Studies on the population dynamics of a thistle-feeding lady beetle,Henosepilachna pustulosa (Kôno) in a cool temperature climax forest

1. A field study was carried out on the population dynamics of a thistle-feeding lady beetle, Henosepilachna pustulosa (Kôno ) living in a cool temperature climax forest in northern Kyoto Prefecture, central Japan.
2. Intensive marking, release and recapture program was carried out to estimate the adult population parameters by usingJolly-Seber method.
3. Sampling ratio was around 50%. Marking ratio rapidly rised as the census progressed and approached to 100%.
4. Sex ratio (% ♀) was 63–69% in both overwintered and new adults.
5. Daily survival rate was as high as 0.95 or more and constant throughout the season. Adult longevity in the spring was longer than 40 days.
6. Reproductive rate i. e., the ratio of the number of newly emerged adults in a given generation to that of overwintered adults in the preceding generation, is very small, ranging 1–3, whereas winter survival is higher than 50%, consequently the size of populations in the study area remain in a remarkably constant size and it never reached a level where intraspesific competition occurred.
7. The population characteristics of H. pustulosa are compared with those of the two closely related species, H. vigintioctopunctata and H. vigintioctomaculata, which are the pests of Solanaceous crops. Hp is more K-strategic than the two pest species.

     

Survivorship and fertility schedules of two Sumatran tortoise beetles,Aspidomorpha miliaris andA. sanctaecrucis (Coleoptera: Chrysomelidae) under laboratory conditions

Two species of tortoise beetles, Aspidomorpha miliaris (AM) and A. sanctaecrucis (AS) feeding on a shrub-like morning glory, Ipomoea carnea, were reared under laboratory conditions to study their survivorship and fertility schedules. AM and AS required 34–39 days and 30–37, respectively, for the development of the immature stages. The mean longevity of the males was 88.4 days in AM and 63.8 in AS, and that of females was 87.9 days in AM and 83.3 in AS. The mean length of the pre-reproductive period (27.2 days in AM and 33.8 in AS) was much longer than that of the post-reproductive period (10.9 days in AM and 14.3 in AS). Females laid eggs at a nearly constant rate throughout their reproductive period. The reproductive value V_x/V₀ of the two species remained high for most of their adult life, as a result of prolonged survivorship and fertility periods. The total number of eggs produced per female was 442.9 (AM) and 80.1 (AS). The intrinsic rate of natural increase r was 0.070 (AM) and 0.044 (AS) per capita per day. The prolonged reproductive schedules, coupled with strong dispersal power, of these species no doubt have an adaptive value for living in highly disturbed tropical environments, where rainfall is ample but unpredictable and food resources are available throughout the year in a wide area, but distributed in widely flung patches.     

Reproductive strategy of insects as adaptation to temporally varying environments

Diversity of oviposition curve observed in 125 insects was analysed using the ratio of the length of period for development to that for reproduction. On the basis of this empirical data, two extreme reproductive patterns were selected: prolonged reproduction with early maturity and concentrated reproduction with late maturity. Population growth of the species with each reproductive pattern was calculated usingLeslie Matrix under some simulated fluctuating environments where the length of time during which the environments change is short compared with the time required to stabilize age structure. These simulation studies show that there is an optimal ratio of the length of period for development to that for reproduction in achieving high population densities and this ratio varies depending on the favorableness of environment for reproductive success: as the environmental favorableness decreases, the optimal ratio becomes larger.     

Effect of population density on reproduction of two sympatric dung beetle species,Aphodius haroldianus andA. Elegans (Coleoptera: Scarabaeidae)

The effect of population density on reproduction as well as the oviposition modes of two sympatric dung beetle species, Aphodius haroldianus (a low fecundity/high parental effort species) and A. elegans (a high fecundity/low parental effort species) was studied at a pasture in central Japan from 1982 to 1986.

1. The adult population density of A. haroldianus was high (>40 per dung pat). The density of A. elegans was low (<5 per dung pat). Oviposition of A. haroldianus was suppressed above the density of about 10 adults of conspecifics but not affected by the density of other species. The intra- and interspecific density effects on oviposition were not detected in A. elegans.
2. The number of A. haroldianus adults per pat decreased with dung age, while that of A. elegans increased until sixth day after deposition and then decreased. The number of eggs laid per pat was not different between 1-day-and 3-day-old pats for A. haroldianus. However, more eggs of A. elegans were found in 3-day-old pats than in 1-day-old ones.
3. In the both species, the amplitude of population fluctuations was not remarkable; the maximum/minimum ratio for five years being 2.3 for A. haroldianus and 2.9 for A. elegans. Different density dependent processes were suggested to function for the small fluctuation of population density between the two species, i.e. intraspecific density effect on oviposition for A. haroldianus and contest type competition for food among larvae of A. elegans.

     

Studies on the population dynamics of a thistle-feeding lady beetle,Henosepilachna pustulosa (Kôno) in a cool temperate climax forest

Population Ecology -     

Breeding and egg survival of a population of a salamander,Hynobius nebulosus tokyoensis Tago

1. A population of a salamander, Hynobius nebulosus tokyoensisTago was studied at Habu, Hinodemachi, in Tokyo during the breeding seasons from 1976 to 1978. Adults appeared in the pond were captured and marked individually by toe-clipping. Egg sacks were counted and tagged with vinyl tapes for the marking.
2. The annual egg production of this population was found to be constant, about 6500 eggs, and total number of breeding adults appeared in the pond also varied little during three years.
3. The appearance of breeding adults in the pond and oviposition seemed to be influenced by the rise of temperature and rainfall.
4. Males appeared earlier in the pond and stayed there for approximately 10 to 20 days. On the other hand, females appeared later in the pond and left there soon after laying eggs.
5. Mean clutch size and body length of breeding adults appeared in the pond showed a tendency to decrease as time proceeded, which seemed to show that the smaller and younger the salamander was, the later it appeared in the pond.
6. Owing to low temperature, the eggs laid earlier took more time to hatch than those laid later.
7. The survival rate of eggs was 67.2% in 1976, 86.4% in 1977 and 81.2% in 1978. The loss rate of eggs laid in the early and late periods of oviposition was higher than that of eggs laid in the middle period.

     

Factors affecting body size variation within a population of an herbivorous lady beetle,Henosepilachna niponica (Lewis)

Size variation in newly-emerged adults was examined in two different local populations of an herbivorous lady beetle, Henosepilachna niponica, for 1976–80. Mean adult size of both sexes changed rather synchronously in the two populations over 5 years. Body size of adult beetles apparently decreased with increasing leaf damage of the plants on which they developed. Adult beetles which emerged late in the season, associated with increasing food deterioration, were smaller than those which emerged early. Ecological consequences of adult size variation is discussed in terms of oviposition site selection.     

Univoltine and bivoltine life cycles in insects: A model with density-dependent selection

Selection for univoltine and bivoltine life cycles in insects under resource-limited but favourable temperature conditions is analyzed with a difference equation model including density-dependent population dynamics based on the conceptual framework of an evolutionarily stable strategy. The model predicts that the bivoltine type can spread in a univoltine population when the fraction of density-independent rate of annual increase by producing a second generation exceeds the survival rate during diapause of the univoltine type, but monopoly of the bivoltine type is not possible unless it attains an equilibrium population density exceeding that of the univoltine type. The applicability of the model prediction in explaining the occurrence of a partial bivoltine cycle in predominantly univoltine population in the temperate zones is discussed.     

Arthropod predation limits the population density of an herbivorous lady beetle,Henosepilachna niponica (Lewis)

1. The possible impact of arthropod predation on inter-population variation in adult density of a thistle-feeding lady beetle, Henosepilachna niponica (Lewis ) was evaluated by means of predator exclusion experiments conducted in the field.
2. The population density of newly-emerged adults at one habitat in the upstream area (site F) was significantly lower than at another in the downstream area (site A) although the egg density was nearly identical in the two habitats.
3. In the habitat with lower adult density, egg mortality was higher due to higher levels of predation. A predator exclusion experiment demonstrated that arthropod predation was the main factor causing high mortality during the immature stages, and physical factors such as heavy rains were unlikely to influence larval survivals.
4. Earwigs, ground beetles, predaceous stink bugs, and spiders were identified as the main predators in the study area. Of these, an earwig, Anechura harmandi (Burr ) was more predominant than other predators and was significantly more abundant in the habitat with low adult densities.