D Y N A M I C S O F F I E L D P O l ’ U L A T I O N O F PINE LOOPER
299
low, though not significant, probabilities (0.09 and 0-08, resp.). It is
suggested by the great variability of the data, that other, and evidently
very variable factors, have an additional effect on the survival of eggs
and young larvae, and the first factor to think of is variation in weather.
Another question which requires an answer concerns the causation of
the relationship. It is obvious to revert to the influence of larval density
on larval growth and adult fecundity (p. 266) that if density has an
additional effect on the viability of the offspring, i.e. on the eggs and
young larvae of the next generation, then the relationship could be
explained. It will be evident from the results reached in the experiments
discussed earlier in the paper (p. 246), that the question raised here can
also be approached experimentally. This has been carried out by Gruys,
and the conclusive results heavily support the hypothesis that in the
field, larval density not only has an influence on larval growth and adult
fecundity, but also on the viability of the offspring (Klomp and Gruys,
1965). Gruys showed that the extent to which the effect is expressed
depends on the conditions of temperature and humidity, and this supports the view that the great variability of at least the juvenile mortality is due to weather influences. Egg mortality cannot be reduced to
the same denominator, because in the field the egg parasite Trichogramma is the main source of the deaths. We have some slight indications, however, that host eggs of crowded parents are more successfully
infected on the averagb than eggs of solitarily reared parents.
All in all, several of the questions raised in this section are still
unsolved, and the proposed answers provide no more than a working
hypothesis for future investigations. They cannot be regarded as establishing a full explanation of how the number cd individuals in a pine
looper population are regulated.
V I I I . FINAL CONSIDERATIONS
The study of the population dynamics of insects has progressed
farther than that of any other animal group excepting perhaps the birds.
Undoubtedly, the cause of this has partly to1 be looked for in the
phenomenon that many insects are pests, thus making a study of the
factors determining their numbers economically worthwhile.
However, the great majority of phytophagous insect species never
reach infestation levels and their numbers fluctuate far below the
capacity of their food plants. It should be evident that improved knowledge of the dynamics of this group of species should be of great help in
understanding the mechanisms which determine the high levels of the
pests. Several theories have been put forward to explain the low population densities of most plant-eating insects. The oldest views state that
the numbers of the insects are kept down through the delayed density
299
low, though not significant, probabilities (0.09 and 0-08, resp.). It is
suggested by the great variability of the data, that other, and evidently
very variable factors, have an additional effect on the survival of eggs
and young larvae, and the first factor to think of is variation in weather.
Another question which requires an answer concerns the causation of
the relationship. It is obvious to revert to the influence of larval density
on larval growth and adult fecundity (p. 266) that if density has an
additional effect on the viability of the offspring, i.e. on the eggs and
young larvae of the next generation, then the relationship could be
explained. It will be evident from the results reached in the experiments
discussed earlier in the paper (p. 246), that the question raised here can
also be approached experimentally. This has been carried out by Gruys,
and the conclusive results heavily support the hypothesis that in the
field, larval density not only has an influence on larval growth and adult
fecundity, but also on the viability of the offspring (Klomp and Gruys,
1965). Gruys showed that the extent to which the effect is expressed
depends on the conditions of temperature and humidity, and this supports the view that the great variability of at least the juvenile mortality is due to weather influences. Egg mortality cannot be reduced to
the same denominator, because in the field the egg parasite Trichogramma is the main source of the deaths. We have some slight indications, however, that host eggs of crowded parents are more successfully
infected on the averagb than eggs of solitarily reared parents.
All in all, several of the questions raised in this section are still
unsolved, and the proposed answers provide no more than a working
hypothesis for future investigations. They cannot be regarded as establishing a full explanation of how the number cd individuals in a pine
looper population are regulated.
V I I I . FINAL CONSIDERATIONS
The study of the population dynamics of insects has progressed
farther than that of any other animal group excepting perhaps the birds.
Undoubtedly, the cause of this has partly to1 be looked for in the
phenomenon that many insects are pests, thus making a study of the
factors determining their numbers economically worthwhile.
However, the great majority of phytophagous insect species never
reach infestation levels and their numbers fluctuate far below the
capacity of their food plants. It should be evident that improved knowledge of the dynamics of this group of species should be of great help in
understanding the mechanisms which determine the high levels of the
pests. Several theories have been put forward to explain the low population densities of most plant-eating insects. The oldest views state that
the numbers of the insects are kept down through the delayed density