ANALYSIS OF PROCESSES IN CONTROL O F INSECTS
53
weather at the time the hosts are passing through the susceptible stage;
and differences in the dispersion of hosts and parasites, so that some
hosts occur in areas with few parasites.
L. Tinbergen (see Klomp, 1958) claimed that in theory an additional
density-dependent mortality of the host could be effective in damping
oscillations if it occurred at medium levels of density; he went on to
relate this idea to his studies of mortality in the caterpillars of woodland
insects due to predation by tits (Parus spp.). Tinbergen and Klomp
(1960) combined a Nicholson parasite-host model with empirical data
on the insects, on their parasites, and on the mortality due to bird
predation; this last was density-dependent so long as host density did
not rise too high. They concluded; “When the birds eliminate a considerable part of the population at intermediate densities of the prey
(more than 25%) this density dependent predation has a damping effect
on the oscillations of a host-parasite model. . . . Consequently, under
certain conditions, the system host-predator-parasite is self-regulating.”
Varley and Gradwell (1963) illustrated a simple model showing how
pupal predation might restrict the population oscillations of the winter
moth and a parasite. They used the Nicholson-Bailey model and substituted constants based on their field observations. These combinations
of theory with empirical data are a welcome and valuable development.
When so many “damping” processes are suggested, one may well
doubt whether any expression of a tendency towards increasing parasitehost oscillations under natural conditions would survive long enough
to be detectable. For that makter, even more “damping” agents could
be listed, for almost any density-dependent processes acting on the
hosts would tend to have this effect. We are concerned here with the
limiting aspects of natural control at high densities, and with the
relaxation of controls, and the operation of protective influences, at
low densities (Solomon, 1949, section on Phases of Control). The
question, whether these processes would be adequate to damp increasing
oscillations before they reached dangerously high and low extremes,
cannot be settled in general, but must be determined according to the
circumstances of particular cases.
It is obvious that hosts do at times overcome any controlling influence
their parasites may have upon them, and reach high levels of density.
But unless there is a history of systematic oscillation with the population of a specific parasite, there is no reason to expect that the parasites
will overtake the host while it is abundant, and cause it to “crash”.
Often some other influence reduces the host to low density, a t which the
effect of parasitism again becomes prominent. This is quite a different
matter from the damping of a host-parasite oscillation.
C
C.E.R.
53
weather at the time the hosts are passing through the susceptible stage;
and differences in the dispersion of hosts and parasites, so that some
hosts occur in areas with few parasites.
L. Tinbergen (see Klomp, 1958) claimed that in theory an additional
density-dependent mortality of the host could be effective in damping
oscillations if it occurred at medium levels of density; he went on to
relate this idea to his studies of mortality in the caterpillars of woodland
insects due to predation by tits (Parus spp.). Tinbergen and Klomp
(1960) combined a Nicholson parasite-host model with empirical data
on the insects, on their parasites, and on the mortality due to bird
predation; this last was density-dependent so long as host density did
not rise too high. They concluded; “When the birds eliminate a considerable part of the population at intermediate densities of the prey
(more than 25%) this density dependent predation has a damping effect
on the oscillations of a host-parasite model. . . . Consequently, under
certain conditions, the system host-predator-parasite is self-regulating.”
Varley and Gradwell (1963) illustrated a simple model showing how
pupal predation might restrict the population oscillations of the winter
moth and a parasite. They used the Nicholson-Bailey model and substituted constants based on their field observations. These combinations
of theory with empirical data are a welcome and valuable development.
When so many “damping” processes are suggested, one may well
doubt whether any expression of a tendency towards increasing parasitehost oscillations under natural conditions would survive long enough
to be detectable. For that makter, even more “damping” agents could
be listed, for almost any density-dependent processes acting on the
hosts would tend to have this effect. We are concerned here with the
limiting aspects of natural control at high densities, and with the
relaxation of controls, and the operation of protective influences, at
low densities (Solomon, 1949, section on Phases of Control). The
question, whether these processes would be adequate to damp increasing
oscillations before they reached dangerously high and low extremes,
cannot be settled in general, but must be determined according to the
circumstances of particular cases.
It is obvious that hosts do at times overcome any controlling influence
their parasites may have upon them, and reach high levels of density.
But unless there is a history of systematic oscillation with the population of a specific parasite, there is no reason to expect that the parasites
will overtake the host while it is abundant, and cause it to “crash”.
Often some other influence reduces the host to low density, a t which the
effect of parasitism again becomes prominent. This is quite a different
matter from the damping of a host-parasite oscillation.
C
C.E.R.