DYNAMICS OF FIELD POPULATION O F P I N E LOOPER
297
density is very high. In 1952, parasitism by this species slightly de
creases, whereas theoretically a considerable increase was to be expected
as a response to larval host density in 1951. Earlier, in a preliminary
discussion of the significance of this parasite (Klomp, 1959), I did suggest that part of the deviations from expectation might be due to a
shortage of sunshine during the period of activity of the fly. However, it
is clear that in later years the parasite did not react to large changes of
host density either (see Fig. 39), and weather cannot be held responsible
in these cases.
The other tachinid, Blondelia, was relatively ineffective up to 1958,
and then started increasing slowly in numbers. This increase may possibly be the result of a numerical response to the simultaneous growth
of host density, but then the question is still unanswered why the parasite did not react to changes of host density in carlier years.
The fluctuations of Poecilostictus cannot be explained satisfactorily
by a numerical response to host density either. True, the high rate of
parasitism in 1957 may be the result of the high host densities in the
foregoing generations. Moreover, the slight and more pronounced increases of 1962 and 1963 may be the result of the high host densities in
1961 and 1962, respectively. However, such an increase of parasitism
does not occur in response to the 1951 densit'y of the host, nor does the
parasite react clearly to the sudden host, declines of 1952 and 1957. It
might be argued that the low rate of 1959 is due to the 1957 fall of host
numbers, but in fact this should have occurred in 1958.
A regulatory influence on host numbers of one or more of the pupal
parasites should necessarily find expression in the total fraction of pupae
infected. The bottom graph of Fig. 39 demonstrates, however, that in
general the overall infection rates fluctuate within relatively narrow
limits, and the pattern of fluctuation does not suggest a delayed density
dependent relationship. Instead it seems to be the result of irregular
fluctuations of the individual parasite species occurring independently
of the host's density.
2. The In$uence of Larval Density on the Viability of Eggs and Larvae in
the Next Generation
Having found that parasites do not play a demonstrable part in the
regulation of numbers of their common host, we have to look for other
possible delayed effects. In the preceding chapters we have repeatedly
argued that where the population declines after having reached a high
level, or increases sharply after a very low level, this did not appear to
be due primarily to the density controlled factors so far found operating.
This question should now be considered more positively by stating
297
density is very high. In 1952, parasitism by this species slightly de
creases, whereas theoretically a considerable increase was to be expected
as a response to larval host density in 1951. Earlier, in a preliminary
discussion of the significance of this parasite (Klomp, 1959), I did suggest that part of the deviations from expectation might be due to a
shortage of sunshine during the period of activity of the fly. However, it
is clear that in later years the parasite did not react to large changes of
host density either (see Fig. 39), and weather cannot be held responsible
in these cases.
The other tachinid, Blondelia, was relatively ineffective up to 1958,
and then started increasing slowly in numbers. This increase may possibly be the result of a numerical response to the simultaneous growth
of host density, but then the question is still unanswered why the parasite did not react to changes of host density in carlier years.
The fluctuations of Poecilostictus cannot be explained satisfactorily
by a numerical response to host density either. True, the high rate of
parasitism in 1957 may be the result of the high host densities in the
foregoing generations. Moreover, the slight and more pronounced increases of 1962 and 1963 may be the result of the high host densities in
1961 and 1962, respectively. However, such an increase of parasitism
does not occur in response to the 1951 densit'y of the host, nor does the
parasite react clearly to the sudden host, declines of 1952 and 1957. It
might be argued that the low rate of 1959 is due to the 1957 fall of host
numbers, but in fact this should have occurred in 1958.
A regulatory influence on host numbers of one or more of the pupal
parasites should necessarily find expression in the total fraction of pupae
infected. The bottom graph of Fig. 39 demonstrates, however, that in
general the overall infection rates fluctuate within relatively narrow
limits, and the pattern of fluctuation does not suggest a delayed density
dependent relationship. Instead it seems to be the result of irregular
fluctuations of the individual parasite species occurring independently
of the host's density.
2. The In$uence of Larval Density on the Viability of Eggs and Larvae in
the Next Generation
Having found that parasites do not play a demonstrable part in the
regulation of numbers of their common host, we have to look for other
possible delayed effects. In the preceding chapters we have repeatedly
argued that where the population declines after having reached a high
level, or increases sharply after a very low level, this did not appear to
be due primarily to the density controlled factors so far found operating.
This question should now be considered more positively by stating