Population studies ofCavelerius saccharivorus Okajima (Heteroptera: Lygaeidae): Adult dispersal in relation to the density

Summary Two adjacent stations (H and L) were set up to study movement ofC. saccharivorus adults in sugarcane fields. At the beginning of the study the density ofC. saccharivorus including all stages of development was quite different between the two. The density of the first generation adult on Station H was about 5 times that on Station L. The number ofC. saccharivorus on both the stations became almost the same one month after the beginning of the study. At the beginning of the study macropterous adults were more numerous in Station H than in Station L. However percentage of macropterous adults on Station L increased after one month whereas that on Station H declined. About 2,000 marked adults were released on each station during the early period of the emergence of the first generation adult. Marked insects were recaptured on both the stations one month after the release. The adults released on the dense population (H) tended to disperse more actively than those on the scarce population (L). Marked macropterous adults moved more actively than brachypterous ones. The density related dispersal of adults was considered to be an important factor to regulate the population density.     

Ecological significance of the wing polymorphism of the oriental chinch bug,Cavelerius saccharivorus Okajima (Heteroptera: Lygaeidae)

The wing polymorphism of the oriental chinch bug, Cavelerius saccharivorus, was studied in relation to its reproductive strategy. The frequency distribution of wing form in terms of the relative wing length (R.W.L.) changed with increase in the population density from a single modality biased toward the brachypterous form to bimodality with both macropterous and more extreme brachypterous forms. Since some evidences in the field showed that fliers are limited to macropters, such a bimodality of wing form indicated that C. saccharivorus employs a mixed strategy with some portion of adults staying on in the same habitat, whereas the rest disperse. Moreover, macropters were larger in body size than brachypters emerging in high density populations, although smaller than brachypters emerging in low density populations. Larger body size as well as the delay of ovarian maturation in macropters was considered to be a beneficial trait for migratory behaviour.     

Dispersal and reproductive properties of sugarcane bugs,Cavelerius saccharivorus

The distribution of sugarcane bugs, Cavelerius saccharivorus is remarkably patchy at time of the peak emergence of the first generation adult in early summer. In population near carrying capacity 70% of adults become macropterous, and they move to sparse populations or vacuum areas. The experiment on the reproductive difference between females from field populations with different densities, showed that the overcrowding restricted their oviposition. On the further experiment it was shown that reproductive ability of macropterous adults or dispersers was not much inferior to that of brachypterous from sparse populations. Overcrowding at time of the peak emergence is reduced by the dispersal. Hence the intraspecific competition is a transient condition. The oviposition is at first restricted in residents of dense populations as compared with brachypterous adults of sparse populations, but the difference becomes small as the former density decreases by the emigration. The last section considers the relation of the reproductive rate and dispersal risk to the rate of dispersal with the simple model, which explains the characteristic dispersal of this species.     

Population studies ofCavelerius saccharivorus Okajima (Heteroptera: Lygaeidae): A few findings on population interchange

The change in population density of Cavelerius saccharivorus was studied in the sugar cane field and the Miscanthus habitats around it. This species spent 2 (or partly 3) generations a year. It was suggested that the density rose in the sugar cane field where the density had been low enough, when the population density increased in the nearby sugar cane fields. Likely, the density in the Miscanthus habitat increased with the density in the nearby sugar cane field. It seems that these are mainly due to flying movement of adults, and that the movement takes place in every season. It was found through the investigation in the sugar cane field that the percentage of long-winged adults increased with the population density. These adults seemed to emmigrate to the nearby sugar cane fields and Miscanthus habitats. The remarkable increase in the percentage of long-winged adults in the sugar cane field was probably due to the immigration into there. In the Miscanthus habitat the adult population was consisted mainly of long-winged ones. Through all the observations, it was suggested that the adult movement was associated with the population regulation and performed chiefly by long-winged adults.     

Genetic correlation of wing polymorphism between females and males in the oriental chinch bug,Cavelerius saccharivorus Okajima (Heteroptera: Lygaeidae)

Genetic correlations of the wing form and the relative wing length between females and males were estimated in the oriental chinch bug, Cavelerius saccharivorus, by calculating the correlation between the sexes of the proportion macropterous or the mean relative wing length in full-sib families obtained from different wing forms of parents emerged in a high density population. There was a significantly positive genetic correlation between the sexes in both the proportion macropterous and the mean relative wing length. However, the appearance rate of macropters tended to be much lower in males than in females under the rearing conditions which promote the appearance of macropters. This was evident especially in the offspring of brachypterous parents. These indicated that in C. saccharivorus the wing polymorphism of males is not a simple result of the genetic correlation of wing morphology between the two sexes. It was considered that both of the female and male fitness advantages to wing reduction, as well as the genetic correlation between the sexes, would influence the evolution of wing polymorphism in this species.     

A male fitness advantage to wing reduction in the oriental chinch bug,Cavelerius saccharivorus okajima (heteroptera: Lygaeidae)

A male fitness advantage to wing reduction was investigated for the oriental chinch bug, Cavelerius saccharivorus, which is wing dimorphic. Field surveys for the frequency of matings between wing morphs showed that the mating probability was much higher in brachypterous males than in macropterous males in the early breeding season. Brachypterous males copulated with not only brachypterous females but also macropterous females in this season. This led to a considerable rate (30%) of insemination of macropterous females just before emigration even in the early breeding season. A rearing experiment for the pre-reproductive period of females revealed that females mated with brachypterous males copulated and oviposited earlier than females mated with macropterous males. These results indicate that there is a fitness advantage to wing reduction in the males of C. saccharivorus in terms of the earlier sexual maturation.     

Reproductive properties of the oriental chinch bug,Cavelerius saccharivorus Okajima (Heteroptera: Lygaeidae), in relation to its wing polymorphism

Reproductive properties of the oriental chinch bug, Cavelerius saccharivorus, were investigated for three types of female adults: brachypters produced in both high and low density populations and macropters produced in high density populations. With respect to egg-laying potential in terms of clutch size, clutch number, and total fecundity, macropters were superior to brachypters produced in the same high density population, although inferior to brachypters produced in the low density population. The mean fecundity of female adults in each type was positively correlated with mean body length. Macropterous females markedly prolonged their initiation of both copulation and oviposition and had a much longer life span compared with both types of brachypters. However, there was no fundamental difference in reproductive schedules between the two types of brachypters.     

Phenological studies on the wing dimorphism of a semi-aquatic bug,Microvelia douglasi (Heteroptera: Veliidae)

Seasonal changes in the percentage of macropterous individuals in natural population of Microvelia douglasi in permanent habitats were investigated in Shimane, western part of Japan. Macropterous forms of 5th instar nymphs occurred most frequently from late June to early July. However, the seasonal appearance of macropterous adults differed between habitats and years. There were significant effects of density, temperature, photoperiod and food availability on the percentage of macropters under laboratory conditions. The percentage of macropterous forms generally increased with density, but the mode of the response was modified by temperature, photoperiod and food availability. When insects were reared under long day photperiod (16L8D) with abundant food (0.5–1.0 g/container/day), the percentage of macroptery varied most among the different density treatments at the intermediate temperature (24°C) and the difference was reduced at higher temperature (28° and 32°C). Effects of temperature were small under the short day photoperiod (12L12D). Under 12L12D photoperiod sensitivity of density responses were higher at 32°C and lower at 24°C than those under 15L8D photoperiod. When the insects were supplied with food every other day, the density response became obscure. Ecological significance of environmental responses in the wing morph determination was discussed in relation to the heterogeneous environment of water surface.     

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.

     

Biology ofhyphantria cunea drury (lepidoptera : Arctiidae) in Japan. IX. population dynamics

Fluctuations in population density of Hyphantria cunea in Japan are characterized by a gradation-like pattern. Analysis of the life table data taken from two stations during eight successive generations showed that (1) mortality during egg and early larval stages was density-independent, (2) mortality during later larval stages was inversely density-dependent, and (3) mortality during prepupal and pupal stages was density-independent. Thus the overall mortality process from egg to adults eclosion was inversely density-dependent. The inverse density-dependence in mortality process during later larval instars was mainly attributed to the ‘escape’ (VOÛTE, 1946) of H. cunea populations from the predation pressure of polyphagous predators such as birds and Politses wasps. This inverse density-dependence was considered to be a cause of gradation-like fluctuation. Field collection of egg-masses showed that the mean number of eggs per egg-mass, which was believed to be a good representation of mean fecundity, varied from 425 to 1050 during 4 years. Density-dependent reduction in the mean number of eggs per egg-mass was demonstrated, and this reduction was a factor regulating the population density. Assuming fixed sex ratio and survival rate of adults, a preliminary population model was constructed. The number of eggs laid in the survey station could be predicted well by the model based on the number of eggs laid in the previous generation, in 9 out of 13 cases. An attempt to apply a model of the same type to mimic the fluctuation of abundance (peak number of larval colonies per tree) on road-side trees suggested another density-dependent process, that is, insecticide application by man. Discussion was also presented on the causes responsible for the turn of population trend from decreasing to increasing in the 1st generation of 1968.     

Damped oscillation of population density at equilibrium

It has been theoretically assumed that the population density at the equilibrium oscillates with damping from generation to generation. In the adult population of the southern cowpea weevil, Callosobruchus maculatus, it was exemplified. But, it was not so clear in the adult population of the azuki bean weevil, C. chinensis as seen in that of C. maculatus. This difference seems to be due to the scramble type of competition that occurs in larval stage in C. maculatus, instead of in the egg stage as C. chinensis. Comparing with the oscillation from generation to generation obtained in the present experiment to that of Lucilia population found byNicholson , the oscillation in Lucilia population is composed of the cycle in a generation and the descending phase of each cycle of it is not regulated density-dependently. The present result seems to be more appropriate for the demonstration of the theory of self-adjustment of population.     

Mortality pattern and age structure in two carabid populations with different seasonal life cycles

1. Adult populations of two carabid species, a spring breeder, Carabus yaconinus, and an autumn breeder, Leptocarabus kumagaii, were studied in a lowland habitat (area: 16,500 m²) by pitfall sampling and mark-recapture method. The seasonal change in population number, age structure and mortality pattern were clarified and related to their seasonal life cycles.
2. The survival rate of new adults from emergence to the first reproductive season was on the average 48% in C. yaconinus and 55% in L. kumagaii. In C. yaconinus, beetles which emerged later in the season survived more to the first reproductive season than those emerged earlier. C. yaconinus which had survived the pre-reproductive period mostly died out during the first reproductive season of 5 months, and about 8% survived until the second season. In L. kumagaii which had a short reproductive period in autumn, about 20% survived to the second reproductive season, and a small proportion even to the third reproductive season. Accordingly, the proportion of old beetles in the reproductive population was higher in L. kumagaii than in C. yaconinus.
3. The reproductive population of C. yaconinus contained on the average 1600 beetles and produced 3300 new adults. the L. kumagaii population contained on the average about 530 reproductives in autumn, and about 820 beetles emerged in the following year. The recruitment rate of new adults of C. yaconinus was higher than that of L. kumagaii, and this resulted in its higher population density. In L. kumagaii, however, the high adult survivorship and iteroparous reproduction were important for its population growth.
4. Relationship between seasonal adaptation and demographic strategies in the carabid populations were discussed.

     

Life table studies on the paddy field population of the green rice leafhopper,Nephotettix cincticeps Uhler (Hemiptera: Cicadellidae), with special reference to the mechanism of population regulation

The population growth of the green rice leafhopper, Nephotettix cincticeps, in the paddy field was analyzed based on the life table data accumulated for six years. The paddy field population, which stems from the invading adults of the first generation (G-I), repeats two complete generations, and the hatchlings of the fourth generation (G-IV) enter diapause and overwinter as the fourth instar nymphs in fallow paddy fields. It was clarified that the density dependent reduction in the mean longevity and oviposition rate of adult females in G-II and III played a primary role in stabilizing the annual population densities. The annual average of the mean longevity of G-II females (3.9 days) was much shorter than that of G-III ones (7.7 days) and thus the density dependent reduction in the mean longevity induced a more prompt regulatory effect on the oviposition of G-II females compared with G-III ones. As the result, two equilibrium densities of eggs were obtained, e.g., ca 100 and 700 eggs per hill in G-III and IV, respectively. Density dependent decrease in the proportion of mature females in the adult population was especially conspicuous in G-II, and this was closely associated with the density dependent reduction in the mean longevity and fecundity. Thus, the density dependent dispersal (emigration) of the adult females by flight in G-II and III was the most convincing factor in the process of population regulation. The density dependent dispersal of the adult females is effective in avoiding the deleterious effects of nymphal crowding in a breeding habitat unit (a paddy field), and may result in a more even distribution of the population over a continuous habitat units in a locality than otherwise.     

Spatio-temporal analysis of the population dynamics of the oriental moth,Monema flavescens (Lepidoptera: Limacodidae)

The comparative analysis of life tables of the oriental moth, Monema flavescens, obtained in 6 patches for 8 generations in 4 years revealed the following:

1. The ratio of maximum to minimum of cocoon density for each patch ranged from 5.34 to 22.50, each value being more than 3.20, the ratio for the whole study area.
2. The density change from adult to cocoon in the next generation caused most of the spatial variation in density change per patch. The rate of adult-to-cocoon population change showed spatial density dependence in some generations but not in others. When the change rate lacked spatial density dependence, it was the key-factor for spatial variation in adult density for the following few generations till the change rate recovered spatial density dependence. This was because of flooding, which killed the spatial density dependence existing potentially in the adult-to-cocoon change rate and damaged the same patches during the few successive generations.
3. The rate of population change from overwintered generation adults (summer ones) to first generation cocoons was not only the key-factor for the rate of throughout-the-year change but temporally density dependent in each patch. Therefore, the density for the whole study area is considered to fluctuate within a range. However, the strong equilibrium seen in the cocoon density for the whole study area was due to the floods that happened to occur when the density was near and at its maximum, and it is considered that such a strong equilibrium does not always occur.
4. In the population change from summer adults to first generation cocoons, temporal density dependence was found in all the patches, but it was found only in one patch in the population change from autumn adults to second generation cocoons. This was because the spatial density dependence seen in the former corresponded to the absolute density of adults, while that in the latter corresponded to the relative density.

     

Studies on the interspecies competition between the azuki bean weevil,Callosobruchus chinensis,and the southern cowpea weevil,C. maculatus V. The role of adult behavior in competition

Summary One strain (iC) ofC. chinensis and two strains (aQ and tQ) ofC. maculatus were used to study the temporal pattern in adult distribution in oviposition site. The density of iC in the oviposition site decreased faster than the density in the population cage, but the pattern was not affected by the presence of the other species. The density in ovipositon site of aQ and tQ controls was far lower than that of iC. The density changes in the controls of both strains were rather parallel to the pattern in the population cage. The density of aQ and tQ in the oviposition site in mixed cultures was affected by the presence of the other species. The density of aQ was less than half that in the controls. The pattern of density change was almost the same as in the controls. The density of tQ in the mixed cultures, although lowered initially by the presence of iC, increased after the density of iC decreased. Coupled with its longer female longevity, this characteristic of tQ behavior at the oviposition site apparently contributed to its winning the competition with iC. The adult distribution pattern observed in the present experiments provides an explanation for the peculiar two-fold increase in mean individual weight ofC. chinensis in mixed cultures compared with controls, whileC. maculatus adults were a little lighter in mixed cultures than the controls. Contribution from the Entomological Laboratory, Kyoto University, No. 432.     

Temperature-dependent wing dimorphism in a Japanese strain of tobacco thrips, Frankliniella fusca (Thysanoptera: Thripidae)

A Japanese strain of Frankliniella fusca (Hinds) was reared under 4 temperature and two photoperiodic conditions on leaves of green bean. On the basis of the ratio of fore wing length to head width (RWL) of adult thrips we defined individuals with RWL above 3.7 as macropterous forms and those with RWL below 3.5 as brachypterous forms. Percentages of macropterous individuals among females were 0 % at 18 °C, 10.6 % at 22.5 °C, 21.8 % at 25 °C, and 77.5 % at 27.5 °C, under 15L–9D. The proportion of macropters among females developing under 15L–9D was no different from that among those developing under 10L–14D. These results indicate that photoperiod does not affect wing form, and suggest that the proportion of macropterous females increases with temperature. Although development time from hatching to adult eclosion, adult pre-oviposition period, adult head width, adult longevity, total fecundity, and fecundity schedule were compared between macropterous and brachypterous females developing at 25 °C and 15L–9D, these traits were not significantly different between the wing forms. Males of this experimental strain were all brachypterous.     

Role of a parasitoid with a low fecundity,Praestochrysis shanghaiensis (Hymenoptera: Chrysididae), in the population dynamics of its host

1. Analysis of life tables of the oriental moth, Monema flavescens, obtained for 8 generations over 4 years, disclosed that the cocoon parasitoid, Praestochrysis shanghaiensis, acted as a density-disruptive factor.
2. The density of the host cocoon remained stable (max./min.=3.2), whereas that of the host adult varied (max./min.=14.3) although both showed similar fluctation patterns.
3. Stability of the host population was associated with the density-dependence in the ratio of first generation cocoons to overwintered generation moths, which was the key factor for the rate of change throughout the year. Chrysidid parasitism among the first generation cocoons ranged from 37.7 to 70.1%, and that among the second generation cocoons from 16.7 to 63.2%, each showing an inverse density-dependence and acting as the main determinant (key-factor) of the between-year variation in the density of the adult moths.
4. The density-dependence of the rate of change from overwintered generation adults to first generation cocoons was so strong that the parasitism on the second generation hosts had not effect on the cocoon density of the first generation. On the other hand, the density-dependence of the rate of change from first generation adults to second generation cocoons was weak, and the parasitism on the first generation hosts became the key factor for the between-year variation of the second generation cocoons.
5. It is suggested that the stability of the parasitoid-host system will be disrupted without three parasitism-restricting factors: asynchrony in the parasitoid attack on the second generation hosts, high mortality among parasitoid larvae of the second generation, and the high proportion of those first generation parasitoids that enter diapause. These factors are considered to be effective only in cooler parts of the distribution of the parasitoid.

     

Life table analysis of the green rice leafhopper,Nephotettix virescens (distant) (Hemiptera: Cicadellidae), and efficient vector of rice tungro disease in asynchronous rice fields in Indonesia

Population dynamics of Nephotettix virescens was studied in 17 paddy fields transplanted at intervals of about 1 month in 1988–1990. The adult density was highest either in the immigrant or the 1st generation and sharply decreased to the 2nd generation. The survival rate of the 1st generation was lowest in the transition season when areal population density increased. Key factor analysis revealed that the nymphal and adult mortality of the 1st generation (kn) was the principal source of population fluctuations. No significant correaltion was found between kn and natural enemy density, natural enemy density/healthy egg density, or the precipitation during the nymphal period. On these bases adult emigration was suspected to be the key factor. Areal population build-up of N. virescens in the transition season was considered to occur as a result of increasing immigration to young stages of rice.     

Differential allocation of protein resources to flight muscles and reproductive organs in the flightless wing-polymorphic bug, <Emphasis Type="Italic">Pyrrhocoris apterus</Emphasis> (L.) (Heteroptera)

Differences in the growth of dorsolongitudinal flight muscles and gonads in 1–28 days old long-winged (macropterous) and short-winged (brachypterous) adults of the firebug (Pyrrhocoris apterus L.) and the resource allocation to these organs were studied by means of total protein analysis. We found predominant allocation of food resources to flight muscles compared to reproductive organs in both macropterous males and females during the first 5 days of adult life. Subsequent histolysis of developed flight muscles coincided with increased total protein content in some reproductive organs. Initiation of intensive food intake after starvation or application of higher dose of methoprene on macropterous adults changed the resource allocation in favour of growth of reproductive organs and induced precocious histolysis of flight muscles. It indicates an involvement of juvenile hormone in wing morph-related differential allocation of resources in the bug. Increased total protein contents in the ovaries and accessory glands of starved macropterous females and males treated with methoprene, respectively, indicate that proteins derived from the methoprene-induced histolysis of the flight muscles are re-utilized for the growth of the reproductive organs. It is the first report of persistence of differential resource allocation to flight muscles and reproductive organs in the wing-polymorphic insects with non-functional macropterism.     

Juveniles stick to adults: recruitment of the tube-dwelling polychaete Lanice conchilega (Pallas, 1766)

Juveniles of the tube-dwelling polychaete L. conchilega were found attached to adult tubes at an exposed beach in South Wales (U.K.). They attached to the upper part of tubes protruding above the sediment. The aim of this study was to determine the timing of juvenile settlement on to adult tubes, their abundance and size distribution, and the time of detachment and densities in the sediment after re-settlement. Furthermore, to establish whether the sediment composition of juvenile tubes differed from those of adults and the surrounding sediment. In 1998, 1999 and 2000, tubes of adult L. conchilegawere sampled and numbers of attached juveniles were quantified and their sizes measured. Densities of solitary L. conchilega in the sediment were estimated, as well as the size distribution of solitary tubes. On average, 5–13 juveniles were attached to single adult tubes in spring, but numbers varied greatly with up to 72 juveniles on a single tube in April 1998. Availability of a hard substratum and chemical cues are discussed as potential factors triggering the attachment of juveniles to adult tubes. Juveniles started to detach and re-settle in the sediment 1 month after attachment. Once settled in the sediment, juvenile numbers decreased steeply, but the size distribution of solitary L. conchilega suggested that some juveniles survived in the population. However, the extent to which numbers of juvenile recruits contribute to the density of adults remained unclear. Juveniles used a smaller mean particle size for tube building than adults, but both actively selected particles.