DYNAMICS OF FIELD POPULATION O F P I ~ Y E LOOPER
273
already been mentioned (p. 240), namely putrefaction and an ichneumon
fly. However, the more important component of the complex is the
elaterid larva. Its feeding stage is still active in May, and when numerous may destroy a considerable proportion of the pupae in the litter.
Samples taken in the second half of May proved that this had occurred,
certainly in 1953 and 1963.
The life tables show in a relatively detailed manner the large losses
which occur in the population. That the generatioi mortality must be
necessarily high is obvious from the simple fact that one female produces some 200 eggs, and where the population density has no upward
trend the total mortality can be expected to fluctuate around 99%
(three examples are shown in Fig. 10).
Though far from being complete, the life tables: are sufficiently detailed to provide us with an insight into the hazards faced by the
population. Their presentation is essential because they contain all the
basic material indispensable for a further analysis.
VI. THE ANALYSIS OF THE CAUSES OF FLUCTUATION
A. THE PATTERN O F FLUCTUATION
The population density of the pine looper shows more or less cyclic
fluctuations (see Fig. 27). Considering egg densities, it appears that the
differences from generation to generation may be considerable. This is
expressed in a meaningful way by the size of the population trend (R),
being the quotient of densities of two successive generations (Balch and
Bird, 1944). The extreme values occur from 1952 to 1953 and from 1953
to 1954, R being 0.17 and 9.7, respectively. This range approximates
with those of other endemic pine forest insects, indicating that the
type of fluctuation found in the pine looper is representative for pine
caterpillars as a whole (Klomp, 1962).
The question may be raised as to whether the pattern of fluctuation
is a statistical reality. This point is considered in Fig. 28, where the
95% confidence intervals of pupal density in April are given (cf.
Table VIII). It is evident that the population lows of 1953 and 1958
are highly significant. Moreover, the steady increase fiom 1959 toward
1963 is significant, as shown by the fact that the intervals of 1959 and
1961, and also those of 1961 and 1963 do not overhp.
The pattern of fluctuation of pupal density is reflected in the pattern
of moth density, and the latter again in that of egg density. Consequently, it seems safe to conclude that the population at least in broad
outline behaved as portrayed in Fig. 27.
273
already been mentioned (p. 240), namely putrefaction and an ichneumon
fly. However, the more important component of the complex is the
elaterid larva. Its feeding stage is still active in May, and when numerous may destroy a considerable proportion of the pupae in the litter.
Samples taken in the second half of May proved that this had occurred,
certainly in 1953 and 1963.
The life tables show in a relatively detailed manner the large losses
which occur in the population. That the generatioi mortality must be
necessarily high is obvious from the simple fact that one female produces some 200 eggs, and where the population density has no upward
trend the total mortality can be expected to fluctuate around 99%
(three examples are shown in Fig. 10).
Though far from being complete, the life tables: are sufficiently detailed to provide us with an insight into the hazards faced by the
population. Their presentation is essential because they contain all the
basic material indispensable for a further analysis.
VI. THE ANALYSIS OF THE CAUSES OF FLUCTUATION
A. THE PATTERN O F FLUCTUATION
The population density of the pine looper shows more or less cyclic
fluctuations (see Fig. 27). Considering egg densities, it appears that the
differences from generation to generation may be considerable. This is
expressed in a meaningful way by the size of the population trend (R),
being the quotient of densities of two successive generations (Balch and
Bird, 1944). The extreme values occur from 1952 to 1953 and from 1953
to 1954, R being 0.17 and 9.7, respectively. This range approximates
with those of other endemic pine forest insects, indicating that the
type of fluctuation found in the pine looper is representative for pine
caterpillars as a whole (Klomp, 1962).
The question may be raised as to whether the pattern of fluctuation
is a statistical reality. This point is considered in Fig. 28, where the
95% confidence intervals of pupal density in April are given (cf.
Table VIII). It is evident that the population lows of 1953 and 1958
are highly significant. Moreover, the steady increase fiom 1959 toward
1963 is significant, as shown by the fact that the intervals of 1959 and
1961, and also those of 1961 and 1963 do not overhp.
The pattern of fluctuation of pupal density is reflected in the pattern
of moth density, and the latter again in that of egg density. Consequently, it seems safe to conclude that the population at least in broad
outline behaved as portrayed in Fig. 27.