In a field population of Bupalus piniarius
in the Netherlands, Klomp (1958a, 1966) found a negative correlation of
population density with growth and adult fecundity. The experimental analysis of this relationship in B. piniarius
is reported in this paper.Part I
describes growth of isolated larvae, under the influence of abiotic factors. The experiments were performed at the former Institute for Biological Field Research (now amalgamated into the Research Institute for Nature Management) at Arnhem, in an outdoor insectary, the laboratory, and a greenhouse.
The number of larval instars varied from four to eight, according to environmental conditions and sex. Warm conditions and long photoperiods tended to increase the number of instars. Females tended to moult one time more than males. Under given environmental conditions, the variation in number of instars was rarely more than one. In the field, growth proceeds through five or six, and, exceptionally through four, instars.
Rate of growth was positively correlated with temperature, especially during the earlier larval stages, and negatively with photoperiod, particularly during the older larval stages. Warmth (e.g., 25° C) and long photoperiods (e.g., 17 hr) became detrimental towards the end of the larval stage.
It is concluded that temperature and photoperiod influence the number of instars in Bupalus
directly, rather than indirectly via the rate of growth.
The period during which external factors determined the ultimate number of larval instars began in the embryonic stage and ended about the middle of the larval stage. The possible endocrine mechanism determining the number of instars is briefly discussed.
Average pupal weight was positively correlated with number of larval instars. For a given number of larval instars, pupal weight was: 1) positively correlated with average temperature during the larval period, within the range of temperatures that permit normal development; 2) reduced by rearing in constant darkness. Slight differences in daylength during the larval stage were not reflected in pupal size.
Morphologically normal pupae were formed after development through four, five, six, and seven larval instars. Moths of four, five, and six-instar larvae were capable of normal reproductive behaviour; seven-instar larvae were not tested in this respect. Sixinstar females produced more ripe eggs than five-instar females. The number of eggs laid did not differ significantly between the two groups, owing to a greater retention of eggs in the former group. No differences in viability of eggs and newly hatched larvae were found between these two categories.Part II
of the present paper reports an experimental analysis of the negative correlation of population density with growth and fecundity in Bupalus piniarius
, and an attempt to evaluate its function. The majority of the experiments, like those of Part I were performed at the former Institute for Biological Field Research. Some were conducted in the field.
Both laboratory and field experiments confirmed the causality of the negative correlation between larval density and growth. In both laboratory and field, the maximum effect was attained at quite low densities, and when food was abundant.
Aggregation under experimental conditions, if not excessively high, did not affect either mortality of larvae and pupae or longevity of the adults. In the first larval instar (L1), aggregation did not influence growth. In all other stages except the last, it retarded and reduced growth, and reduced the efficiency of conversion of food absorbed through the intestine. In the last larval stage, the rate of growth and efficiency of food conversion were about the same in aggregated and in isolated larvae, but weight of the former remained lower because aggregation reduced the duration of this stage.
In some identical experiments, the average reduction in pupal weight due to crowding varied from 12 to 24% in females and from 9 to 17% in males.
The females showed significant individual differences in the effect of aggregation on growth, probably reflecting genetic differences in susceptibility to the density effect.
Aggregated larvae tended to develop through one fewer instar than isolated specimens. This is probably an indirect effect of retarded development caused by crowding.
The behaviour of the larvae is described in some detail. They rested for by far the greater part of the day. Activity was restricted mainly to the night, especially during twilight, except in L1, when there was considerable diurnal activity. Crowded L1 larvae seemed somewhat more mobile than isolated specimens, but behavioural differences between the two categories could not be detected in the other larval stages.
A preliminary analysis of the chemical composition of pupae showed no obvious differences between density categories in content of dry matter, carbohydrates, fat, and nitrogen.
Moth eclosion was slightly retarded after larval crowding.
In the adults, all the linear dimensions considered (including wing loading) were reduced by about the same degree by larval crowding.
Fecundity of females was reduced by larval crowding, as expected from its positive correlation with pupal weight. Females from aggregated cultures laid somewhat smaller eggs, and produced more eggs per unit body weight, than those reared in isolation. Egg retention was reduced by grouping of the larvae. On average, however, fewer eggs were actually laid by females from aggregated cultures.
Tests of offspring viability relative to density conditions in the parental generation yielded contradictory results. All the evidence presently available disproves an earlier conclusion (Klomp & Gruys, 1965) about reduced viability as a result of aggregation. An analysis of field data showed that larval density had a much greater influence on growth than did temperature and daylength, whose effect in the field proved to be insignificant. (Chapter 2)
The stimulus that the larvae exert upon each other did not consist of contamination of the food nor did it involve olfactory or visual perception. Only when the larvae had direct bodily contacts with each other during the night was growth reduced. Growth was not reduced if such contacts were restricted to the daytime.
Regurgitated fluid gut contents, transferred between one another during contacts, were an essential component in the growth-reducing stimulus. Mere mechanical disturbance (to which the larvae showed the same behavioural reactions as to disturbance by other larvae) had no effect on growth.
It is concluded that the essential stimulus induces a physiological change which reduces the rates of feeding and of growth; direct interference with feeding, as a result of mutual disturbance, is not the primary cause of growth reduction.
Even when grouping was restricted to a relatively short period (i.e. one of the larval stages, or five days out of a total larval life of 100 days) there was a lasting reduction in size except when aggregation was restricted to L1.
The number of encounters required for growth reduction is rather low, and the area of pine foliage visited by half-grown larvae is sufficient to permit the occurrence of effective mutual interference at the intermediate and high densities found in the field.
The induction of the density effect is not completely specific. Of four species of pine- inhabiting larvae examined, one geometrid reduced growth as effectively as Bupalus
itself. (Chapter 3)
The expression and the underlying mechanism of the density effect in Bupalus
are compared with similar phenomena in other insect species, and its function is discussed.
High larval density may increase the tendency of female moths to disperse. Density-induced reduction of weight would at least contribute to the realization of this tendency and may even be one of the causes of the increased tendency to disperse. Dispersal from highly populated areas, with a fair chance of reaching areas of low population density, presumably enhances the chance of survival of the offspring. Reduction in fecundity owing to high density seems a small price to pay for more effective reproduction. Some points relevant to this hypothesis are discussed (adult dispersal in relation to larval population density; local variations in population density; survival in relation to population density).
It is tentatively concluded that, rather than being a mechanism for self-regulation of population density, the density effect is an adaptation that avoids mortality due to density-related processes by exploiting the heterogeneity of density occurring over extensive areas. (Chapter 4)