300
H . KLOMP
dependent action of predators or parasites. In spite of thorough research
in this field this conception has never been definitely established.
Another theory emphasizes the significance of an increase in the interaction between the individuals of a population with growing density,
and the consequent deterioration of the physiological condition of the
animals, giving rise to self-regulation (Chitty, 1957; Wynne-Edwards,
1962). However, the advocates of the former theory sometimes claim
that, because of the low population density of plant-eating insects,
direct competition, e.g. for food, can be excluded as a causative agent,
and the influence of mutual contact interference can, in their opinion,
be minimized for the same reason. This conception has definitely no
general validity, as was pointed out recently by Kennedy and Way
(1964), who cite several instances of interference at low density levels in
various insects. It is therefore worthwhile to examine processes of this
type to assess their importance for the regulation of insect numbers.
The results of the present study clearly support the theory of selfregulation more than that of the parasite-predator thesis. It is true
that the advanced larvae of the pine looper are subject to density
dependent predation, but the regulatory effect of this relationship was
not of primary importance during the period of observation. In addition, it was found that the larvae interfere a t densities far below the
food capacity of the pine trees, thus giving rise to an inhibition of larval
growth and a decreased fecundity at the higher levels. Moreover, there
is strong evidence that in addition to the effects just mentioned, the
viability of eggs and larvae of the next generation is affected disadvantageously, phenomena which show much agreement with those observed
by Chitty (1955, 1957) in the vole, Microtus agrestis.
If we assume for the moment that the numbers of Bupalus piniarius
are regulated by this principle - the reader is reminded that the final
proof for an influence of density on the viability of the offspring under
field conditions has still to be provided - then at least two questions
arise. The first is concerned with the fact that in Germany the pine
looper is a serious pest. How can it reach such extremely high levels
without being suppressed by the effects of the growing mutual contact
between larvae? There is some evidence indicating that the inhibition
of growth is intensified up to a maximum density. Above this level
there seems to be no further decrease of pupal size, and the same may
hold for the viability of the offspring. Further, the data available
suggest that the density induced decrease of the viability is dependent
on weather conditions and it might be that in the foci of Bupa1u.s
infestations in Germany, the results of larval interference are suppressed by the prevailing weather at critical times, thus giving rise to
infestations within two or three generations.
H . KLOMP
dependent action of predators or parasites. In spite of thorough research
in this field this conception has never been definitely established.
Another theory emphasizes the significance of an increase in the interaction between the individuals of a population with growing density,
and the consequent deterioration of the physiological condition of the
animals, giving rise to self-regulation (Chitty, 1957; Wynne-Edwards,
1962). However, the advocates of the former theory sometimes claim
that, because of the low population density of plant-eating insects,
direct competition, e.g. for food, can be excluded as a causative agent,
and the influence of mutual contact interference can, in their opinion,
be minimized for the same reason. This conception has definitely no
general validity, as was pointed out recently by Kennedy and Way
(1964), who cite several instances of interference at low density levels in
various insects. It is therefore worthwhile to examine processes of this
type to assess their importance for the regulation of insect numbers.
The results of the present study clearly support the theory of selfregulation more than that of the parasite-predator thesis. It is true
that the advanced larvae of the pine looper are subject to density
dependent predation, but the regulatory effect of this relationship was
not of primary importance during the period of observation. In addition, it was found that the larvae interfere a t densities far below the
food capacity of the pine trees, thus giving rise to an inhibition of larval
growth and a decreased fecundity at the higher levels. Moreover, there
is strong evidence that in addition to the effects just mentioned, the
viability of eggs and larvae of the next generation is affected disadvantageously, phenomena which show much agreement with those observed
by Chitty (1955, 1957) in the vole, Microtus agrestis.
If we assume for the moment that the numbers of Bupalus piniarius
are regulated by this principle - the reader is reminded that the final
proof for an influence of density on the viability of the offspring under
field conditions has still to be provided - then at least two questions
arise. The first is concerned with the fact that in Germany the pine
looper is a serious pest. How can it reach such extremely high levels
without being suppressed by the effects of the growing mutual contact
between larvae? There is some evidence indicating that the inhibition
of growth is intensified up to a maximum density. Above this level
there seems to be no further decrease of pupal size, and the same may
hold for the viability of the offspring. Further, the data available
suggest that the density induced decrease of the viability is dependent
on weather conditions and it might be that in the foci of Bupa1u.s
infestations in Germany, the results of larval interference are suppressed by the prevailing weather at critical times, thus giving rise to
infestations within two or three generations.