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

Population Ecology - 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...     

Host-dependent association and its implications on the relative abundance of blue alfalfa aphid and pea aphid (Homoptera: Aphididae) on alfalfa in Oklahoma

Field populations of blue alfalfa aphid and pea aphid on alfalfa were sampled during 1985 and 1986 to determine the association of co-occurrence, interspecific interactions and comparative temporal variations in the spatial dispersion patterns of these species in Oklahoma. Relative abundance of these species is discussed in the light of above analyses.Cole' s coefficient revealed a high degree of association between these species in terms of their occurrence on the same alfalfa stems in the field. Regression analyses indicated that the species populations tended to increase in concert on the same stems without evidence of competitive displacement. Spatial dispersion patterns of both species were highly aggregated at low population densities early in the season. Over time, both species tended to disperse and became less aggregated as numbers increased. It was concluded that magnitude of interspecific interactions between the blue alfalfa aphid and the pea aphid were not of a nature that they could be termed as competing species. On the contrary, a concept of an “ecospecies” is proposed for practical applications such as sampling plans and economic threshold determinations.     

Population biology of the helminth parasites of Phoxinus phoxinus (L.), the minnow, in a Cardiganshire lake

Specimens of Phoxinus phoxinus (L.) were collected over a period of a year from September 1967 to September 1968 from Frongoch Lake, Cardiganshire, Wales. Of a total of 650 fish examined for helminth parasites, 32 % were definitive hosts for Macrolecithus papilliger , and 16·5% for Neoechinorhynchus rutili (Müller), both parasites of the alimentary canal. All the fish examined were second intermediate host for Diplostomum phoxini (Faust) in the brain and 1·67% for Ligula intestinalis (L.) in the coelom. An increase in incidence of infestation with M. papilliger in spring and autumn and with N. rutili in summer were evident. The sex of the fish had no apparent effect on worm burden whereas size was more significant. There was a decrease in incidence of infestation with M. papilliger and an increase with N. rutili together with an increase in intensity of infestation with D. phoxini as the fish became larger. The latter species accumulated in the brain as the host increased in age.     

Aphid honeydew and its effect on the phyllosphere microflora of Picea abies (L.) Karst

Aphids of the genus Cinara, feeding on Norway spruce, excrete copious amounts of honeydew, a carbon-rich waste product, which accumulates locally on needles and twigs. We investigated the role of honeydew as a potential source of energy which might promote the growth of micro-organisms in the phyllosphere of conifer trees. To approach this question, we followed the population dynamics of Cinara spp. in a natural forest stand over two seasons. We also studied the amounts of honeydew produced by individual aphids and identified potential parameters which might influence honeydew production. Finally, we determined the growth of micro-organisms on infested and uninfested needles of Norway spruce during the growing season. Confined to Picea abies, the investigated Cinara species only became abundant in midsummer, when needles and shoots were expanding. The populations showed only a single peak in abundance, the timing and magnitude of which may vary from year to year due to weather conditions, changes in plant quality in a yearly cycle or the impact of natural enemies. The amount of honeydew produced by individual aphids was dependent on the developmental stage of the aphid, the nutritional supply of its host plant and on the developmental state of the Norway spruce (e.g. bud burst, end of shoot extension). The presence of honeydew significantly increased the growth of bacteria, yeast and filamentous fungi on the surface of needles and there was a pronounced seasonal trend, with the highest abundance in midsummer correlating with the period of peak aphid abundance. Taken together, these findings indicate that aphids have an influence on microbial ecology in the phyllosphere of trees. The implication of our study, from interactions at the population level to effects and potential consequences for C and N fluxes at the level of forest ecosystems, is discussed.     

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.

     

Population genetic studies of the crown-of-thorns starfish,Acanthaster planci (L.), in the Great Barrier Reef region

Seven populations of the crown-of-thorns starfish, Acanthaster planci, were compared genetically using starch gel electrophoresis in order to investigate the extent of genetic exchange throughout the Great Barrier Reef (GBR) region. These populations extended from Lizard Island in the north to One Tree Island in the south, a distance of approximately 1300 km. Thirteen of 36 enzymes assayed were genetically interpretable, and 10 (77%) of these were polymorphic. Mean heterozygosity over all loci was 0.225. An analysis of the gene frequency heterogeneity between the populations using Wright's F _ST statistic gave an overall F _ST of 0.019. The mean unbiased value of Nei's genetic distance between the populations was 0.009. These values indicate a homogeneous genetic composition throughout the range, and are consistent with the hypothesis that gene flow between these populations is high, and that A. planci throughout the GBR region are members of a single, effectively panmictic population. Within this group, the Green Island population was most distinct genetically because of differences in allele frequencies at the MDH-1 locus. Although there is no rigorous method for determining the selective basis for such differences, it is argued that the differences observed in the Green Island population were the result of selection. The basis for selective differences was possibly food availability since, at the time of sampling, the Green Island A. planci were the remnants of a large, high-density population that caused extensive coral mortality, and suffered severe population decline as food became scarce. These findings are consistent with observations of a relatively ordered sequence of outbreaks from north to south along the GBR, suggesting that all outbreaks but the first are secondary. Control measures, both on the GBR and elsewhere, have been unsuccessful except on a very small scale. Unless a vulnerable part of the A. planci life cycle can be identified, it would seem that the greatest chance for successful control would be to identify and control the causes of the primary population outbreak.     

Infection rates and pathogenicity of trypanosomatid gut parasites in the water strider Gerris odontogaster (Zett.) (Heteroptera: Gerridae)

Summary Trypanosomatid flagellates are common protozoan gut parasites of a wide range of insect species. Water striders (Gerridae) harbour the trypanosomatid Blastocrithidia gerridis. Three different populations of the water strider Gerris odontogaster in northern Sweden were sampled to assess the infection rate dynamics of trypanosomatids. The initially very low infection rates (0%–15%) early in the season were followed by a rapid increase during the reproductive period of the water striders, reaching very high levels (80%–90%). The pathogenic effects of trypanosomatids on G. odontogaster adults were studied in laboratory experiments. The parasites caused a general reduction of host vigour. Male skating endurance was negatively correlated with the intensity of the trypanosomatid infection. However, infection of trypanosomatids increased the mortality among adults only when the water striders were subjected to food stress. The trypanosomatids did not reduce the fecundity of females provided with food. We suggest that trypanosomatid gut parasites may be an important mortality factor in water strider populations. Since the pathogenicity of the parasites is enhanced by stress, parasitism by trypanosomatids may contribute to the regulation of host populations.     

Distribution and variation in the growth of roach, Rutilus rutilus (L.), and dace, Leuciscus leuciscus (L.), in a river catchment in south-west England

The distribution and growth of roach and dace in the R. Exe catchment was studied. Distribution conformed to the classic theories of zonation, although human interference extended or reduced the range of species. Distinct differences were found in the growth rates of roach and dace between different regions of the catchment. Both species achieved their fastest growth in zones where the river topography portrayed their 'preferred' habitat characteristics, i.e. the fast-flowing middle and slow-flowing lower reaches for dace and roach, respectively. Female roach were found to grow considerably faster than male fish, whilst no sexual differentiation in growth rates was found in dace. Considerable fluctuations in year class strength were observed in both species.     

Population fluctuations of cabbage aphid,Brevicoryne brassicae (L.) (Hom.: Aphididae) and its natural parasitism rate on different canola cultivars

The cabbage aphid, Brevicoryne brassicae is one of the most destructive pests in Cruciferaceae or Brassicaceae plant family around the world. According to this pest rapid resistance to insecticide, using the resistant cultivars should be evaluated. In the present study, the different canola cultivars resistance against the cabbage aphid and the population fluctuations in canola fields and also the natural parasitism are evaluated in Shahre-rey region. To evaluate the population fluctuations and its natural parasitism on the RGS003, SLMOO46, Karag2, Licord, Sarigol, Opera, Ocapi and Talaei canola cultivars were cultivated in four replications using the complete randomised block design. The population fluctuations studies showed that the pest is active during the canola growth period in studied region, but the peak of population and damage is found in 6 May month. The results showed that natural parasitism percentage of cabbage aphid on different canola cultivars has not significant difference, but the parasitism amount in different sampling dates was significantly different, so that the natural parasitism maximum observed in date 29 April. These results showed that the different canola cultivars have significant effects on cabbage aphid and parasitoids activity amount. Using the present results, the recognised cultivars in integrated pest management programmes can be used to grow the laboratory aphids as the parasitoids hosts.     

Predation by Nile perch Lates niloticus (L.) on Oreochromis niloticus (L.), Cyprinus carpio (L.), Mugil sp. and its role in controlling tilapia recruitment in Egypt

Vulnerability of three fish groups (tilapia, common carp, mullet) to predation by Nile perch Lates niloticus was evaluated. The study was conducted in aquaria and in cages placed in concrete ponds. Common carp Cyprinus curpio was the more vulnerable to predation by Nile perch followed in order by mullet ( Mugil cephulus and M. cupito ) and Nile tilapia Oreochromis niloricus . Medium-size Nile perch (30–32 cm t.l. ) were able to consume tilapia up to 14 cm t.l. , while tilapia of 15 cm total length were not eaten. Ratios of 1 : 10 to 1 : 15 between Nile perch and ready-to-spawn females of O. niloticus were found to be adequate to control tilapia reproduction.     

Computer simulation of family selection schemes suitable for kale (Brassica oleracea L.), involving half-sib,full-sib and selfed families

Summary Three recurrent selection schemes suitable for kale (Brassica oleracea L.), involving half-sib (HS), full-sib (FS) and selfed (S) families, were compared by computer simulation. All combinations of 6, 12 and 24 families selected, out of 120 and 240 assessed, were investigated for a range of genetical models. Selection was simulated for 20 generations from an initial allele frequency of 0.05 and for 16 generations from an initial frequency of 0.20. With an initial frequency of 0.05 there was a serious loss of desired alleles ranging from 0.31 out of 20 for the HS scheme with 24 out of 240 families selected to 9.19 for the S scheme with 6 out of 120 families selected. It was concluded that if as many as 20 cultivars were included in the initial population the selection scheme should be chosen to minimise the loss. With an initial frequency of 0.20 there were no losses with 12 and 24 families selected in the HS and FS schemes respectively, and the highest loss was 2.88 for the S scheme with 6 out of 120 families selected. It was concluded that if as few as five cultivars were included in the initial population a compromise between the initial response to selection and the loss of desired alleles should be sought. Selecting 6, 12 and 24 families for the HS, FS and S schemes respectively, resulted in average relative responses per generation of 2.28, 2.74 and 2.76, respectively for the first five generations, and losses of 0.22, 0.13 and 0.35, respectively after 16 generations. Practical considerations favour the FS scheme over the S scheme.     

The growth and reproductive strategy of the gudgeon, Gobio gobio (L.), in two hard-water rivers in southern England

Scales from 682 gudgeon from the River Frome and 504 from the River Stour were used for age determinations. Growth in length was confined to the period May to October, but some weight increase occurred during the winter because of gonad development. Growth rates in the two rivers were the same, and were higher than most of those reported from other European rivers. Wide fluctuations, up to 12-fold, occurred in the year-class strengths of Frome gudgeon, caused by variations in the survival of juvenile gudgeon in their first year of life. Density-independent factors, particularly water temperature, were major influences. Female gudgeon grew marginally slower than the males, but they matured earlier and at a smaller size. They were fractional spawners but their total reproductive effort each season was high. Both sexes had low survival rates and their reproductive life spans were rarely more than three years. These life history traits support the premise that these gudgeon populations are r-selected.     

Effect of Tillage and Water Management on the Population Behaviour of Root-Knot Nematodes Meloidogyne Triticoryzae in Rice Crop

The effects of puddling and water regimes on hydraulic conductivity (cm/day) of soil and on bulk density (mg/m 3 ) during rice culture, soil physical characteristics of the experimental field population densities of plant-parasitic nematodes have been studied. Puddling reduced the bulk density of soil and decreased the hydraulic conductivity in the upper layers but not in the deeper layers of soil aeration was reduced due to high moisture levels retained in the puddled soil. Population density of M. triticoryzae declined in puddled soil. The invasion of the roots by the second-generation infective juveniles was reduced. The population density of the root-knot nematodes was higher in the non-puddled soil especially in unsubmerged condition compared to puddled and submerged soil. However, if the seedlings were already infected before transplanting and submergence, the nematode could survive well and reproduce within the aerenchyma of the root.     

Population dynamics ofAzospirillum brasilense and its bacteriophage in soil

Summary This study examined the ecology and interaction ofAzospirillum brasilense and its bacteriophage in soil. Four Chernozemic soils from Canada, a Latosol and three Podzolic soils from Brazil were assayed for phage. Only the Latosol containedA. brasilense phage. None of the soils contained phage for otherA. brasilense orA. lipoferum strains tested. Recovery of phage from soil depended on the growth of indigenous or added host cells. A phage isolated from the Latosol had a hexagonal head of 100 nm and a tail of 200 nm. This phage was morphologically distinct from previously described Azospirillum phage and its host range was limited toA. brasilense strains 29145 and 29711.Survival and recovery of phage added to phage-free soil was dependent on the phage, the initial phage population, the presence of host cells and nutrients, and the soil. Phage persisted in soils at undetectable levels for at least seven weeks, but were still able to interact with multiplying host cells and exhibit a 1000-fold increase in number. Phage required a host cell population of at least 100–1000 per g of soil in order to multiply. The phage burst detected under these conditions increased as the cell to phage ratio increased. Long term incubation studies showed that the activity of phage in soil closely followed the activity of host cells and thus both were manipulated by appropriate amendments to soil.     

Occurrence and seasonal dynamics of Pseudodactylogyrus anguillae (Yin & Sproston) (Monogenea) in eel, Anguilla anguilla (L.), in England

Established populations of Pseudodacrylogyrus anguillae are reported from eels in England for the first time. Prevalence and abundance peak in late summer in all three localities and the parasite overwinters at low levels on eels.     

Preliminary studies on sap-transmissible viruses of red clover (Trifolium pratense L.) in England and Wales

Red clover plants, collected from nine widely separated permanent pastures in England and Wales, were tested for sap-transmissible viruses. Viruses were identified by the symptoms they caused in test plants, by electron microscopy, and by serological tests. Of the 265 plants tested 14% were infected. Only pea mosaic virus was common and widespread; it was found in 8% of the plants, and in seven of the fields. Other viruses isolated were arabis mosaic, bean yellow mosaic, red clover mottle, and red clover vein mosaic; only red clover mottle virus produced diagnostic symptoms in red clover. No viruses were detected in seedlings grown from seed from eighty-nine commercial seed crops. Attempts to transmit red clover mottle virus by the Collembolan Sminthurus viridis L., which is common on red clover, failed.     

A note on the growth-rate and year-class strength of bream, Abramis brama (L.), in three eutrophic lakes, England

In this study some comparative observations on the growth-rate and year-class strength of bream, Abramis brama , in three Shropshire-Cheshire meres are given. All populations are exposed to variable recruitment. The occurrence of four strong year-classes (1959, 1966, 1969,1973) suggests that climate is an important factor influencing spawning success. Water temperatures ≥16°C in July and August may be particularly important. All other intervening year-classes are absent. The growth-rate of bream in Tatton Mere and Ellesmere Mere is good ( L ∞= 541–561 mm) but comparatively poor ( L ∞= 448 mm) in Cole Mere. The significance of population density, food supply and parasitic infestation by Ligula intestinalis is discussed.     

The age, growth and diet of a freshwater population of the flounder, Platichthys flesus (L.), in southern England

Otoliths from 640 flounders from the River Frome, a chalk river in southern England, were used for age and growth studies together with length-frequency data. Mean lengths at ages I–V were 89, 151, 225, 304 and 323 mm respectively; these are higher than have been reported for marine populations. All flounders caught were immature and, on reaching the sea, both sexes probably became sexually mature a year older than did the marine populations. Flounder diets in freshwater comprised largely aquatic insects, Mollusca and Crustacea but there were differences between large and small fish; seasonal changes in diet largely reflected prey availability. It is suggested that competition for food and space in estuaries between 0 group flounders have given rise to the migration into fresh water.