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H. KLOMP
(c) Mortality of Later Larval Instars. During the first half of August
mortality is a t least partly due to the same factors as in the first instar.
Later, birds come into consideration, especially in September when the
larvae are of an attractive size, and not only flocks of titmice, but also
migrants such as chaffinches and bramblings, will occasionally hunt
in the crowns. These predators most probably also account for most of
the losses occurring after the October census, because nymphal density
is considerably lower than larval density in many years. In some years
(1955 and 1956) a polyhedral disease also plays a significant role (see
also Table XIV).
( d ) Mortality of Prepupae. In nearly every year there is a big difference
between nymphal density in November and pupal density in December.
Predators hardly deserve consideration in this case, because the nymphs
fall off the trees and disappear into the moss usually within minutes,
and probably in most cases they descend during the night. When reared,
nymphs sometimes fail to develop a normal pupal skin and die in the
prepupal stage. We are inclined to assume that some of the losses in the
field are due to this inability, but more precise information is wanted.
( e ) Winter Mortality of Pupae. Having once reached the pupal stage
the insects generally survive the winter very well. When measured
directly in the study area there is no mortality in a statistical sense, as
shown in the first four years of our investigations. If a large collection
of pupae is made in April, then a small fraction is nearly always
damaged. (The only exception occurred in 1953.) Most of the damage is
due to larvae of an elaterid, Athous subfuscus (which become active in
March and April) as shown by the fact that the larvae are often found
preying upon the pupae. They bite a hole between the segments of the
abdomen, leaving a characteristic and recognizable cause of death
which is apparent even after the insect leaves the pupal skin. A very
low proportion of the pupae is diseased. They are either covered by
mould and already dead, or still alive but with their abdominal segments stretched out and this results in death some weeks later.
(f) Post-census Mortality of Pupae. The difference between pupal and
moth density is nearly always striking. Most of the mortality is due to
parasites which infect the larvae and kill the host at reactivation after
hibernation. The parasitism according to species is given in Table XV.
With one exception (1958) it cannot account for the total mortality
occurring in this time interval.
The additional mortality is computed in an indirect way from the
difference between pupal and moth density estimates and after allowance for pupal parasites. It is due to various factors, two of which have
H. KLOMP
(c) Mortality of Later Larval Instars. During the first half of August
mortality is a t least partly due to the same factors as in the first instar.
Later, birds come into consideration, especially in September when the
larvae are of an attractive size, and not only flocks of titmice, but also
migrants such as chaffinches and bramblings, will occasionally hunt
in the crowns. These predators most probably also account for most of
the losses occurring after the October census, because nymphal density
is considerably lower than larval density in many years. In some years
(1955 and 1956) a polyhedral disease also plays a significant role (see
also Table XIV).
( d ) Mortality of Prepupae. In nearly every year there is a big difference
between nymphal density in November and pupal density in December.
Predators hardly deserve consideration in this case, because the nymphs
fall off the trees and disappear into the moss usually within minutes,
and probably in most cases they descend during the night. When reared,
nymphs sometimes fail to develop a normal pupal skin and die in the
prepupal stage. We are inclined to assume that some of the losses in the
field are due to this inability, but more precise information is wanted.
( e ) Winter Mortality of Pupae. Having once reached the pupal stage
the insects generally survive the winter very well. When measured
directly in the study area there is no mortality in a statistical sense, as
shown in the first four years of our investigations. If a large collection
of pupae is made in April, then a small fraction is nearly always
damaged. (The only exception occurred in 1953.) Most of the damage is
due to larvae of an elaterid, Athous subfuscus (which become active in
March and April) as shown by the fact that the larvae are often found
preying upon the pupae. They bite a hole between the segments of the
abdomen, leaving a characteristic and recognizable cause of death
which is apparent even after the insect leaves the pupal skin. A very
low proportion of the pupae is diseased. They are either covered by
mould and already dead, or still alive but with their abdominal segments stretched out and this results in death some weeks later.
(f) Post-census Mortality of Pupae. The difference between pupal and
moth density is nearly always striking. Most of the mortality is due to
parasites which infect the larvae and kill the host at reactivation after
hibernation. The parasitism according to species is given in Table XV.
With one exception (1958) it cannot account for the total mortality
occurring in this time interval.
The additional mortality is computed in an indirect way from the
difference between pupal and moth density estimates and after allowance for pupal parasites. It is due to various factors, two of which have