Comparison of native and non‐native predator consumption rates and prey avoidance behavior in North America and Europe

Novel predator–prey interactions can contribute to the invasion success of non‐native predators. For example, native prey can fail to recognize and avoid non‐native predators due to a lack of co‐evolutionary history and cue dissimilarity with native predators. This might result in a competitive advantage for non‐native predators. Numerous lady beetle species were globally redistributed as biological control agents against aphids, resulting in novel predator–prey interactions. Here, we investigated the strength of avoidance behavior of the pea aphid (Acyrthosiphon pisum) toward chemical cues of native lady beetles and non‐native Asian Harmonia axyridis and European Coccinella septempunctata and Hippodamia variegata in North America, hypothesizing that cues of non‐native lady beetles induce weaker avoidance behavior than cues of co‐evolved native lady beetles. Additionally, we compared aphid consumption of lady beetles, examining potential predation advantages of non‐native lady beetles. Finally, we compared cue avoidance behavior between North American and European pea aphid populations and aphid consumption of native and non‐native lady beetles in North America and Europe. In North America, pea aphids avoided chemical cues of all ladybeetle species tested, regardless of their origin. In contrast to pea aphids in North America, European pea aphids did not avoid cues of the non‐native H. axyridis. The non‐native H. axyridis and C. septempunctata were among the largest and most voracious lady beetle species tested, on both continents. Consequently, in North America non‐native lady beetle species might have a competitive advantage on shared food resources due to their relatively large body size, compared to several native American lady beetle species. In Europe, however, non‐native H. axyridis might benefit from missing aphid cue avoidance as well as a large body size. The co‐evolutionary time gap between the European and North American invasion of H. axyridis likely explains the intercontinental differences in cue avoidance behavior and might indicate evolution in aphids toward non‐native predators.     

Consumption,developmental and reproductive attributes of two con-generic ladybird predators under variable prey supply

Availability of aphid prey in habitat is often heterogeneous in space and time and its deprivation causes severe effects on life attributes of ladybird predators. Sometimes ladybirds locate high prey biomass while on certain occasions prey biomass may be either medium/low or altogether absent. Present study has been designed in view of three prey biomass conditions in nature, viz. low, medium and high, selecting Coccinella septempunctata and Coccinella transversalis as experimental ladybirds. Results revealed that consumption, developmental and reproductive attributes of both ladybirds changed in response to prey availability. On high prey biomass larvae had higher consumption and growth rates, developed faster and had low mortality, while emerging adults were large in size, had high consumption rates and utilized prey biomass maximally on production of eggs, self maintenance and survival. In contrast low prey biomass reduced chances of larval survival and emerging adults were small in size, had poor prey consumption rates, low fecundity, egg fertility and short life span. Females exhibited triangular fecundity and egg fertility functions and plateau shaped prey consumption rate function with age, indicating towards their highest reproductive performance during middle age on three prey biomasses; being highest on high prey biomass. On three prey biomasses, females had higher body biomass conversion efficiency during pre- and post-oviposition periods and higher egg biomass conversion efficiency during oviposition period; being highest on high prey biomass. Thus middle aged ladybirds reared on high prey biomass may suppress pest populations better than those reared on low/medium prey biomass.     

Population field studies on the aphidophagous ladybird beetle Harmonia axyridis (Coleoptera: Coccinellidae): resource tracking and population characteristics

To clarify functional and numerical responses to temporal and spatial variations of resources (resource tracking), and the population characteristics of the ladybird beetle Harmonia axyridis, I analyzed the results of a 3-year field observation at 24 sites (seven plant species) on eight species of aphids. The seasonal changes in the number of beetles estimated by the Jolly–Seber method were significantly correlated with those of aphids in the total area. The estimated values of population parameters suggested frequent immigration and emigration of the H. axyridis population, although reproductive rates between spring and summer were rather stable all 3 years (1.87–3.49). The staying time and the daily number of adults and eggs at each site were influenced not only by a single factor but also by interactions among time and quantity and quality of the prey. The adult movement showed two patterns, which corresponded with the movement within and between the subpopulations when an assemblage of H. axyridis occurring on the plants of the same species or genus was regarded as a subpopulation. Adult movement intensely occurred within a subpopulation, although the beetles moving between subpopulations had a significantly greater chance to reach the habitat with a high aphid density. The habitats of H. axyridis could be categorized into a suitable habitat for survival and reproduction and a temporal refuge. The results obtained here suggest that H. axyridis, with high ability of prey searching and reproduction, maintains a stable population in heterogeneous and temporal habitats by its resource tracking mechanisms. Received: March 8, 1999 / Accepted: April 25, 2000     

Effects of photoperiod and diet on diapause tendency,maturation and fecundity in Harmonia axyridis (Coleoptera: Coccinellidae)

The effects of photoperiodic conditions of larval development and adult maturation (L : D = 12 : 12 vs. 18 : 6) and different diets (sugar solution, frozen eggs of Sitotroga cerealella, different numbers of aphids Myzus persicae, and their combinations) on survival, reproductive maturation and fecundity of Harmonia axyridis were studied in laboratory conditions. The fundamental aim of the work was to distinguish between cue effect of diet (neurohormonal triggering of reproduction) and direct effect of diet (nutritional maintenance of reproduction). When adults were kept under short‐day conditions, the proportion of ovipositing females decreased and the duration of the pre‐oviposition period increased. Moreover, a strong reaction to the direction of changes in the day length was demonstrated: when larvae and pupae developed at long day and adults were transferred to short day, the proportion of ovipositing females was much lower than in individuals that were permanently kept under short‐day conditions. The percentage of ovipositing females, the rate of their reproductive maturation and the average daily fecundity gradually increased in the following succession of diets: ‘sugar + 5 aphids per day < sugar + eggs < sugar + eggs + 5 aphids per day < sugar + 100 aphids per day’. However, dissection showed that most of the non‐laying females fed on these diets (particularly those kept under long‐day conditions) have started reproductive maturation, while even first stages of oogenesis were not found in females fed on sugar solution alone. We conclude that cue effect of diet (reproductive activation) can be achieved almost independently of the number of prey consumed, while nutritional effects (the rate of reproductive maturation and fecundity) are sensitive both to the quality and quantity of food.     

Survival costs of reproduction predict age-dependent variation in maternal investment

Life-history theory predicts that older females will increase reproductive effort through increased fecundity. Unless offspring survival is density dependent or female size constrains offspring size, theory does not predict variation in offspring size. However, empirical data suggest that females of differing age or condition produce offspring of different sizes. We used a dynamic state-variable model to determine when variable offspring sizes can be explained by an interaction between female age, female state and survival costs of reproduction. We found that when costs depend on fecundity, young females with surplus state increase offspring size and reduce number to minimize fitness penalties. When costs depend on total reproductive effort, only older females increase offspring size. Young females produce small offspring, because decreasing offspring size is less expensive than number, as fitness from offspring investment is nonlinear. Finally, allocation patterns are relatively stable when older females are better at acquiring food and are therefore in better condition. Our approach revealed an interaction between female state, age and survival costs, providing a novel explanation for observed variation in reproductive traits.     

Rain,prey and predators: climatically driven shifts in frog abundance modify reproductive allometry in a tropical snake

To predict the impacts of climate change on animal populations, we need long-term data sets on the effects of annual climatic variation on the demographic traits (growth, survival, reproductive output) that determine population viability. One frequent complication is that fecundity also depends upon maternal body size, a trait that often spans a wide range within a single population. During an eight-year field study, we measured annual variation in weather conditions, frog abundance and snake reproduction on a floodplain in the Australian wet-dry tropics. Frog numbers varied considerably from year to year, and were highest in years with hotter wetter conditions during the monsoonal season (“wet season”). Mean maternal body sizes, egg sizes and post-partum maternal body conditions of frog-eating snakes (keelback, Tropidonophis mairii, Colubridae) showed no significant annual variation over this period, but mean clutch sizes were higher in years with higher prey abundance. Larger females were more sensitive to frog abundance in this respect than were smaller conspecifics, so that the rate at which fecundity increased with body size varied among years, and was highest when prey availability was greatest. Thus, the link between female body size and reproductive output varied among years, with climatic factors modifying the relative reproductive rates of larger (older) versus smaller (younger) animals within the keelback population.     

Reaction norms of size and age at maturity of Pomacea canaliculata (Gastropoda: Ampullariidae) under a gradient of food deprivation

Pomacea canaliculata, an apple snail native to South America,has become a serious pest of aquatic crops and a promoter ofecosystem changes in natural wetlands worldwide. Its successas an invader has been attributed to its great phenotypic plasticityin life-history traits. Our aims were to determine the reactionnorms of size and age at maturity under a gradient of food deprivation.Full sibling experimental snails were reared in isolation fromhatching and maintained until maturity under seven differentlevels of relative food deprivation based on size-specific ingestionrates. To detect the onset of sexual activity of experimentalsnails, fully mature virgin snails reared in the laboratorywere used as consorts. The reaction norms for age and size atmaturity of P. canaliculata showed marked sexual dimorphism.Shell length was the main component of variation in the malereaction norms for both copulation and egg-laying by femaleconsorts, whereas age was the main component of variation forfemales. Irrespective of the intensity of food deprivation,males mature at the same age at the expense of size, since sizeis apparently irrelevant in the access to females and male fitnesscan be maximized through fast maturation. In contrast, a minimumsize is required for females to reach maturity, perhaps as aresult of their higher reproductive costs. The highly dimorphicreaction norms lead to an increasing lag between male and femalematurity as deprivation increases; in temperate regions, malesborn early in the reproductive season would mature in the sameseason irrespective of food availability, while most femaleswould have to overwinter before attaining sexual maturity inunproductive habitats or those dominated by unpalatable macrophytes.The great life-history plasticity reported in invaded areascould be a heritage from populations in the native range. (Received 10 August 2007; accepted 30 July 2008)     

Resource availability modulates the effect of body size on reproductive development

Within-species variation in animal body size predicts major differences in life history, for example, in reproductive development, fecundity, and even longevity. Purely from an energetic perspective, large size could entail larger energy reserves, fuelling different life functions, such as reproduction and survival (the “energy reserve” hypothesis). Conversely, larger body size could demand more energy for maintenance, and larger individuals might do worse in reproduction and survival under resource shortage (the “energy demand” hypothesis). Disentangling these alternative hypotheses is difficult because large size often correlates with better resource availability during growth, which could mask direct effects of body size on fitness traits. Here, we used experimental body size manipulation in the freshwater cnidarian Hydra oligactis, coupled with manipulation of resource (food) availability to separate direct effects of body size from resource availability on fitness traits (sexual development time, fecundity, and survival). We found significant interaction between body size and food availability in sexual development time in both males and females, such that large individuals responded less strongly to variation in resource availability. These results are consistent with an energy reserve effect of large size in Hydra. Surprisingly, the response was different in males and females: small and starved females delayed their reproduction, while small and starved males developed reproductive organs faster. In case of fecundity and survival, both size and food availability had significant effects, but we detected no interaction between them. Our observations suggest that in Hydra, small individuals are sensitive to fluctuations in resource availability, but these small individuals are able to adjust their reproductive development to maintain fitness.     

Effects of mummy consumption on fitness and oviposition site selection on Harmonia axyridis

Intraguild predation (IGP) has been commonly reported between predators and parasitoids used as biological control agents as predators consuming parasitoids within their hosts. However, the effect of parasitoid–mummy consumption on the fitness of the predator and subsequent oviposition site selection have not been well studied. In our study, we conducted two laboratory experiments to examine the influence of Aphidius gifuensis Ashmead (Hymenoptera: Braconidae) mummies as prey on fitness and subsequently oviposition site selection of Harmonia axyridis (Pallas) (Coleoptera: Coccinellidae). Results indicate that when H. axyridis was reared on A. gifuensis mummies only, its larval development was prolonged, and body weight of the 4th instar larvae and newly emerged adults, and fecundity decreased. Moreover, H. axyridis did not exhibit oviposition preference on plants infested with unparasitized aphids or aphids parasitized for shorter than 9 days. However, compared with plants with mummies (parasitized ≥9 days), H. axyridis laid more eggs on plants with unparasitized aphids. In contrast, H. axyridis previously fed with A. gifuensis mummies did not show a significant oviposition preference between plants with unparasitized aphids and those with mummies (parasitized ≥9 days). Overall, our results suggest that mummy consumption reduced the fitness of H. axyridis. Although H. axyridis avoided laying eggs on plants with A. gifuensis mummies, prior feeding experience on A. gifuensis mummies could alter the oviposition site preference. Thus, in biological control practice, prior feeding experience of H. axyridis should be carefully considered for reduction of IGP and increase of fitness of H. axyridis on A. gifuensis.     

Occurrence of cotton aphids (Aphis gossypii) and lady beetles (Hamonia axyridis) on Hibiscus syriacus Linne: Are the aphids a pest of cucurbits?

Populations of cotton aphid on Hibiscus syriacus increased rapidly from 17 to 24 May 2007, and then decreased as its predator, the lady beetle Hamonia axyridis, increased in number. There was a 10 day time lag between peak populations of aphids and lady beetles. The infestation of aphids on H. syriacus produced some damage, but H. syriacus recovered soon after the lady beetles arrived. Cotton aphid clones from H. syriacus were transferred to other summer host plants: to five different vegetables on two dates, and to cucumber on three dates. Apart from one case where reproduction occurred on eggplant, most H. syriacus aphid clones did not survive on the vegetables. The cotton aphid on H. syriancus was prey and a food source for H. axyridis and acted to conserve natural enemies.     

Transgenerational effects of nutrition are different for sons and daughters

Food shortage is an important selective factor shaping animal life‐history trajectories. Yet, despite its role, many aspects of the interaction between parental and offspring food environments remain unclear. In this study, we measured developmental plasticity in response to food availability over two generations and tested the relative contribution of paternal and maternal food availability to the performance of offspring reared under matched and mismatched food environments. We applied a cross‐generational split‐brood design using the springtail Orchesella cincta, which is found in the litter layer of temperate forests. The results show adverse effects of food limitation on several life‐history traits and reproductive performance of both parental sexes. Food conditions of both parents contributed to the offspring phenotypic variation, providing evidence for transgenerational effects of diet. Parental diet influenced sons’ age at maturity and daughters’ weight at maturity. Specifically, being born to food‐restricted parents allowed offspring to alleviate the adverse effects of food limitation, without reducing their performance under well‐fed conditions. Thus, parents raised on a poor diet primed their offspring for a more efficient resource use. However, a mismatch between maternal and offspring food environments generated sex‐specific adverse effects: female offspring born to well‐fed mothers showed a decreased flexibility to deal with low‐food conditions. Notably, these maternal effects of food availability were not observed in the sons. Finally, we found that the relationship between age and size at maturity differed between males and females and showed that offspring life‐history strategies in O. cincta are primed differently by the parents.     

Effects of food variability on growth and reproduction of Aedes aegypti

Despite a large body of knowledge about the evolution of life histories, we know little about how variable food availability during an individual's development affects its life history. We measured the effects of manipulating food levels during early and late larval development of the mosquito Aedes aegypti on its growth rate, life history and reproductive success. Switching from low to high food led to compensatory growth: individuals grew more rapidly during late larval development and emerged at a size close to that of mosquitoes consistently reared at high food. However, switching to high food had very little effect on longevity, and fecundity and reproductive success were considerably lower than in consistently well‐fed mosquitoes. Changing from high to low food led to adults with similar size as in consistently badly nourished mosquitoes, but even lower fecundity and reproductive success. A rapid response of growth to changing resources can thus have unexpected effects in later life and in lifetime reproductive success. More generally, our study emphasizes the importance of varying developmental conditions for the evolutionary pressures underlying life‐history evolution.     

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

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.     

Pollen as an alternative food for Harmonia axyridis

The current study examines the potential of the multicoloured Asian lady beetle Harmonia axyridis (Pallas) (Coleoptera: Coccinellidae) to use pollen as a food to sustain development and reproduction in the absence of insect prey. Three populations of H. axyridis were used in this study: a long-term laboratory population (since 1998) and a melanic and non-melanic population originating from field collected individuals in Belgium. The insects were allowed to develop and reproduce on frozen eggs of Ephestia kuehniella Zeller (Lepidoptera: Phycitidae), frozen moist bee pollen or an even mixture of the two. Females of the field population offered the mixed diet initiated oviposition sooner than those fed only E. kuehniella eggs, but other developmental and reproductive traits were similar on these diets. A diet of pollen alone allowed 35–48% of the larvae of the field population of H. axyridis to successfully reach adulthood. However, developmental time for these individuals was prolonged by 31–49% and adult body weight was reduced by 37–68%, compared to individuals offered the diets containing E. kuehniella eggs. When fed exclusively on pollen in their larval and adult life, about 40% of the adult females of either field population were able to produce a small number of viable eggs. The laboratory and field strains differed in their response to diet for a number of developmental and reproductive traits. The exploitation of pollen and other plant foods at times when insect prey is scarce, may offer a further competitive advantage to the non-indigenous coccinellid H. axyridis over native European predatory lady beetles that share the same niche and are less capable of using pollen as an alternative food.     

Predation and avoidance behaviour in aphid‐ladybird interactions of native and invasive ladybirds in Europe

1. While detrimental effects of invasive predators on native species are well documented, we often lack a mechanistic understanding of the invasion success. Lack of prey avoidance behaviour can lead to higher consumption rates by invasive predators compared to native predators. This competitive advantage is expected to contribute to the invasion success of non‐native predators.
2. We compared aphid consumption and cue avoidance behaviour of aphids between four native ladybird species (Coccinella septempunctata, Adalia bipunctata, Propylea quatuordecimpunctata, and Hippodamia variegata) and the invasive Asian ladybird Harmonia axyridis.
3. The invasive H. axyridis and the native C. septempunctata consumed more aphids than the three smaller native ladybird species. In line with our expectations, aphids avoided leaves bearing cues of most native ladybird species but not of the invasive H. axyridis.
4. Our results indicate that body size rather than ladybird origin determined aphid predation rates. The lack of aphid avoidance behaviour towards cues of H. axyridis indicates that they were not able to recognise the chemical cues of the invasive predator.
5. Relatively large body size and the absence of cue avoidance in aphids might benefit the invasive H. axyridis, particularly in comparison to smaller native ladybird species. The absence of avoidance behaviour in aphids might lead to even higher predation rates of H. axyridis under more natural conditions.

     

Suitability of greenbugs, cotton aphids, andHeliothis virescens eggs for development and reproduction ofOrius insidiosus

Development, fecundity, and longevity of the predator,Orius insidiosus (Say) (Heteroptera: Anthocoridae), when reared on greenbug [Schizaphis graminum (Rondani)], cotton aphid (Aphis gossypii Glover), or eggs of tobacco budworm [Heliothis virescens (F.)] with green beans or water were examined. Developmental time was shortest when predators were reared onH. virescens eggs and beans and longest when reared on cotton aphids and water. Predators were most fecund when fedH. virescens eggs. The inclusion of beans in the nymphal diet further enhanced fecundity when fed eggs. Longevity of both females and males was significantly shorter when reared on aphids than on eggs. Beans in the nymphal diet enhanced longevity of female predators only in combination with budworm eggs. FemaleO. insidiosus were largest when reared onH. virescens eggs. Addition of green beans in aphid treatments resulted in increased size ofO. insidiosus when compared to aphids and free water.     

Some factors affecting reproduction and sex-ratio of the predacious mite Amblyseius deleoni (Muma and Denmark) (Acari: Phytoseiidae)

Abstract

Several factors were examined to determine their effect on the reproduction and sex ratio in the predacious mite Amblyseius deleoni (Muma and Denmark), in the laboratory. The factors investigated included multiple matings, duration of copulation, capacity of male for mating in excess of females and age of mating females and males. The factors included also, the host plant leaf texture, food deprivation during immature and adult stages, and prey (Tetranychus urticae Koch) density. The results indicated that females of A. deleoni require multiple matings to maximize their reproductive potential, also when copulation was allowed for increasing periods of time, there was a gradual increase in total egg production and oviposition period. A male showed a high reproductive ability for more than 15 days and was able to mate more than once in excess of females. Age of females has an influence on fecundity and sex ratio; old females decreased egg production and produced proportionally more male progeny compared with young females. Similarly, the highest number of eggs deposited per female A. deleoni was reported, when female mated with a young male (0-day old). In addition, males of A. deleoni (at any age) were able to inseminate the females. Results from host plant leaf texture indicated that guava leaf gave the highest reproduction rate, while the fig leaf gave the least female fecundity. Neither the reproductive rate nor the sex ratio of the progeny of females crossed by normal or experimental males had been influenced by the food deprivation during immature stages. A significant lower fecundity was recorded on female's A. deleoni when exposed to different food deprivation programmes during adult stage. The number of eggs laid by the predator female increased with increasing prey density of T. urtice to a maximum of 2.04 eggs deposited per day at a prey density of 30 protonymphs of T. urticae as a prey. As the level of prey density was increased, there was a shift in sex ratio towards an increased proportion of females.     

Temporal changes in fecundity and age at sexual maturity of southern elephant seals at Marion Island

Our objective was to examine the effect of variation in reproductive parameters on the demography of southern elephant seals at Marion Island. We used age-specific capture probabilities of breeding females in a Cormack-Jolly-Seber context to derive reproductive rates. We found that age at maturity declined and fecundity rates increased as the population declined, indicating a compensatory response. Fecundity rates ranged from 0.03 to 0.29 among 3-year-olds (mean=0.16), 0.18 to 0.50 in 4-year-olds (mean=0.40), and 0.28 to 0.50 in 5-year-olds (mean=0.45). We think that a relative increase in food availability, concomitant with the population decline, promoted earlier sexual maturity correlated with more rapid growth of juveniles when population abundance was lower. It is suggested that the relative importance of fecundity in population regulation in elephant seals has been underestimated. Moreover, it appears that the onset of sexual maturity may be the first demographic variable to change in response to a change in population density.     

The effect of rearing the ladybirdHarmonia axyridis onEphestia kuehniella eggs on the response of its larvae to aphid tracks

Larvae ofHarmonia axyridis Pallas (Col., Coccinellidae) exhibited two walking patterns during prey search. Extensive search occurred when searching for prey patches and was characterized by long linear paths and a fast speed. Intesive search, which appeared after the ingestion of a prey in a patch, resulted from a lowering of the linear speed and an increase in the number of stops and angular speed. When larvae reared on the aphidAcyrthosiphum pisum Harris (Hom., Aphidae) crossed an artificial substratum previously contaminated by this prey, they changed their path direction and adopted intensive search. They probably perceived aphid odor tracks and consequently modified their walking pattern. This gustatory capacity probably allowed very mobile larvae to locate prey patches more rapidly and improve encounter with preys in every patch.H. axyridis larvae reared on a substitute prey, the eggs ofEphestia kuehniella Zeller (Lep., Pyralidae), for more than a hundred generations, also changed their path orientation but retained extensive search. The weak response of these larvae to aphid tracks may have resulted from either a decrease in their sensitivity to gustatory aphid stimuli or their difficulty in associating aphid odor with aphid presence. These larvae needed more time and more preliminary encounters than larvae reared on aphids before catching prey.     

Population stability in relation to resource availability in an introduced population of an herbivorous lady beetle

Mechanisms responsible for population stability in relation to resource availability were studied in an introduced herbivorous lady beetle,Epilachna niponica. The introduced population was relatively constant over a seven-year study period. Egg density was related to the variation in host-plant abundance in different years, and was highly stabilized during the period from reproductive adult to egg stage. Two density-dependent processes were identified in the reproductive season: (1) density-dependent reduction in fecundity and (2) density-dependent increase in female mortality and/or emigration, all of which operated early in the season. As a result, temporal variability in cumulative egg density was greatly reduced by mid-May, by which time approximately40% of total eggs were laid. A field cage experiment demonstrated that egg-laying of individual females was largely limited by resource availability even at low levels of leaf herbivory. Since movement activity of ovipositing females increased in a density-dependent manner, inter-plant movement is more likely to cause density-dependent reduction in fecundity and female loss, due to enhanced energy expendiditure. The introduced population was less stable than the source population, probably because of decreased inter-plant movement of females and the unlikelihood of egg resorption, both of which contribute significantly to the temporal stability ofE. niponica population densities.