Delayed density-dependent parasitism of eggs and pupae as a contributor to the cyclic population dynamics of the autumnal moth

Many populations of forest Lepidoptera exhibit 10-year cycles in densities, with impressive outbreaks across large regions. Delayed density-dependent interactions with natural enemies are recognized as key factors driving these cyclic population dynamics, but emphasis has typically been on the larval stages. Eggs, pupae and adults also suffer mortality from predators, parasitoids and pathogens, but little is known about possible density relationships between mortality factors and these non-feeding life stages. In a long-term field study, we experimentally deployed autumnal moth (Epirrita autumnata) eggs and pupae to their natural enemies yearly throughout the 10-year population cycle in northern Norway. The abundance of another geometrid, the winter moth (Operophtera brumata), increased in the study area, permitting comparisons between the two moth species in predation and parasitism. Survival of autumnal moth eggs and pupae was related to the moth abundance in an inverse and delayed manner. Egg and pupal parasitoids dominated as density-dependent mortality factors and predicted the subsequent growth rate of the host population size. In contrast, effects of egg and pupal predators were weakly density dependent, and generally predation remained low. Parasitism rates did not differ between the autumnal and winter moth pupae, whereas predators preferred winter moth pupae over those of the autumnal moth. We conclude that parasitism of the autumnal moth by egg and pupal parasitoids can be related to the changes of the moth density in a delayed density-dependent manner. Furthermore, egg and pupal parasitoids cannot be overlooked as causal factors for the population cycles of forest Lepidoptera in general.     

LIFE-HISTORY EVOLUTION IN GUPPIES (POECILIA RETICULATA) 6. DIFFERENTIAL MORTALITY AS A MECHANISM FOR NATURAL SELECTION

We have previously reported a correlation between the life-history patterns of guppies and the types of predators with which they coexist. Guppies from localities with an abundance of large predators (high predation localities) mature at an earlier age and devote more resources to reproduction than those found in localities with only a single, small species of predator (low predation localities). We also found that when guppies were introduced from a high to low predation locality, the guppy life history evolved to resemble what was normally found in this low predation locality. The presumed mechanism of natural selection is differences among localities in age/size-specific mortality (the age/size-specific mortality hypothesis); in high predation localities we assumed that guppies experienced high adult mortality rates while in the low predation localities we assumed that guppies experienced high juvenile mortality rates. These assumptions were based on stomach content analyses of wild-caught predators and on laboratory experiments. Here, we evaluate these assumptions by directly estimating the mortality rates of guppies in natural populations. We found that guppies from high predation localities experience significantly higher mortality rates than their counterparts from low predation localities, but that these higher mortality rates are uniformly distributed across all size classes, rather than being concentrated in the larger size classes. This result appears to contradict the predictions of the age/size-specific predation hypothesis. However, we argue, using additional data on growth rates and the probabilities of survival to maturity in each type of locality, that the age-specific mortality hypothesis remains plausible. This is because the probability of survival to first reproduction is very similar in each type of locality, but the guppies from high predation localities have a much lower probability of survival per unit time after maturity. We also argue for the plausibility of two other mechanisms of natural selection. These results thus reveal mortality patterns that provide a potential cause of natural selection, but expand, rather than narrow, the number of possible mechanisms responsible for life-history evolution in guppies.     

Key-factor analysis of immature stages of Aedes scapularis (Diptera: Culicidae) populations in southeastern Brazil

Development times and mortality factors were investigated for populations of Aedes scapularis (Rondani) occurring in natural temporary ground pools in southeastern Brazil. Analysis of life tables by the key-factor method showed that mortality caused by desiccation of breeding sites was the principal factor accounting for fluctuations in population sizes throughout the study period. For cohorts that completed development, mortality attributed to predation by aquatic insects ranged from 68 to 96% and was the most important cause of death. In these cohorts, final population size was determined by the high mortality rates that occurred during the fourth larval instar and pupal stages; however, the key-factor best accounting for the population fluctuations was predation of fourth instar larvae. The short mean time of 7.1 days for synchronous development of the larvae and the lack of evidence of density-dependent mortality suggested that the A. scapularis populations were not strongly regulated and can attain relatively high densities.     

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

The relationships between egg predation of an herbivorous lady beetle Epilachna niponica (Lewis ) and its predator, the earwig Anechura 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.     

A life table for the cinnabar mothTyria jacobaeae,in Oregon

The cinnabar moth,Tyria jacobaeae L. (Arctiidae), was introduced into Oregon in 1960 against the weed tansy ragwort,Senecio jacobaea L., and in 1970 an intensive study of a population of this biological control agent was initiated. Field sampling methods were devised, and laboratory investigations of feeding and larval development were conducted. Results of the study were analyzed and reported as partial life tables, where possible mortality factors were identified and quantified. The most important mortality factor in both 1970 and 1971 was starvation after defoliation of host plants. Other factors identified included pupal death, emergence failure, egg predation, and larval ingestion of eggs.     

Ecological Life Table of Tuta absoluta (Meyrick) (Lepidoptera: Gelechiidae)

An ecological life table was constructed, aiming to determine the critical stages and key mortality factors of Tuta absoluta (Lepidoptera: Gelechiidae). The total population mortality of this tomato leafminer was 92.3%. During the egg stage the mortality was 58.7%, mainly due to egg inviability. A total of 8.6% egg parasitism by Trichogramma pretiosum (Riley) (Hymenoptera: Trichogrammatidae) and 5.0% egg predation by Xylocoris sp. (Heteroptera: Anthocoridae), Cycloneda sanguinea (L.) (Coleoptera: Coccinellidae) and members of the family Phlaeothripidae (Thysanoptera) was observed. The mortality of the larval stage was 33.0%. This was considered to be the critical stage as it showed the highest apparent mortality (79.8%). Larval parasitism was low (0.1%), and was only found with Goniozus nigrifemur Ashmead (Hymenoptera: Bethylidae). Predators were responsible for 79.4% of larval mortality. Therefore, their attraction to and maintenance in the target area are important management tactics to be considered for T. absoluta control. The first and second instars were considered to be the most critical, and predation by the above mentioned species was the key mortality factor. The mortality at the pupal stage was low (0.6%) and was due to malformation.     

Predation on Colorado Potato Beetle Eggs by Generalist Predators in Research and Commercial Potato Plantings

Field studies quantified predation on Colorado potato beetle [Leptinotarsa decemlineata(Say)] eggs and determined the relationship between predation and egg mass abundance in research and commercial potato plantings in eastern North Carolina. Predator exclusion experiments were conducted weekly in research plantings. In addition, egg mass density and predation on egg masses were monitored throughout the season in research plots and commercial potato fields. Predation was an important source of mortality for Colorado potato beetle eggs. Survivorship of eggs exposed to predators was consistently, significantly lower than survivorship of eggs protected from predation. Averaged over 2 years, the mean survivorship of eggs protected from predation was 69%, compared with 26% survivorship of eggs exposed to predation. Regression analysis failed to detect any relationship between egg mortality due to predation and egg abundance. These results imply that efforts to reduce Colorado potato beetle populations selectively will not be offset by an according decline in abundance of natural enemies and therefore should be fully compatible with naturally occurring biological control.     

Integrated control of apple pests in New Zealand 10. Population dynamics of codling moth in Nelson

Abstract

Sampling methods are described for estimating the population density, mortality, and natality of a univoltine population of codling moth attacking mature apple trees (cv. ‘Delicious’) at Nelson, New Zealand. These methods were used to construct life tables for the species over eight generations (1967–68 to 1974–75) on trees variously sprayed and not sprayed with ryania in an integrated control programme. Bait traps provided a sensitive measure of seasonal adult population density. Analysis of the life tables shows that migration of adults was the main key factor and that overwintering larval mortality (particularly that due to bird predation), fecundity, and ryania also made a major contribution to variation in generation mortality. In the absence of ryania the resident population usually increased between generations, whereas it usually decreased when ryania sprays were applied. The density dependence of overwintering larval mortality was due to bird predation, and the inverse density dependence of larval mortality from ryania was due to changes in the site of fruit entry with larval population density. Fecundity was density independent, and inconclusive evidence was obtained on the density dependence of migration. The wide variation in fecundity is attributed primarily to weather conditions. The impact on control strategy of the above key factors, density dependence, and total natural mortality is discussed. Ryania is found to be uneconomic, whereas the granulosis virus of codling moth and male removal with pheromone traps show promise as future control methods. The need to eliminate reservoirs of codling moth close to orchards under integrated pest control is emphasised. Regulation of codling moth populations at Nelson on neglected, unsprayed trees appears to result from intraspecific competition for fruits and cocooning sites, and weakly density-dependent mortality of mature larvae when seeking cocooning sites and while overwintering in their cocoons. Variation in fecundity also cohtributes to fluctuations in abundance of the species. In contrast, at low density in an integrated control programme no intraspecific competition was evident; migration, winter mortality, and fecundity were the main determinants of abundance. This illustrates the need to study pest populations at densities similar to those tolerable commercially.     

Larval survival in populations of the large copper butterfly Lycaena dispar batavus

Lycaena dispar is recorded as an early case of extinction and subsequent re-introduction in the UK. However, repeated establishment attempts have not resulted in self-sustaining populations, including those having taken place at Woodwalton Fen (Cambridgeshire, UK).
Earlier studies at Woodwalton Fen showed that the highest mortalities occurred between the egg stage and resumption of larval feeding in spring.
This study was designed; firstly, to investigate factors causing field mortality during different larval stages; secondly, to compare survival on Rumex hydrolapathum foodplants in different habitat situations; and thirdly, to compare survival in a natural population (Weerribben, The Netherlands) with the introduced Woodwalton Fen population. Experiments employed exclusion cages in order to examine the relative roles of vertebrate and invertebrate natural enemies, and survival on food-plants in open fen and waterside situations was compared.
Results suggest invertebrate predation to be the dominant mortality factor acting upon pre-diapause larvae. Over the winter diapause natural enemies do not have a significant role. However during both these stages losses still occur that are unaccounted for. Extensive winter flooding appears to increase overwintering losses. Vertebrate predators cause significant mortality of post-diapause larvae.
No significant difference was found between larval survival on open fen versus waterside plants.
Comparison of introduced and native populations reveal that overwintering survival was significantly higher in the latter; potential reasons for this difference are discussed.     

Neither the devil nor the deep blue sea: larval mortality factors fail to explain the abundance and distribution of Tischeria ekebladella

Abstract.  1. The distribution, abundance and population dynamics of herbivorous insects may be affected by trophic interactions, by abiotic influences, or by intra-specific processes. Relatively little is known about how trophic influences vary across space. Here, we investigate spatial variation in mortality in the oak-feeding leaf miner Tischeria ekebladella as attributable to individual causal agents.
2. Leaf miners were experimentally introduced on 67 trees on an island 5 km² in area in south-western Finland. On each tree, some larvae were protected by a muslin bag, others by a glue barrier around the branch and some left exposed.
3. In the bagged transplants, 78.4% of larvae survived, compared with only 9.6% in the other two treatments. Most of the mortality was because of airborne agents: mortality on branches sheltered by a glue barrier was as high as on fully exposed branch tips.
4. We consider mortality caused by parasitoid wasps to be the main source of larval death and the primary factor driving general patterns of survival. The effects of bird predation and premature leaf abscission were negligible.
5. We detected spatial aggregation in larval survival and parasitism rates at the level of individual trees, but not across the landscape.
6. Spatial variation in overall leaf miner survival, parasitism and leaf abscission does not suffice to explain patterns of incidence and abundance of wild T. ekebladella on experimental trees. Rather, we identify metapopulation dynamics as a likely determinant of the spatial distribution of T. ekebladella in the landscape.     

Mortality Dynamics of Spodoptera frugiperda (Lepidoptera: Noctuidae) Immatures in Maize

We characterized the dynamics of mortality factors affecting immature developmental stages of the fall armyworm, Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae). Multiple decrement life tables for egg and early larval stages of S. frugiperda in maize (Zea mays L.) fields were developed with and without augmentative releases of Telenomus remus Nixon (Hymenoptera: Platygastridae) from 2009 to 2011. Total egg mortality ranged from 73 to 81% and the greatest egg mortality was due to inviability, dislodgement, and predation. Parasitoids did not cause significant mortality in egg or early larval stages and the releases of T. remus did not increase egg mortality. Greater than 95% of early larvae died from predation, drowning, and dislodgment by rainfall. Total mortality due to these factors was largely irreplaceable. Results indicate that a greater effect in reducing generational survival may be achieved by adding mortality to the early larval stage of S. frugiperda.     

Factors influencing development and survival ofCulex pipiens pallens larvae (Diptera: Culicidae) in polluted urban creeks

Factors influencing development and survival of Culex pipiens pallens immatures in polluted urban creeks were studied in Saga, Japan. Addition of food shortened developmental durations and increased pupal size in floating cages at only the least polluted site out of 4 study sites. There was no evidence for developmental delay due to overcrowding in natural populations. Survival was not increased by artificial feeding at any site. Using the developmental parameters obtained for caged cohorts, survival in natural populations was estimated from differences between the actual number and the expectation if there were no mortality. Pupation rates ranged from 1> to >10% by site and season. Mortality due to predation, evaluated from the difference in pupation rates between caged and natural populations, exceeded 30% in 4 out of 6 cases (twice at 2 sites and once at the other 2 sites). Lethal factors in the creek water caused the predominant mortality in 2 other cases. Regulation of C. pipiens pallens population breeding in eutrophic, open creeks was discussed in the context of larval dispersal through behavioral interference.     

Oviposition site preference and larval mortality in a leaf-mining moth

Abstract. 1. The univoltine leaf-mining moth, Lithocolletis quercus Ams., is endemic to Israel, where it spends its 10.5 month larval period feeding only in the leaves of Quercus calliprinos Webb.
2. We compared patterns of egg deposition and sources of larval mortality to test whether oviposition patterns and site preferences confer an enhanced likelihood of larval survival.
3. Dominant sources of larval mortality were premature leaf abscission and death from unknown causes, whereas predation, parasitism and intraspecific interference accounted for relatively little larval mortality.
4. Eggs, and thus mines, were aggregated among leaves of host trees even though premature leaf abscission was positively correlated with density of mines per leaf. Interference competition among larvae was the only other density-dependent mortality factor.
5. Oviposition patterns within leaves mitigated the probability of death from larval interference, and probably also from early leaf abscission.
6. Despite these density-dependent mortality factors, overall probability of larval survival to pupation was independent of initial density of mines on a leaf.
7. The long larval period allows synchrony between oviposition flights and times of predictable resource availability.     

Feeding while evading predators by a lotic mayfly: linking short-term foraging behaviours to long-term fitness consequences

Laboratory experiments were performed to quantify the combined effects of food abundance (low, high) and predation threat, imposed using a model fish (safe, risky) on fitness correlates (i.e. growth, time to emergence, adult body mass, fecundity, egg size) of the mayfly Baetis tricaudatus. These effects were determined by rearing larvae under different combinations of food abundance and predator threat. Fitness correlates were significantly affected by food abundance, predation threat or the interaction of these factors. High food abundance and the absence of predation threat significantly increased larval growth rates, adult body mass, fecundity, egg size, and decreased time to emergence. Long-term effects of predator threat and food abundance on fitness correlates of B. tricaudatus were compared to previously conducted short-term patch choice trials to test for concordance between short-term measures of patch choice and their potential longer-term fitness consequences. This comparison indicated that patches that were utilized the most were those that yielded the highest fitness benefits. These results suggest that behavioural costs of balancing mortality risk due to predation against food acquisition by B. tricaudatus can have strong fitness consequences.     

Seasonal mortality factors of the coffee leafminer, Leucoptera coffeella

Seasonal population fluctuation of the coffee leafminer, Leucoptera coffeella (Guérin-Méneville & Perrottet) (Lepidoptera: Lyonetiidae), led to an investigation of its natural mortality factors during the rainy season when the population level is low and during the dry season when population peaks occur. Life-table data were collected from insecticide-free plots within a 3 ha coffee plantation on the upper, medium and lower canopy. Leafminer mortality was similar among the canopy parts but varied in the two seasons studied. During the rainy season, the generational mortality averaged 94.3%, with 50.2, 33.7 and 10.4% occurring during egg, larval and pupal stages, respectively. During the dry season, total mortality was 89%, with 13.2, 61.0 and 14.8% occurring during egg, larval and pupal stages, respectively. Marginal mortality rates during the rainy season were highest for physiological disturbances, rainfall and egg inviability; but, in the dry season, they were highest for predaceous wasps, physiological disturbances and parasitoids. Egg and larval stages accounted for most of the mortality variation in the rainy season, while the combination of larval and pupal mortality better described the generational mortality in the dry season. Variation in mortality during the rainy season was primarily associated with egg inviability, rainfall and parasitoids. In contrast, predatory wasps and physiological disturbances were the main factors associated with mortality variation during the dry season. These results suggest that weather conditions, natural enemies and plant quality attributes are the main determinants of the population dynamics of L. coffeella.     

Testing of selected insecticides to assess the viability of the integrated pest management of the Eucalyptus snout-beetle Gonipterus scutellatus in north-west Spain

Abstract:  We investigated, by means of topical, residual and field assays, the toxicity of four insecticides ( Bacillus thuringiensis , azadirachtin, flufenoxuron and ethofenprox) to adult, larvae and eggs of the Eucalyptus snout-beetle Gonipterus scutellatus and their side-effects on the adults of the solitary egg parasitoid Anaphes nitens , its natural enemy. In the topical toxicity experiment, both flufenoxuron and ethofenprox severely affected G. scutellatus , causing 100% adult mortality after 7 days, and 75 and 100% larval mortality, respectively. Ethofenprox also caused 93% larval pre-eclosion mortality and 100% parasitoid mortality. In the residual experiment, azadirachtin acted as an effective larvicide when ingested (73% mortality after 7 days). Ethofenprox showed the highest toxicity, with 100% mortality of adults and larvae of G. scutellatus after 24 h. On the day 7 of observation, flufenoxuron resulted in 93% adult mortality and 100% larval mortality. After contact with residuals, only ethofenprox and flufenoxuron had adversely affected A. nitens (98% mortality after 48 h). Bacillus thuringiensis exhibited only a slight activity against the pest or the parasitoid. The field experiment was carried out on three localities, with a preliminary sampling, and three samplings after the applications of the products, on days 7, 14 and 30. We evaluated the parasitism rate, parasitoid sex ratio, parasitoid pre-eclosion mortality, the abundance of G. scutellatus adults, larvae and egg capsules, the pre-eclosion mortality rate of weevil eggs and larvae and the foliar damage. Only fluenoxuron and ethofenprox reduced the pest population and the foliar damages, but ethofenprox also significantly decreased the rate of parasitism (17%) after 7 days. Given our experimental results, we suggest that flufenoxuron and azadirachtin could be used in association with the natural enemy.     

Spatial distribution, life history and estimates of survivorship in a fourteen-species assemblage of larval dragonflies (Odonata: Anisoptera)

SUMMARY. 1. Spatial and temporal changes in larval densities were used to infer patterns of habitat use and survivorship in a fourteen-species assemblage of dragonflies (Odonata: Anisoptera) in a small fishless pond. The density of all species combined peaked at >1000 m⁻² in late summer, Most species (e.g. Libellula spp.) were restricted to shallow, nearshore habitats (<1.0m in depth), but a few (e.g. Epitheca spp.) also used deeper areas of the pond. Only Perithemis tenera was most abundant in deep habitats.
2. Because many species exhibited temporal shifts in their use of habitats, it was necessary to estimate survival from changes in population size, calculated as the product of density and habitat area, summed across habitats. In most species, periods of high mortality in autumn and spring were separated by 3–4 months of negligible mortality in winter. Survivorship was linear only in the two species that completed all of larval development in summer ( Sympetrum vicinum and Pantala flavescens) . Average survival rates for these two species (−0.0049 and −0.0079 log density d⁻¹) were similar to those in previous studies (Lawton, 1970; Benke & Benke, 1975).
3. Survivorship in many species was confounded by other life history phenomena such as (i) mixed voltinism, (ii) overlapping migrant and resident cohorts, and (iii) asynchronous development within species. Asynchrony made it difficult to estimate initial and final population sizes, hence total larval survivorship. However, based on emergence data, only 0.4–3% of larvae survived after peak abundance. None of this mortality can be ascribed to vertebrate predation, and only a little to overwintering stress and starvation. Thus, predation by invertebrates might play a major role in the regulation of these populations.     

Mortality and production of 0+ perch, Perca fluviatilis L., in two Scottish lakes

Mortality of 0+ Eurasian perch was measured directly from the decline in catch in a high-speed plankton sampler and described by a negative exponential model. A mean mortality of 0.063 day ¹ (range=0.020–0.089) was calculated; for 0+ juveniles it was 0.021 day ¹. Mean larval production was 0.85–1.30 mg dry weight m⁻² day⁻¹ in Loch Kinord and 0.08–0.12 in Loch Davan. Annual production of 0+ juveniles was 1.97–7.55 kg wet weight ha ⁻¹ at L. Kinord and 1.01 kg at L. Davan. The contribution of 0+ perch, including the egg stages, to total perch production was 88–96%. Availability of food items on transition to exogenous feeding and through the larval period did not limit survival. Cannibalism by adults accounted for most of larval mortality, and predation by pike and adult perch were responsible for the majority of juvenile losses. The population structure of adult perch, for 12 year classes, was stable at L. Kinord, while year classes at L. Davan exhibited fluctuations in abundance and in two sampling years were eliminated in the first summer. Survival in the eleutheroembryonic phase was variable at L. Davan and observed to be low compared with L. Kinord. It is suggested that this was a result of water turbulence on the exposed and shallow spawning areas at L. Davan.     

Impact of natural enemies on coffee leafminer Leucoptera coffeella (Lepidoptera: Lyonetiidae) population dynamics in Chiapas, Mexico

Coffee leafminer, Leucoptera coffeella, is a pest in many New World coffee growing areas. Previous studies suggested that its population dynamics were strongly affected by natural enemies, particularly of larvae, and physical environmental conditions. Our study documented through field surveys and life table analyses (i) the natural enemy complex associated with coffee leafminer and (ii) the impacts of natural enemies on the population dynamics of coffee leafminer, on coffee (Coffea arabica) at two elevations and two rainfall levels in the Soconusco region of Chiapas, Mexico. Twenty-two larval parasitoid species (including 14 morphospecies) were collected. Egg and pupal parasitoids were not recovered. Life table analyses showed that parasitism contributed 10% of real mortality, and parasitism rates were 8–10-fold higher at the low (<550 m) versus high (>950 m) elevation; parasitism rates were similar under low (<100 mm) and high (>400 mm) rainfall. Seventeen predator species (including five morphospecies) were collected, of which most were ants (Formicidae, 14 species) that contributed >58% of real mortality. Life table analyses showed that predation rates were higher at high versus low elevation and under high versus low rainfall. Independently of elevation and rainfall, egg predation (likely by ants) was the most important source of indispensable mortality (range = 0.13–0.30), except at low elevation and high rainfall where pupal predation (=0.14) was similarly important. Also, predation was the main source of coffee leafminer larval and pupal mortality during a 13-month period in a low elevation coffee farm and was highest during the rainy season (>400 mm rainfall/month), when coffee leafminer prevalence was highest. Overall, predation of eggs and pupae (the latter particularly at low elevation), mostly by ants, were the most important sources of coffee leafminer mortality. Because ants were the main source of coffee leafminer egg and pupal mortality, their importance and potential role in coffee pest management strategies were discussed.     

Density-related mortality of the flour beetle,Tribolium confusum duval

Life tables were constructed to assess the relative importance of some factors causing mortality of Tribolium confusum and to gauge their response to increasing population density. Observations focussed on three population densities (100, 400 and 800 individual/8 g medium) from the egg to the adult stage. The medium was not renewed in order to maximize predatory interactions. Generation mortality at densities 100, 400 and 800 was 42%, 50% and 74% respectively, i. e. density-dependent. Mortality in the first 10 days was also density-dependent reaching a maximum of 27% at density 800: predation by small larvae on eggs seemed the principal causative factor. The overall pattern of larval mortality was density-independent. Data on the mortality of pupae and callows were ultimately consistent with an inversely density-dependent pattern. Apparently, only mortality occurring within the first 10 days was capable of population regulation.