A SYNOPSIS O F THE PESTICIDE PROBLEM
107
insecticides DDT, aldrin and dieldrin, the organophosphorus insecticides demeton methyl (metasystox) and dimethoate, and among the
rodenticides zinc phosphide, strychnine, thallium sulphate and sodium
fluoroacetate (1080). “Delayed toxicity” and “delayed expression”
have only been observed among the organochlorine insecticides and
among the organomercury fungicides.
Concentration of organochlorine insecticides in food chains has been
demonstrated in many different types of environment. Since the process
results in concentration at the end of food chains, predators are potentially at greater risk than species at lower trophic levels. There is insufficient evidence to say categorically that predators are declining at
a greater rate than other species, but the evidence suggests that this
is happening (see p. 119). For example, much work in agricultural
entomology shows that many insect predators are severely affected by
pesticides (see p. 110). There has almost certainly been an unprecedented
decline of predatory birds in the northern hemisphere in recent years
(Prestt, 1965; Ratcliffe, 1963). Some of the declines may be due to
other factors, but since so many of the avian predators contain residues
at, or approaching levels of toxicological significance (Moore, 1965b),
the suspicion that indirect poisoning by organochlorine insecticides is
an important cause is well based. The effects observed may be due to
birds eating a few highly contaminated specimens (Jefferies and Prestt,
1966) or to consuming many less contaminated specimens over a long
time. The effects may be direct and/or due to effects on reproduction
(Ratcliffe, 1965). As shown above, delayed effects rarely operate through
more than five stages in a food chain. Nevertheless considerable
concentration of pesticides can occur (see Table IV).
Delayed effects are not restricted to animal/animal relationships or
to unintentional effects. For example, aphids are poisoned by sucking
the juices of plants which have taken up systemic organophosphorus
insecticides from the soil. Recently C. H. Walker (pers. comm.) has
shown that sufficient BHC can be taken up by brussels sprouts from the
soil to kill lepidopterous larvae feeding on them. Little is known about
the ecological effects of intentional and unintentional systemic action.
D. REDUCTION O F FOOD SPECIES
In all countries where herbicides are widely used there has been an
extensive reduction in certain weed species. Animal species dependent
on these weeds must have suffered a considerable loss of potential food.
For example, all insects dependent on poppies (PupaveT spp.) have
suffered an enormous reduction in their food supply in East Anglia,
where poppies were once common in many fields but are now banished
from most of them, Similarly predators and parasiteswhich are dependent
107
insecticides DDT, aldrin and dieldrin, the organophosphorus insecticides demeton methyl (metasystox) and dimethoate, and among the
rodenticides zinc phosphide, strychnine, thallium sulphate and sodium
fluoroacetate (1080). “Delayed toxicity” and “delayed expression”
have only been observed among the organochlorine insecticides and
among the organomercury fungicides.
Concentration of organochlorine insecticides in food chains has been
demonstrated in many different types of environment. Since the process
results in concentration at the end of food chains, predators are potentially at greater risk than species at lower trophic levels. There is insufficient evidence to say categorically that predators are declining at
a greater rate than other species, but the evidence suggests that this
is happening (see p. 119). For example, much work in agricultural
entomology shows that many insect predators are severely affected by
pesticides (see p. 110). There has almost certainly been an unprecedented
decline of predatory birds in the northern hemisphere in recent years
(Prestt, 1965; Ratcliffe, 1963). Some of the declines may be due to
other factors, but since so many of the avian predators contain residues
at, or approaching levels of toxicological significance (Moore, 1965b),
the suspicion that indirect poisoning by organochlorine insecticides is
an important cause is well based. The effects observed may be due to
birds eating a few highly contaminated specimens (Jefferies and Prestt,
1966) or to consuming many less contaminated specimens over a long
time. The effects may be direct and/or due to effects on reproduction
(Ratcliffe, 1965). As shown above, delayed effects rarely operate through
more than five stages in a food chain. Nevertheless considerable
concentration of pesticides can occur (see Table IV).
Delayed effects are not restricted to animal/animal relationships or
to unintentional effects. For example, aphids are poisoned by sucking
the juices of plants which have taken up systemic organophosphorus
insecticides from the soil. Recently C. H. Walker (pers. comm.) has
shown that sufficient BHC can be taken up by brussels sprouts from the
soil to kill lepidopterous larvae feeding on them. Little is known about
the ecological effects of intentional and unintentional systemic action.
D. REDUCTION O F FOOD SPECIES
In all countries where herbicides are widely used there has been an
extensive reduction in certain weed species. Animal species dependent
on these weeds must have suffered a considerable loss of potential food.
For example, all insects dependent on poppies (PupaveT spp.) have
suffered an enormous reduction in their food supply in East Anglia,
where poppies were once common in many fields but are now banished
from most of them, Similarly predators and parasiteswhich are dependent