DYNAMICS O F FIELD POPULATION O F P I N E LOOPER
295
brumata, and its tachinid, Cyzenis albicuns, such oscillations might be
damped by density dependent predation occurring in the pupal stage
during hibernation.
There is as yet very little evidence supporting the theory of Nicholson.
Such evidence should comprise field-collected da,ta on the functional
and numerical responses of the parasites over a series of successive
generations, and material of this kind can in most cases be obtained
only by the application of time-consuming sampliiig techniques. Morris
(1959, 1963b) has shown that parasites are responsible for most of the
density variability in some forest insects, which makes a regulatory
influence of parasites appear likely. Other evidence in favour of a regulatory effect of parasitoids is provided by the results of some biological
control measures. In these cases the introduction of a specific parasite
has resulted in a stabilization of the density of the pest at a level far
below the economically intolerable one. Strictly speaking this fact does
not, however, provide proof of regulation by parasites (Klomp, 1962,
p. 105).
Thus it is clear that there is great need for a be1;ter understanding of
the part played by insect parasites in natural populations. Therefore we
will consider the parasites of the pine looper in some detail.
(a) Egg Parasites. There is one species infecting eggs: Trichogramma
embryophagum (Table XIII). This species is polyphagous and has 5 to 6
generations per year on different hosts in succetsion (Klomp, 1956).
This implies that it is not synchronizcd with eii$er of its hosts and
cannot be delayed density dependent with reference to the pine looper,
because the numbers of the parasite are not only determined by the
density of this host species, but also by the numbers of the others.
( b ) Larval Parasites. So far two species have been met infecting and
killing the larval stage: a tachinid, Strobliomyia fissicornis, and a
braconid, Apanteles caberae (Table XIV and p. :!37). These parasites
have no regulatory significance. The tachinid occurred in 1954 and 1955
only. In the years studied the braconid never infected more than 9%
of the larvae, and there is no indication of a numerical response to increases of host density.
( c ) Pupal Parasites. Of the species listed in Tab113 XV, Cratichneumn
is said not to be synchronized with Bupalus (Thdenhorst, 1939). The
adults emerge in May and the females of the first generation fly in May
and June, infecting pupae of Bupalus, which at the time have not yet
emerged, and some alternative host species. In addition, the fraction
of pupae parasitized by this species never exceeded 6p/,, and consequently it needs no further consideration from the viewpoint of regulation.
295
brumata, and its tachinid, Cyzenis albicuns, such oscillations might be
damped by density dependent predation occurring in the pupal stage
during hibernation.
There is as yet very little evidence supporting the theory of Nicholson.
Such evidence should comprise field-collected da,ta on the functional
and numerical responses of the parasites over a series of successive
generations, and material of this kind can in most cases be obtained
only by the application of time-consuming sampliiig techniques. Morris
(1959, 1963b) has shown that parasites are responsible for most of the
density variability in some forest insects, which makes a regulatory
influence of parasites appear likely. Other evidence in favour of a regulatory effect of parasitoids is provided by the results of some biological
control measures. In these cases the introduction of a specific parasite
has resulted in a stabilization of the density of the pest at a level far
below the economically intolerable one. Strictly speaking this fact does
not, however, provide proof of regulation by parasites (Klomp, 1962,
p. 105).
Thus it is clear that there is great need for a be1;ter understanding of
the part played by insect parasites in natural populations. Therefore we
will consider the parasites of the pine looper in some detail.
(a) Egg Parasites. There is one species infecting eggs: Trichogramma
embryophagum (Table XIII). This species is polyphagous and has 5 to 6
generations per year on different hosts in succetsion (Klomp, 1956).
This implies that it is not synchronizcd with eii$er of its hosts and
cannot be delayed density dependent with reference to the pine looper,
because the numbers of the parasite are not only determined by the
density of this host species, but also by the numbers of the others.
( b ) Larval Parasites. So far two species have been met infecting and
killing the larval stage: a tachinid, Strobliomyia fissicornis, and a
braconid, Apanteles caberae (Table XIV and p. :!37). These parasites
have no regulatory significance. The tachinid occurred in 1954 and 1955
only. In the years studied the braconid never infected more than 9%
of the larvae, and there is no indication of a numerical response to increases of host density.
( c ) Pupal Parasites. Of the species listed in Tab113 XV, Cratichneumn
is said not to be synchronized with Bupalus (Thdenhorst, 1939). The
adults emerge in May and the females of the first generation fly in May
and June, infecting pupae of Bupalus, which at the time have not yet
emerged, and some alternative host species. In addition, the fraction
of pupae parasitized by this species never exceeded 6p/,, and consequently it needs no further consideration from the viewpoint of regulation.