110
N . W . MOORV:
their numbers. The effects of changes in predator pressure are discussed
in the next section.
G. R E M O V A L O F A PREDATOR
For many years it has been known that resurgences of pests may
occur after an application of an insecticide. Theoretically (see, for
example, Kuenen and Post, 1958) these may be due to:
1. A stimulatory effect of the pesticide on the pest’s reproduction.
2. A stimulatory effect of the pesticide on the food of the pest.
3. A reduction in competition.
4. A reduction in predators.
In most cases there is strong circumstantiiil evidencc that, resurgences
havc been caused mainly by a rediiction of predators. In many other
cases the usc of ;L pesticide has resulted in the rise of another pest, i.e.
substitution (see above) has occurred.
The most studied case is that of the tetranychid mites, in particular
Pnnon ychus ulmi. These animals have become important economic
pests as a result of the use of pesticides (see, for example, Collyer, 1964
and Muir, 1965).
Of the forty-five species of predator found in orchards with P. uimi,
two species, the mirid Blepharidopterus angulntus and the phytoseiid
mitc T?yphlodromus pyri, appear to be important, in controlling the red
spider mite. The work of Collyer (1964) shows that whereas the univoltine mirid rcquires morr than onc season to 1)uild up numbers large
enough to control P. srlrni, the prrdnceouq mite brceds fast enough to
control itI in most cascs. In an insectary experiment another mite, Aculus
fockeri, was introduced into the system. Despite the presence of this
alternative prey, T. pyri continued to control P. ulmi. Spray programmes directed against other pests of orrhiwds reduce the populations
of the predators including T. pyri and so cause eruption of P. ulmi.
Kuenen and Post (1958), however, point out that low populations of
P. ulmi can occur when predator populations are very low and so other
factors may sometimes operate. They suggest that the reduction of
competitors, changes in chemical composition in the leaves due to
fertilizers applied at the same time as the pesticides, and stimulation of
cgg production 1)y DDT i L t low levels, may also be important.
V I . I ~ ~ ~ ~ - F I w T s O N ECOSYSTEMS
A. INTRODUCTION
The ways in which a pesticide can affect a species or simple relationships between species have been outlined above. Again it must be
emphasized that studies of this sort are likely to be misleading unless
they are related to the whole situation. I n this section the effects of
N . W . MOORV:
their numbers. The effects of changes in predator pressure are discussed
in the next section.
G. R E M O V A L O F A PREDATOR
For many years it has been known that resurgences of pests may
occur after an application of an insecticide. Theoretically (see, for
example, Kuenen and Post, 1958) these may be due to:
1. A stimulatory effect of the pesticide on the pest’s reproduction.
2. A stimulatory effect of the pesticide on the food of the pest.
3. A reduction in competition.
4. A reduction in predators.
In most cases there is strong circumstantiiil evidencc that, resurgences
havc been caused mainly by a rediiction of predators. In many other
cases the usc of ;L pesticide has resulted in the rise of another pest, i.e.
substitution (see above) has occurred.
The most studied case is that of the tetranychid mites, in particular
Pnnon ychus ulmi. These animals have become important economic
pests as a result of the use of pesticides (see, for example, Collyer, 1964
and Muir, 1965).
Of the forty-five species of predator found in orchards with P. uimi,
two species, the mirid Blepharidopterus angulntus and the phytoseiid
mitc T?yphlodromus pyri, appear to be important, in controlling the red
spider mite. The work of Collyer (1964) shows that whereas the univoltine mirid rcquires morr than onc season to 1)uild up numbers large
enough to control P. srlrni, the prrdnceouq mite brceds fast enough to
control itI in most cascs. In an insectary experiment another mite, Aculus
fockeri, was introduced into the system. Despite the presence of this
alternative prey, T. pyri continued to control P. ulmi. Spray programmes directed against other pests of orrhiwds reduce the populations
of the predators including T. pyri and so cause eruption of P. ulmi.
Kuenen and Post (1958), however, point out that low populations of
P. ulmi can occur when predator populations are very low and so other
factors may sometimes operate. They suggest that the reduction of
competitors, changes in chemical composition in the leaves due to
fertilizers applied at the same time as the pesticides, and stimulation of
cgg production 1)y DDT i L t low levels, may also be important.
V I . I ~ ~ ~ ~ - F I w T s O N ECOSYSTEMS
A. INTRODUCTION
The ways in which a pesticide can affect a species or simple relationships between species have been outlined above. Again it must be
emphasized that studies of this sort are likely to be misleading unless
they are related to the whole situation. I n this section the effects of