Longevity, flight ability and reproductive performance of the diamondback moth,Plutella xylostella (L.) (Lepidoptera: Yponomeutidae), related to adult body size

Plutella xylostella in the temperate zone shows a clear seasonal change in adult body size. In the laboratory, large and small moths were produced during immature stages at 15°C and 25°C, respectively. These moths were then used to evaluate longevity, age-specific flight ability, flight ability of mated and unmated females, and the influence of flight experience on the subsequent reproductive success. The large moths lived longer and displayed a greater flight ability over 3 weeks. Irrespective of body size, unmated females flew for a longer time than mated females, and flight experience affected their subsequent reproductive success. Females of both sizes mated and laid eggs soon after emergence, without any obvious pre-reproductive period. More flight experience did not delay oviposition, but did reduce egg production. It is likely that large moths with a longer adult life span and greater flight ability are better fitted for long-distance flight and more fecund than small ones. These experimental results may explain why long-distance migration ofP. xylostella is mostly seen during cool seasons, when relatively large moths with long forewing appear in the field.     

Population dynamics of a phytophagous lady-beetle,Epilachna vigintioctopunctata (Fabricius), living in spatio-temporally heterogeneous habitats. III. Effects of habitat structure on population dynamics

Temporal changes in the population size of a phytophagous lady-beetle were analyzed to identify mechanisms affecting lady-beetle population dynamics at different spatial scales. The study area (15 ha) included 18 habitat patches. The major host plants were potato for first generation larvae and eggplant for second generation larvae. The habitat patches were classified into three groups according to the major host plants in each patch: P-E patches (both host plants available), P patches (potato only), and E patches (eggplant only). The winter disappearance of adults in the whole study area, and larval mortality in E patches were apparently the most important factors disturbing the overall population density. Density-dependent movement of females appeared to have the greatest stabilizing effect on the yearly fluctuation of population density. Rate of increase of female adults from the first to the second generation,R, was generally higher on eggplants in E patches than in P-E patches because the adult density of the first generation was much higher in P-E patches. The yearly fluctuation of adult density in each generation tended to be less in patches with all habitat components necessary for the full life cycle (P-E patches). However, such patches were not favorable for first generation females, as indicated by the lower rate of increase from the first to the second generation. The density and stability of lady-beetle populations is discussed in relation to habitat structure.     

Spatial variation in female fertility related to interactions with flower consumers and pathogens in a forest metapopulation ofPrimula sieboldii

Antagonistic biological interactions with flower consumers and pathogens may influence reproductive success of flowering plants, affecting population dynamics and natural selection for floral traits. However, ecological and evolutionary consequences of the interactions may depend on both spatial and temporal patterns of the interactions. In a forest metapopulation ofPrimula sieboldii E. Morren, an endangered clonal plant species, we measured between-subpopulation patterns of seed sets and interactions with an influential flower consumer, a rove beetle,Eusphalerum bosatsu Watanabe, and a specific smut fungal pathogen,Urocystis tranzschelina (Lavrov) Zundel (Ustilaginales), for three years. Mean female fertility (seed set per flower) for individual subpopulations fluctuated moderately among years but was highly variable within each year among the five subpopulations studied. In two subpopulations, the impact ofEusphalerum beetle, was sufficiently large to result in almost complete failure in seed production over eight years including the three study and five previous preliminary observation years. In the two other subpopulations, seed set failure was caused by infection by the smut fungus. Infected capsules which constitute 10–30% of the capsules produced in the subpopulations were filled with ustilospores instead of seeds. In the subpopulation that escaped flower damage byEusphalerum beetles and smut fungal infection, seed sets of both pin and thrum flowers were much higher than in the other subpopulations. The spatial restriction of individual antagonistic agents to a part of subpopulations suggest that dispersal of the agents, as well as the mode of spatial subdivision of the plant population would be important for determining the overall effects of antagonistic interactions on plant performances at the metapopulation level.     

Differences in adult and reproductive lifespan in the two male forms ofMnais pruinosa costalis selys (Odonata: Calopterygidae)

The damselflyMnais pruinosa costalis (Odonata: Calopterygidae) is unusual in that males are dimorphic, existing as clear-winged non-territorial ‘sneaks’ and orange-winged territorial ‘fighters’. Here we report the results of population census data and behavioural observations in the field and laboratory, and present estimates of emergence period, reproductive period, total lifespan, and reproductive success of each morph. Clear-winged males are smaller and have lower daily reproductive success than orange-winged males, but live for longer in the field and laboratory. We accounted for the difference in the ‘operational reproductive life’ of the two morphs and estimated lifetime reproductive success: there was no difference between clear-winged and orange-winged males. We discuss possible mechanisms for the maintenance of the two forms.     

A galling aphid with extra life-cycle complexity: Population ecology and evolutionary considerations

Summary In most gall-forming aphids, only the fundatrix is able to induce a gall on the host plant. InSmynthurodes betae Westw. (and a few other species), F₂ descendants emerge from the mother gall and induce their own, morphologically different galls. This constitutes an added complexity to the already very complex life cycle of gall-forming aphids. We investigated the ecology ofS. betae on marked trees and shoots at four sites in Israel. Gall initiation, gall distribution and density, and temporal changes in clone size within the galls were investigated during two consecutive years. We discuss the possibility that the two-gall life cycle evolved from the typical one-gall system of most gall aphids, and the possible selective advantage of this added complexity in the life-history strategy of gall aphids. Although the total reproductive output ofS. betae is not higher than in related species with a single gall per life cycle, there seems to be an advantage in the subdivision of each aphid clone into several galls, thus reducing the risk of the accidental extinction of the clone (genotype) by environmental factors, including parasites and predators.     

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

Summary 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 thatC. 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.     

Selection and reproductive success in males of the dragonfly,Orthetrum japonicum (Odonata: Libellulidae)

Reproductive success, copulation success, and mating success were measured for a population of male dragonflies,Orthetrum japonicum. Copulation success explained the greatest variation in reproductive success. The proportion of copulations followed by oviposition was positively correlated with the number of oviposited eggs per mating. Directional selection on four morphological characters was estimated. The effect of selection on correlated traits was comparable to that of direct selection. Directional selection varied between traits and between episodes in a single trait. The probability that the observed directional selection on the four morphological traits was expected under the condition of the selective neutrality of traits was not smaller than 5%.     

Food limitation of population density in the orb-wed spider,Nephila clavata

Summary Field studies were conducted to clarify whether variation in food availability among habitats influences population density, and whether population density has a negative effect on foraging success in the orb-web spider,Nephila clavata. Lifetime food consumption per individual (i.e., foraging success) strongly correlated with mean body size of adult females and mean fecundity in populations. Also, there was a positive correlation between foraging success and population density. Since foraging success reflected potential prey availability in the habitat, food resource appeared to be a limiting factor for populations in this spider. Mean fecundity per individual correlated with population density of the following year, suggesting that decreased reproduction is a major component of food limitation on population density. Consistent defferences in mean body size between particular sites were observed over years, while such difference was less obvious in density. Thus, ranking of food abundance among habitats seems to be predictable between years. A field experiment revealed that an artificial increase in population density had no negative effect on the feeding rate of individuals, suggesting that intraspecific competition for food is not important in this species.     

Spatio-temporal variation of natural and sexual selection in breeding populations of the coreid bug,Colpula lativentris (Heteroptera: Coreidae)

Spatio-temporal variations of lifetime reproductive succes (LRS) of both male and female individuals of a coreid bugColpula lativentris were measured and analyzed using the multiple regression method of Arnold and Wade (1984a, b). The standardized variance of LRS was larger in males than that in females as males often to secure mates for a long period whereas females could easily find mates and oviposit simply dependent on ovarial maturation. LRS was partitioned into 4 consecutive fitness components: (1) reproductive lifespan, (2) copulating efficiency, (3) guarding efficiency (for males) or oviposition efficiency (for females), and (4) number of eggs per clutch. In males copulating efficiency was the largest determining factor of LRS, whereas in females reproductive lifespan was the most important factor. Such tendencies were stable on both a yearly and local basis. Patterns of relative contribution of natural selection (reproductive lifespan and number of eggs per clutch) and sexual selection (copulating efficiency and guarding or oviposition efficiency) to LRS were clearly different between males and females. This sexual difference is, at least to some extent, thought to be brought about by sexual selection among males for mating opportunity, though no physical fight was observed among males. Directional selection on body length was found only in relation to the clutch size of females because large females tended to lay larger clutches. No significant directional selection was found in other fitness components.     

Effects of gall volume on survival and fecundity of gall-making aphidsAdelges japonicus (Homoptera: Adelgidae)

Summary The number of larvae reaching maturity within the gall ofAdelges japonicus was positively related to gall volume, and the relationship between the number of mature larvae and gall volume did not change with different densities of colonized larvae. The population changes in the bud galls ofA. japonicus were surveyed by collecting the galls, which did not suffer predation or parasitism within the galls, from young artificial plantations ofPicea jezoensis over two years. In the year when the density of colonized larvae was high, they suffered a 42% mortality within the galls, whereas mortality was nearly zero in the low density year. The numbers of larvae per gall were positively correlated with gall volume. The regression lines of the number of colonized larvae on gall volume did not differ significantly in the regression coefficients between the two years, whereas the intercept was significantly higher in the year when the density of colonized larvae was higher. However, different within-gall mortality nullified this difference, and the regression lines of the number of mature larvae on gall volume had no significant difference both in the regression coefficients and the intercepts. This suggests that the number of mature larvae per gall was limited by available resources within the gall which were positively related to gall volume. In 25% of the galls in which mature larvae inhabited, the space within the galls were completely filled by the larvae, indicating that space was one of the limiting resources. Gall volume also affected the number of adults that emerged from the gall and the potential number of their progeny.