The effects of age and weather on egg laying inDanaus plexippus L. (Lepidoptera: Danaidae)

Summary The effects of age and weather conditions on egg laying inD. plexippus were determined for caged females. Age (measured in physiological time), temperature and solar radiation influence egg laying in this species of butterfly. An algorithm taking these factors into account in presented and accounts for 88% of the daily variation in egg laying. CagedD. plexippus begin to lay eggs six—seven days after emergence, peak egg production (about 60 eggs/♀) occurs about 15 days later. Females continue to lay eggs throughout their adult life, which in a flight cage was about 40 days. This egg laying pattern is compared with other published fecundity schedules. The effect and importance of a female being prevented from laying her eggs, on her life-time egg production, is also discussed.     

Density-dependent growth and reproduction of the apple snail, Pomacea canaliculata : a density manipulation experiment in a paddy field

To examine density dependence in the survival, growth, and reproduction of Pomacea canaliculata, we conducted an experiment in which snail densities were manipulated in a paddy field. We released paint-marked snails of 15–20 mm shell height into 12 enclosures (pens) of 16 m² at one of five densities – 8, 16, 32, 64, or 128 snails per pen. The survival rate of released snails was 95% and was independent of snail density. The snail density had a significant effect on the growth and egg production of individual snails. This density dependence may have been caused by reduced food availability. The females at high density deposited fewer and smaller egg masses than those at low density, and consequently produced fewer eggs. The females at densities 8 and 16 deposited more than 3000 eggs per female, while the females at density 128 oviposited only 414 eggs. The total egg production per pen was, however, higher at higher snail density. The survival rates of juvenile snails were 21%–37% and were independent of adult density. The juvenile density was positively correlated with the total egg production per pen and hence was higher at higher adult density. However, the density of juveniles larger than 5 mm in shell height, i.e., juveniles that can survive an overwintering period, was not significantly different among density treatments. These results suggest that snail density after the overwintering period is independent of the density in the previous year. Thus, density dependence in growth and reproduction might regulate the population of P. canaliculata in paddies. Received: October 23, 1998 / Accepted: July 16, 1999     

Egg cluster size variation in relation to the larval food abundance inLuehdorfia puziloi (Lepidoptera: Papilionidae)

Summary Mean egg cluster size ofLuehdorfia puziloi yessoensis varied among habitats. The mean egg cluster size tended to be large when abundance of the larval food leaves expressed as the fresh weight of leaves per unit area at a given habitat was high. Since this variation was observed among closely located study plots (butterflies can easily move between study plots), the egg cluster size variation among habitats is likely to be a result of flexible response by females to varying food abundance for larvae.     

Seasonal life cycle and autogeny in the mosquitoAedes togoi in northern Kyushu, Japan, with experimental analysis of the effects of temperature, photoperiod and food on life-history traits

We studied seasonal changes in the larval population structure, adult size, and autogeny (egg production without a bloodmeal) of the mosquitoAedes togoi on the seacoast of northern Kyushu, Japan. The effects of temperature, photoperiod and food conditions on larval development, adult size and autogeny were studied in the laboratory.Aedes togoi overwintered in both the egg and larval stages and was multivoltine. Adult size was greater in spring and autumn and smaller in summer. Autogeny occurred in spring and autumn but not in summer. Autogenous females were larger than anautogenous females, and larger autogenous females produced more eggs. Laboratory experiments showed that autogeny was promoted under low temperature and short-day conditions which corresponded with spring/autumn conditions in the field. Experimental food limitation in the larval stage greatly reduced adult size and autogenous egg production. In the field, large inter-pool variations in adult size and autogeny rate probably resulted from variations in nutrient and crowding conditions. Seasonal autogeny ofAedes togoi was discussed in terms of life-history strategy under fluctuating environmental conditions where relative advantages of autogenous and anautogenous reproduction alternate seasonally.     

Why doLuehdorfia butterflies lay eggs in clusters?

Summary Luehdorfia butterflies lay eggs in clusters. Clones of their host plants (Asiasarum andHeterotropa) are distributed pacthily among the understory of deciduous forests. Groups ofLuehdorfia larvae often exhaust the clones and may wander over the forest floor seeking new clones. The highest mortality observed is during this wandering period. To elucidate whyLuehdorfia butterflies lay eggs in clusters, a simulation experiment was made for hypothetical populations which lay eggs in clusters or singly. Field data on larval mortality, consumption, density of host clones and leaf weight forLuehdorfia japonica were incorporated into the model. The predictions of the simulation were: (1) When the egg density is low, the single egg type could leave many more pupae than the egg clustering type, but when the egg density is high, the former might leave smaller number of pupae than the latter; and (2) There are optimal sizes of egg clusters for different egg densities and the optimal size becomes larger as the egg density increased. This work was supported in part by Grant-in-Aid No. 439017 and No. 56480039 from the Ministry of Education, Science and Culture, Japan.     

Inheritance of sex ratio in the Kanzawa spider mite,Tetranychus kanzawai Kishida

Summary The frequency distributions in sex ratios of offspring from 4 fieldTetranychus kanzawai populations were examined. No significant difference was observed between average sex ratios of the 4 populations, although there was a great variability within each population, especially in the population fromPueraia lobata. Using the population fromP. lobata, inbred lines with high (H) and low (L) sex ratios were selected. Crossing experiments between H and L showed that sex ratio is determined by the genotype of mothers. It seemed that sex ratio is cointrolled by several genes, with no cytoplasmic factor involved.     

Delayed maturation in the colonial coralGoniastrea aspera (Scleractinia): whole-colony mortality, colony growth and polyp egg production

The present study examines (1) the cost of reproduction on colony growth, and (2) relationships among sexual maturity, whole-colony mortality rate and colony growth rate inGoniastrea aspera free from external influences by macrobenthos. Survival of colonies in permanent plots was followed for two years. Egg production by polyps in colonies collected just before the first spawning of a year was estimated by dissecting the polyps. Growth of the colonies (increase in number of polyps) was followed over one annual reproductive cycle. The cost of egg production on colony growth was apparent through colony ontogeny: (1) immature colonies had a greater annual growth rate than mature colonies, but produced almost no eggs; (2) in mature colonies, growth rate was negatively correlated with NE/PV (number of eggs per polyp volume mm^-3). Annual whole-colony mortality was high in colonies with fewer than11 polyps in initial colony size, while mortality was extremely low once a colony grew beyond this size. This critical size for low whole-colony mortality was much smaller than the colony size (40 polyps) which would attain maturity one year later. Age at maturity was estimated as six years. While survival to maturity may be a selective force for the evolution of delayed maturation, the present data suggest that high colony fecundity, achieved after a long growth period as an immature colony, and an abrupt decrease of colony growth rate after maturation are the crucial forces.     

A genetic perspective on mating systems and sex ratios of parasitoid wasps

Parasitoid sex ratios are influenced by mating systems, whether complete inbreeding, partial inbreeding, complete inbreeding avoidance, or production of all-male broods by unmated females. Population genetic theory demonstrates that inbreeding is possible in haplodiploids because the purging of deleterious and lethal mutations through haploid males reduces inbreeding depression. However, this purging does not act quickly for deleterious mutations or female-limited traits (e.g., fecundity, host searching, sex ratio). The relationship between sex ratio, inbreeding, and inbreeding depression has not been explored in depth in parasitoids. The gregarious egg parasitoid, Trichogramma pretiosum Riley, collected from Riverside, CA (USA) produced a female-biased sex ratio of 0.24 (proportion of males). Six generations of sibling mating in the laboratory uncovered considerable inbreeding depression (∼ 20%) in fecundity and sex ratio. A population genetic study (based upon allozymes) showed the population was inbred (F _it = 0.246), which corresponds to 56.6% sib-mating. However, average relatedness among females emerging from the same host egg was only 0.646, which is less than expected (0.75) if ovipositing females mate randomly. This lower relatedness could arise from inbreeding avoidance, multiple mating by females, or superparasitism. A review of the literature in general shows relatively low inbreeding depression in haplodiploid species, but indicates that inbreeding depression can be as high as that found in Drosophila. Finally, mating systems and inbreeding depression are thought to evolve in concert (in plants), but similar dynamic models of the joint evolution of sex ratio, mating systems, and inbreeding depression have not been developed for parasitoid wasps. Received: November 13, 1998 /Accepted: January 8, 1999     

Population dynamics of a migrant skipper butterflyParnara guttata (Lepidoptera: Hesperiidae) II. Survival rates of immature stages in paddy fields

Summary Survival rates and mortality factors of a migrant skipperParnara guttata were censused in paddy fields in 4 localities of central and western Japan during 1975–1980, and 10 life tables were developed for 3 generations. Mortality rates of eggs, older larvae and pupae were high but those of younger larvae were very low. The high mortality rate was caused by parasitoids. Ten primary and 6 secondary parasitoids were recorded. Out of three egg parasitoids,Telenomus sp. was predominant. Larval parasitoids,Apanteles baoris andPediobus mitsukurii which are specific to genusParnara were predominant in wet land habitat, i.e. paddy land. On the other hand, tachinid flies which have a wide host range and a high searching ability were predominant in dry land habitats. Mortalities of eggs and pupae contribute more to the change in the total mortality of immature stages than those of other stages. egg parasitoids which are the major mortality factor of eggs acted density-dependently but larval and pupal parasitoids did not do so. The total mortality of immature stages occurred more or less density-dependently. Larval density on rice plants scarecely affected the survival rate of larvae.     

Temporal and spatial relationships between an herbivorous lady beetleEpilachna niponica and its predator, the earwigAnechura harmandi

Summary The relationships between egg predation of an herbivorous lady beetleEpilachna niponica (Lewis) and its predator, the earwigAnechura harmandi (Burr), were examined in both time and space. In spite of little annual, changes in egg densities, egg mortality due to predation varied considerably. There was no, clear relationship between the earwig density and the proportionate predation over the five years. The seasonal occurrence of earwig nymphs on thistle plants, however, was closely synchronized with that of egg predation. Predator attacks on the beetle occurred in a time-restricted manner. Thus, later cohorts mostly escape from heavy predatory pressure. No spatially density-depent egg predation was detected at the level of either thistle plants or thistle patches. Furthermore, there was no indication of aggregative behaviour of the earwig in response to local egg density. The earwig density was more likely to be associated with particular localities with sandy deposits available for its nest site.     

Selection for divergent emigration rates in laboratory populations ofMusca domestica

Summary High and low emigration rates through a laboratory system were selected for in populations of house flies (Musca domestica L.). Emigration consisted of movement of flies from on plastic box to another by way of a connecting tube. Selection was carried out by first dividing a wild population of flies into two lines and then selecting for movement from the box in one line and against movement from the box in the other line. The selection experiment was performed twice. In both experiments a statistically significant difference between the two strains was obtained in three to four generations of selection. In the second experiment, after 5 genrations the mean number of high emigration rate flies emigrating in 1 hour was 4.37 times the mean number of low emigration rate flies and in 24 hours was 2.81 times the number of low emigration rate flies. The second experiment was terminated after 6 generations, but the first experiment was continued for 35 generations. In this case, divergence ceased in roughly 15 generations. The possible relevance of the findings to laboratory population experiments in which spatial discontinuities are included is discussed. This study was supported by a research grant of the National Science Foundation (Environmental Biology GB 4567) to Prof. DavidPimentel. The author wishes to thank Prof.Pimentel for his aid and encouragement.     

One-host one-parasitoid system: Population dynamics of a zygaenid mothPryeria sinica Moore in an undisturbed habitat

Summary The population dynamics ofPryeria sinica was investigated in an undisturbed area in 1976–1979. We analyzed the process stabilizing the local population by the life table approach for immature stages and the mark-recapture method for the adult stage. Females usually layed about 130 eggs in an egg-mass. The shape of the survivorship curve was convex and was characterized by a relatively low mortality in the egg and larval stages and by a relatively high mortality in the prepupal and pupal stages. The low mortality in the early stage seemed to be not only due to the peculiar life cycle of this species (larvae develop in early spring when natural enemies are not active) but due to their protective nest-webs, larval warning coloration and repellent smell. The high mortality after cocooning was caused by severe parasitization byAgrothereutes minousubae. The number of adult in the population varied by 2.10-fold, which was less than that of other gregarious moths. The life table data and field observations suggest that adult female dispersal would have acted as a stabilizing factor, andA. minousubae as a conditioning factor on the dynamics of the moth population.     

Behavior and mortality of the southern stink bugNezara viridula in Hawaii

Summary This paper is concerned with some aspects of the ecology of the southern green stink bug,Nezara viridula, in Hawaii. Dispersal studies on adults showed an asymetrical dispersal pattern. The rate of plant to plant to movement by the adults was 3.3–6.7 feet per day. Mortality studies showed that egg predation by ants,Pheidole megancephala, was as high as 80–90 per cent. Egg parasitism byTelenomus basalis was highest during February, 1965, and adult parasitism byTrichopoda pennipes was highest during July, 1964, to January of the following year. The population ofN. viridula declined during May, 1964, to May, 1965. This decline occurred simultaneously with the decline in the number of eggs produced per female. It has been speculated that this reduction in egg production was caused by parasitism byT. pennipes. Published with the approval of the Director of the Hawaii Agricultural Experiment Station as Technical Paper No. 816. It is with great pleasure that I acknowledge with thanks the help that I received in various ways from the following: Dr. WallaceC. Mitchell, Dr. MervinKamran., Mr HarryKaya and Mr. HitoshiKamasaki.     

Modelling and analyzing density-dependent population processes: Comparison between wild and laboratory strains of the bean weevil,Callosobruchus chinensis (L.)

Three models were constructed for analyzing the population characteristics ofC. chinensis on stored beans; model A describing the whole reproductive process with a single equation, model B describing the three age-specific processes (oviposition, egg survival and larval survival) with separate equations, and model C which describes all these processes not for the whole habitat but for the individual beans comprizing it. The logit equation was employed here as a common basis to describe the density-response relationship involved. All three models showed very good fit to the experimental data obtained for both laboratory and wild strains of the weevil. The parameter values characterizing the population dynamics were, however, widely different between the two strains; the laboratory one which had been reared for some 500 generations showed significantly higher reproductive capacity, less sensitive and gentler response to crowding in both adult and egg stages, and more uniform egg distribution among individual beans, as compared with the wild strain newly introduced. Sensitivity analyses using these models suggested that these changes in population characteristics have been attained by the process of domestication or adaptation to stable laboratory conditions through a long period of time. This process seemed in effect to have optimized the population's performances in the laboratory environment. Evolutionary significance of such optimization was discussed with reference to the selection pressure which may have acted upon individuals.     

Biology ofHyphantria cuneaDrury (Lepidoptera: arctiidae) in Japan. V. Preliminary life tables and mortality data in urban areas

Summary Studies on the population dynamics of the fall webworm,Hyphantria cunea have been carried out at three survey stations and along selected roads in the urban area of Tokyo since 1966. Twelve survivorship curves obtained during two years and 8 life tables show that the mortality rate in early developmental stages of the fall webworm is remarkably low as compared with that of other lepidopterous defoliaters and the mortality rate in later developmental stages is compensatory high. The low mortality rate in early stages is considered to be due to the protective role of the nest-web and the lack of egg and larval parasites. All but one parasitic species emerge from prepupae and pupae. Spiders living in the nest-web of the fall webworm play an important role in reducing the number of young larvae. Direct observations and caging experiments showed that relatively high mortality during later larval stages is mainly due to predation by birds (in the first generation) and wasps (in the second generation). The generation mortality in the survey stations always exceeded the level where the population is kept at the steady state, and the outbreak of this moth is considered to be continued by the immigration of adults from large trees growing in gardens on which the larvae can escape from predation pressure. Contributions from JIBP-PT No. 51. A part of this study was supported by the special project research, ‘Studies on the dynamic status of biosphere’, sponsored by the Ministry of Education.     

The screw-worm flyChrysomya bezziana villeneuve (Diptera: Calliphoridae) in a sterile insect release trial in Papua New Guinea

Summary The feasibility of the sterile insect release method (SIRM) was tested against natural populations of the Old World screw-worm fly,Chrysomya bezziana in the Musa Valley, Papua New Guinea. Sterile mating frequencies were determined from egg masses laid by native females on wounded, sentinel cattle. The aerial release of sterilised puparia resulted in low frequencies of sterile matings and few trap recaptures of released material. The release of chilled adult flies resulted in higher frequencies of sterile matings and many trap recaptures. The mean density of males released was 230 males per km² per week over a target area of 361 km² (48% of the valley). Sterile masses were first detected 2 weeks after the release of chill flies commenced, reaching a weekly peak of 33% after 5 weeks of releases with 15% of egg masses found to be sterile during the final month. The levels of sterile matings achieved in this trial were similar to comparable SIRM studies made in the USA, Mexico and Guatemala to control the New World screw-worm fly,Cochliomyia hominivorax.     

Incorporating physiology into parasitoid behavioral ecology: the allocation of nutritional resources

A critical problem faced by most theoretical studies of parasitoid behavior and population dynamics has been the paucity of empirically obtained information about the pattern of resource allocation to egg production and metabolic maintenance in relation to adult diet in female parasitoids. This review calls for a shift from traditional manipulative feeding studies to studies that quantify the energetic budget of parasitoids and which take into account the dynamic nature of metabolic processes. As guidelines, we highlight the advances made along these lines with other insect groups and some of the simplest tools already available today for fulfilling this goal. Received: December 22, 1998 / Accepted: January 12, 1999     

Comparison of life tables between the solitary eumenid waspAnterhynchium flavomarginatum and the subsocial eumenid waspOrancistrocerus drewseni to evaluate the adaptive significance of maternal care

Summary I compared life tables between the solitary eumenid waspAnterhynchium flavomarginatum Smith and the subsocial eumenid waspOrancistrocerus drewseni Saussure in Kyoto, Japan, during 1980–1983. The subsocial eumenid is parthenogenetic in this study area. There were 9 identified mortality factors in the solitary eumenid and 7 in the subsocial eumenid, 6 of which were common to the two eumenids. The important differences of mortality between the two eumenids were seen in the egg, larval, and prepupal stages. In the egg stage, mortality by the phorid flyMegaselia sp. was much lower in the subsocial eumenid (1.4%) than in the solitary eumenid (15.0%) likely because of the matenal care of the subsocial eumenid (progressive provisioning and other related behavior), which reduced predation pressure. In the larval stage, mortality by the miltogrammine flyAmobia distorta was also lower in the subsocial eumenid (8.1%) than in the solitary eumenid (23.8%) also probably because of the maternal care of the subsocial eumenid. A comparison of mortality in the two eumenids between the stable, long continuing natural nest sites and the additional temporal ones showed that the phorid fly remained near its birth place and parasitized stable nest sites. The miltogrammine fly followed returning eumenid wasps and parasitized those nest sites that have a high host density. In the prepupal stage, mortality by endogenous death was higher in the subsocial eumenid than in the solitary eumenid. Mortality due to the rhipiphorid beetle was also higher in the subsocial eumenid probably due to more frequent flower-visits by the subsocial eumenid. The defense mechanism of the subsocial eumenid was discussed in relation to the evolution of subsociality. Contribution to the ecological studies of the eumenid wasps. I.     

Postmetamorphic growth, survival, and age at first reproduction of the salamander,hynobius nebulosus tokyoensis Tago in relation to a consideration on the optimal timing of first reproduction

Summary The postmetamorphic growth and survival of the salamanderHynobius nebulosus tokyoentis Tago were surveyed in the study site located in Habu village of Hinodemachi, a suburb of Tokyo City, during 1975–1981. A laboratory experiment on the growth rate of juveniles was conducted in parallel with the field survey. The result indicated that this salamander grew at the rate of 8,mm in s.v.l. per year during the juvenile stage, but its growth rate decreased markedly as low as 1.8 mm for males and 1.1 mm for females, once it had attained sexual maturity. According to the “capture-recapture” procedure the annual survival rate after metamorphosis was found to be quite high; that is, approximately 0.7. By using the growth rate of juveniles and the difference between the sizes at metamorphosis and sexual maturity, the age at first reproduction was estimated to be 4 year for males and 5 year for females. From the data obtained in this study, the intrinsic rates of increase (r) were calculated for various values of age at first reproduction under different survival schedules, and the relationship between the age at first reproduction and fitness as measured byr was examined. The result indicated that an optimal age maximizing fitness always existed under respective survival schedules, and the observed age at first reproduction of this salamandei was found to coincide well with the predicted optimal age.     

Evaluation of the role ofMicrovelia douglasi atrolineata (Bergroth) (Heteroptera: Veliidae) as predator of the brown planthopperNilaparvata lugens (St?l) (Homoptera: Delphacidae)

Summary The biology of the veliid bugM. d. atrolineata, its predatory behavior, and the effects of plural hunting were studied to evaluate its role as a predator of the brown planthopperNilaparvata lugens in the Philippines. The probability of planthoppers falling onto the water surface and provision of habitat continuity was measured by a sticky trap placed at the base of rice hills in a greenhouse and in paddy fields. The developmental period of immature stages combined was 21 days. If given prey, females laid 25 eggs on the average during an adult life span of 18 days. Starved adults could survive for only 3–5 days. The functional response to prey density was sigmoid, and the maximum number of prey killed was 7 per day. Prey feeding was completed in 12–36 min. The percentage of successful prey attacks averaged 5–8%, decreasing with higher (and larger) developmental. stages of prey, but adult prey were found the soonest. Plural hunting increased the probability of capturing prey by as much as 2.5 times that by individual hunting. Late-instar nymphs, which may be more active, fell from rice hills in a greenhouse more than early-instar nymphs, and the number falling increased with density. In the field the percentage of planthoppers falling to the water in 1 day varied considerably, from 1% for nymphs in one field to 67% for adults in another field. On the basis of work described above and given the high density of veliid predators in flooded paddy fields of tropical Asia,M. d. atrolineata is considered one of the most important natural enemies of the brown planthopper.