DYNAMICS OF FIELD POPULATION OF P I N E LOOPER
237
The survival and mortality rates given in Titble XI11 have been computed by adding the numbers per category occurring in the appropriate
samples (cf. Table XII). Thus the lowest number of eggs upon which the
rates are based amounts to 134 in 1958. For that year the standard
deviation of the survival rate is:
s = .\/{P(lOO - P)/n} = .\/{89.6(100 -- 89.6)/134} = 2.6,
and the confidence interval is (89.6 f 2 x 2.6). In 1953 the standard
deviation is of the same order. In all other years it is noticeably smaller
because nearly all numbers of eggs amounted to 500 or more. Thus in
1951:
s = .\/{58-7(100 - 58*7)/3010} = 0.9.
B. REARING O F LARVAE
In most years 100-300 larvae were collected and reared in jars in an
insectory. These rearings were far from uniform. Firstly, there was some
variation in the period of sampling, which sometimes fell in mid August,
but sometimes was as late as mid September. Secondly, larvae were
collected in various ways. In some years all larvae found in the twig
samples were reared, viz. in 1950 and 1951, when they were not yet
shaken off the twigs (cf. p. 212). In others they were obtained by shaking
the crowns of several trees scattered through the study area. Thirdly,
the number of larvae per jar varied year by year, and it appears
that the density in the jars had a significant el€ect on larval mortality.
Another objection which can be raised against the usefulness of rearing larvae was the fact that the primary cause of death could not always
be established. Some larvae were found dead and covered by mould,
but this does not necessarily mean that a fungal disease was the primary
agent killing the insect. Other larvae stop taking in food and after some
days die and shrivel up. The cause of this disease is unknown. In 1955
and 1956 many larvae died from a cytoplasmic polyhedral virus (Smith
and Rivers, 1956). Very exceptionally larvae died as a result of their
inability to strip the old skin off the abdominal segments.
A survey of the rearings is given in Table XIW. This shows that parasitic mortality also occurs. There is one braconid (determined by Dr.
G . E. J. Nixon, British Museum (Natural History) as Apanteles caberae,
Marshall, 1885), which is present in most years in low numbers. In 1963
it parasitized about 10% of the laxvae. The parasitic tachinid Strobliomyia Jissicornis (Strobl.) (determined by Dr. F. van Emden, British
Museum (Nat. Hist.), occurred only in 1954 and 1955, being very
numerous in 1954.
The data on “numbers diseased” in Table XIV do not represent the
incidence of disease in the field since the concmtration of larvae in jars
237
The survival and mortality rates given in Titble XI11 have been computed by adding the numbers per category occurring in the appropriate
samples (cf. Table XII). Thus the lowest number of eggs upon which the
rates are based amounts to 134 in 1958. For that year the standard
deviation of the survival rate is:
s = .\/{P(lOO - P)/n} = .\/{89.6(100 -- 89.6)/134} = 2.6,
and the confidence interval is (89.6 f 2 x 2.6). In 1953 the standard
deviation is of the same order. In all other years it is noticeably smaller
because nearly all numbers of eggs amounted to 500 or more. Thus in
1951:
s = .\/{58-7(100 - 58*7)/3010} = 0.9.
B. REARING O F LARVAE
In most years 100-300 larvae were collected and reared in jars in an
insectory. These rearings were far from uniform. Firstly, there was some
variation in the period of sampling, which sometimes fell in mid August,
but sometimes was as late as mid September. Secondly, larvae were
collected in various ways. In some years all larvae found in the twig
samples were reared, viz. in 1950 and 1951, when they were not yet
shaken off the twigs (cf. p. 212). In others they were obtained by shaking
the crowns of several trees scattered through the study area. Thirdly,
the number of larvae per jar varied year by year, and it appears
that the density in the jars had a significant el€ect on larval mortality.
Another objection which can be raised against the usefulness of rearing larvae was the fact that the primary cause of death could not always
be established. Some larvae were found dead and covered by mould,
but this does not necessarily mean that a fungal disease was the primary
agent killing the insect. Other larvae stop taking in food and after some
days die and shrivel up. The cause of this disease is unknown. In 1955
and 1956 many larvae died from a cytoplasmic polyhedral virus (Smith
and Rivers, 1956). Very exceptionally larvae died as a result of their
inability to strip the old skin off the abdominal segments.
A survey of the rearings is given in Table XIW. This shows that parasitic mortality also occurs. There is one braconid (determined by Dr.
G . E. J. Nixon, British Museum (Natural History) as Apanteles caberae,
Marshall, 1885), which is present in most years in low numbers. In 1963
it parasitized about 10% of the laxvae. The parasitic tachinid Strobliomyia Jissicornis (Strobl.) (determined by Dr. F. van Emden, British
Museum (Nat. Hist.), occurred only in 1954 and 1955, being very
numerous in 1954.
The data on “numbers diseased” in Table XIV do not represent the
incidence of disease in the field since the concmtration of larvae in jars