on the reproduction of galliform birds (DeWitt, 1955,
1956; Rudd and Genelly, 1956, etc.). Effects on the number of eggs
laid, hatching and the viability of the progeny of birds dosed with
organochlorine and other insecticides have been demonstrated. Viability
of the young appears to be the most sensitive of these reactions in
laboratory studies although the exact cause of death in these circumstances was not recorded. On the other hand in a well planned and
thorough study of the effects of severe DDT contamination of a colony
of herring gulls in Lake Michigan, U.S.A., J. A. Keith (1966) found
exceptional egg mortality, while chick mortality was high but not exceptionally high. Work on trout by Burdick et al. (1964) showed that
concentrations of 4.75 ppm of DDT in trout eggs resulted in high
mortality of the young fish hatching from them. Work on insects shows
that insecticides may have both favourable and unfavourable effects
on reproduction. For example Knutson (1955) showed that dieldrin
exposure which produced a 66-99% mortality in Drosophila melanogaster resulted in 7.6% more eggs being laid by the survivors than by
the untreated controls, because the dieldrin-treated flies lived longer
than the controls. Georghiou (1965b) showed that the carbamate Isolan
reduced egg production in the house fly (Musca domestica) but that
mating, longevity and egg fertility were not affected.
95
measure can be obtained which both approaches natural conditions and
fulfils requirements of Probit Analysis (Heath and Stickel, 1965).
2. Sublethal Eflects
In the course of toxicity tests many individuals that survive high
doses exhibit pathological symptoms. I n general, it is safe to assume
that a population in which some individuals are killed by a pesticide
will also contain individuals which exhibit sublethal effects. I n the easy
conditions of laboratory life the consequences of sublethal poisoning are
unlikely to be serious, but in the field, a very slight loss in efficiency or an
alteration in behaviour may have important consequences. Sublethal
effects, therefore, are of great potential ecological importance. They are
notoriously difficult to study and assess, and there is little evidence to show
how important in fact they are in the field compared to the effects of acute
toxicity. I n many cases a hard division cannot be made between lethal and
sublethal effects. For example, a sublethal dose may make an animal more
susceptible to disease; thus both the pesticide and the disease are causes
of death. Cases where pesticides are one of several causes of death are a t
least as likely as those in which pesticides are the sole cause.
(i) Eflects on reproduction, development. A very wide range of sublethal
effects attributable to pesticides have been observed but few have
received systematic study. Most of the published work is on the effects
of insecticides on the reproduction of galliform birds (DeWitt, 1955,
1956; Rudd and Genelly, 1956, etc.). Effects on the number of eggs
laid, hatching and the viability of the progeny of birds dosed with
organochlorine and other insecticides have been demonstrated. Viability
of the young appears to be the most sensitive of these reactions in
laboratory studies although the exact cause of death in these circumstances was not recorded. On the other hand in a well planned and
thorough study of the effects of severe DDT contamination of a colony
of herring gulls in Lake Michigan, U.S.A., J. A. Keith (1966) found
exceptional egg mortality, while chick mortality was high but not exceptionally high. Work on trout by Burdick et al. (1964) showed that
concentrations of 4.75 ppm of DDT in trout eggs resulted in high
mortality of the young fish hatching from them. Work on insects shows
that insecticides may have both favourable and unfavourable effects
on reproduction. For example Knutson (1955) showed that dieldrin
exposure which produced a 66-99% mortality in Drosophila melanogaster resulted in 7.6% more eggs being laid by the survivors than by
the untreated controls, because the dieldrin-treated flies lived longer
than the controls. Georghiou (1965b) showed that the carbamate Isolan
reduced egg production in the house fly (Musca domestica) but that
mating, longevity and egg fertility were not affected.