Distribution and dispersal in populations capable of resource depletion

Summary A simulation model has been used to investigate the influence of animal (insect) distribution and dispersal among exhaustable resource units (food plants). Population size and stability were used as measures of success. The results showed that population size and stability are highest when egg batch size is as large as can be supported by the average food plant or slightly larger if larval dispersal occurs. Clumping of egg batches of food plants increases population stability when egg batches are small by insuring that some food plants will not be overcrowded. Increasing the proportion of larval dispersers or the success of dispersers can increase or decrease population size and stability depending on the original egg batch distribution, but individuals which produce offspring some of which disperse, generally have a selective advantage. Density dependent larval dispersal decreases population stability. Finally, individuals with lower reproductive capacities can have a selective advantage over those with higher reproductive capacities under certain conditions of egg batch size and larval dispersal.     

Is the insect or the plant the driving force in the cinnabar moth — Tansy ragwort system?

Summary The interactions between cinnabar moth, Tyria jacobaeae L. and its food plant, tansy ragwort, Senecio jacobaea L. were studied for 4 to 6 years at 9 sites in North America to assess if the herbivore drove the dynamics of the plants or if the plants determined the dynamics of the insects. Cinnabar moth larval density is not closely related to changes in the size and spacing of tansy ragwort plants although high densities of larvae were associated with a high proportion of rosettes the next year. Fluctuating moth populations live in areas where rosettes are small, closely spaced and numerous compared to flowering stem plants. This situation is also associated with greater larval dispersal which may lead to over exploitation of the food supply. The coefficient of variation of both the size of rosettes and the distance between clumps of plants is associated with the coefficient of variation in moth density. This suggests that the plants may be driving the dynamics of the insect populations. The size of the moth egg batches is correlated to the size of the flowering stem plants in the previous year, indicating an adjustment between moth fecundity and food plant size. The conclusion is that environmental factors such as weather and soil type determine to a large degree the characteristics and variation in the plant populations and this in turn controls the dynamics of the insects. The relation of this situation to the biological control of weeds is discussed.     

The population ecology of the Cinnabar Moth,Tyria jacobaeae L. (Lepidoptera,Arctiidae)

Summary This paper describes the results of a study of the factors determining the abundance and distribution of the Cinnabar Moth in Britain. The main part of the study was on a population of the moth at Weeting Heath in Norfolk. This is an area of sandy heath which is heavily overgrazed by rabbits. Here the moth fluctuates violently in number and periodically it completely defoliates its food plant (ragwort, Senecio jacobaea L.) over large areas. This happened in 1960, 1961, 1967, and 1968.Since 1966, the numbers of the moth have been studied in detail and Life Tables are presented for five years.Adult fecundity varies considerably between years. This is due mainly to fluctuations in adult size resulting from changes in larval density. By comparison adult mortality and dispersal have little effect on fecundity; although there is some evidence to suggest that dispersal is density dependent. Because the number of eggs laid in one generation depends on the size of the previous generation, fecundity tends to be acting as a delayed density dependent factor.Mortality is low during the egg stage, but is high amongst young larvae, due mainly to arthropod predation. As the caterpillars grow they become immune from this predation. They are distasteful to vertebrate predators.A larval parasite, Apanteles popularis, kills many of the fully grown larvae. The highest rates of parasitism have coincided with the lowest densities of the moth, however, so that over the five years it has acted as an inverse density dependent factor.In 1967, the population became so large that the ragwort was completely defoliated, and about 20% of the caterpillars died from starvation. In the following year the population was still large and the ragwort plants were small, owing to the effects of defoliation in the previous year. Food ran out early in the season and about 50% of the larvae starved. Because of the overriding effect of starvation, total larval mortality tends to be density dependent.Mortality is high at, or immediately after, pupation and this is thought to be due primarily to predation by moles (Talpa europaea). Pupal mortality does not appear to be density dependent.The upward growth of the population is limited by food supply. Starvation led to a population crash, so that in 1969 only 62 eggs/150 m² were laid compared with 17110 and 16493 in the previous two years. The rate of recovery of the population after this crash was dependent upon the rapid recovery of the ragwort plants. Owing to the wet summer in 1968, plant numbers actually increased after defoliation, due to regeneration from root buds. This was in spite of the fact that no seed was produced in the area in either 1967 or 1968.The only factor which appears to buffer the population against extinction in years when food runs out early in the season, is the heterogeneity within the moth and ragwort populations. The earliest individuals manage to obtain sufficient food in those patches of ragwort which survive longest.Large fluctuations in number only occur in some localities. Other populations of the moth persist at low density and never eat out their food supply. Some data are presented from such a population at Monks Wood. This site is on a heavy clay soil, rabbit grazing is less marked than at Weeting, and ragwort occurs only at a low density. The lusher vegetation supports a very large population of arthropod predators and these take a higher percentage of the young caterpillars than was found at Weeting. Pupal survival is also low due probably to waterlogging of the soil. Pupae can withstand considerable desiccation, but excessive moisture soon leads to their death.The distribution of the moth in Britain and its use for the biological control of ragwort are discussed.     

Dispersal of first-instar gypsy moth larvae in relation to population quality

Summary Field studies of dispersal by first instar gypsy moth larvae indicate that almost all larvae undergo an initial dispersal episode. However, in laboratory studies large larvae (from large eggs) disperse more frequently than small larvae (from small eggs) in the presence of favored food. Large larvae may be better adapted for dispersal. When larvae encounter unacceptable food or are denied food, larvae disperse more frequently and dispersal by small larvae is nearly as frequent as dispersal by large larvae. Factors affecting egg size may contribute to shifts in dispersal patterns of gypsy moth larvae and distribution of populations.Paper No. 2041, Massachusetts Agricultural Experiment Station, University of Massachusetts at Amherst. This research supported (in part) from Experiment Station Project No. 355     

Fecundity, lifespan and egg mass in butterflies: effects of male-derived nutrients and female size

1. Effects of larval reserves and nutrients received as adults on fecundity and lifespan in female Danaus plexippus (the Monarch Butterfly) were measured to determine the relative importance of different sources of nutrients for reproduction and somatic maintenance.
2. Egg-laying lifespan was correlated with female size but not with the amount of male-derived nutrients or adult food concentration.
3. Lifetime fecundity was higher when females received a large first spermatophore, but was not affected by female size when lifespan was controlled or by adult food concentration.
4. At the end of their lives, females contained unlaid eggs and retained, on average, 88% of their initial mass. This proportion was unchanged in two years, although mean egg-laying lifespan varied from 22·5 to 28·7 days.
5. Egg mass decreased over the female lifespan, and was correlated with female size.
6. These results suggest that larval reserves are more important for somatic maintenance than adult income, but that the protein-rich nutrients received from males contribute to egg production. This supports theoretical predictions and empirical studies of other Lepidoptera showing that larval reserves are less likely to affect fecundity when the adult income can contribute substantially to egg production.     

Local adaptation in oviposition choice of a specialist herbivore: The cinnabar moth

Specialist herbivores feed on a restricted number of related plant species and may suffer food shortage if overexploitation leads to periodic defoliation of their food plants. The density, size and quality of food plants are important factors that determine the host plant choice of specialist herbivores. To explore how all these factors influence their oviposition behaviour, we used the cinnabar moth Tyria jacobaeae and the hybrids of a cross between Jacobaea vulgaris and J. aquatica as a study system. While defoliation by the cinnabar moth is common in the coastal area of The Netherlands, it is relatively rare in inland ragwort population. Ragworts contain pyrrolizidine alkaloids (PAs) and those that are found in coastal areas are rich in jacobine-like PAs while those that occur inland are rich in erucifoline-like PAs. We tested how the oviposition preference was influenced by plant size, nitrogen and water content and PA composition. We used cinnabar moth populations from a regularly defoliated area, Meijendel, and Bertogne, a rarely defoliated area. Our results revealed no effects of nitrogen or water content on oviposition preference. Moths from both populations laid larger egg batches on the plants rich in jacobine-like PAs. Moths from Meijendel preferred larger plants and spread their eggs over more egg batches that were, on average, smaller than those of Bertogne moths. These results suggest that Meijendel moths adopted a oviposition strategy to cope with potential defoliation.     

Wise flies: a pre‐dispersal seed predator prefers hermaphrodites over females in the gynodioecious Polemonium foliosissimum

1. Oviposition choices can profoundly affect offspring performance. Oviposition decisions of the dipteran pre‐dispersal seed predator, Hylemya sp. (Diptera: Anthomyiidae), when choosing among sex morphs of their host‐plant—Polemonium foliosissimum Hook—were evaluated. Polemonium foliosissimum is gynodioecious, with female and hermaphrodite sex morphs that differ in flower size. 2. It was asked: Do female flies preferentially oviposit on hermaphrodite plants and, if so, are oviposition choices correlated with flower size? Is larval survivorship higher on hermaphrodite plants and, if so, is larval success correlated with flower size? Can the differences in oviposition and/or larval success be explained by slight differences in flowering phenology between the sexes? 3. Hermaphrodite flowers received 45% more Hylemya eggs than females. Although hermaphrodites had larger petals and sepals than females, egg loads were not correlated with petal or sepal size. Larval survival was 30% greater on hermaphrodites than females and higher on plants with larger sepals. However, the difference in sepal area between genders did not fully explain larval survival differences. Egg numbers declined over weeks, but differences in egg loads between the sex morphs were not attributable to a slight phenological delay of females. Larval survival declined over the season; however, larval survival differences between sex morphs were consistent. 4. Hylemya preferentially oviposited on hermaphrodites where their larvae had a significantly greater survival rate compared with females. The present results add to the knowledge that intra‐specific choices have consequences for phytophagous insects and that the relationship between antagonists and their gynodioecious host plants is complex.     

Variation in trichome density and resistance against a specialist insect herbivore in natural populations of Arabidopsis thaliana

Abstract.  1. The annual herb Arabidopsis thaliana is a prime model organism of plant molecular genetics, and is currently used to explore the molecular basis of resistance to herbivores. However, both the magnitude and the causes of variation in resistance among natural populations of A. thaliana are poorly known. The hypotheses (a) that resistance to a specialist herbivore, the diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae), is positively related to the density of leaf trichomes, and (b) that oviposition preference by female moths is positively correlated to larval performance on different populations and maternal lines of the host plant were examined.
2. Variation in leaf trichome density and resistance to P. xylostella within and among six natural populations of A. thaliana in Sweden was quantified . Resistance was quantified by examining the number of eggs laid on plants exposed to ovipositing female moths and by monitoring larval development on plants of different origin.
3. Trichome density varied significantly among populations; for 4-week-old plants (exposed to ovipositing moths), it also varied significantly among maternal families within populations. The rate of oviposition varied significantly both among populations and among families within populations. This variation could partly be explained by a negative relationship between trichome density and egg number, and a positive relationship between plant size and egg number. Time to pupation, pupal mass, and time to adult eclosion did not vary among populations or maternal lines of the host plant, and offspring performance was not related to P. xylostella oviposition preference . The results indicate that A. thaliana populations may respond to selection for increased resistance to P. xylostella , and suggest that trichome production contributes to resistance against this specialist herbivore.     

On the evolution of egg placement and gregariousness of caterpillars in the Lepidoptera

Drawing heavily upon natural history data from the Neotropical butterfly fauna, an attempt is made to develop a model, with testable hypotheses, to account for the evolution of egg-clustering and larval gregariousness. Given the high diversity of both plant and butterfly species in the American tropics, there is a higher incidence of egg-clustering there, including some species with aposematically-colored immature stages. Emphasis is placed on the need to examine both the physical (mechanical) toughness of larval food plants for larval feeding success, and the spatial structuring of food plant populations. It is argued that when food plant spatial patchiness is high, the breadth of local food plant species usage low, plant growth rates high, fed-upon structures tough, butterfly dispersal low, and reproductive potential low, egg-clustering evolves. Such ecological character states, coupled with the occurrence of food plant patches in large sizes relative to the nutritional and fecundity needs of the butterfly specialist on the species, select for egg-clustering to ensure survival of the butterfly population in that habitat. It is emphasized that the model proposed here requires considerable examination through field studies.     

Larval host plant affects fitness consequences of egg size variation in the seed beetle Stator limbatus

Egg size variation often has large effects on the fitness of progeny in insects. However, many studies have been unable to detect an advantage of developing from large eggs, suggesting that egg size variation has implications for offspring performance only under adverse conditions, such as during larval competition, periods of starvation, desiccation, or when larvae feed on low-quality resources. We test this hypothesis by examining the consequences of egg size variation for survivorship and development of a seed-feeding insect, Stator limbatus, on both a low-quality (Cercidium floridum) and a high-quality (Acacia greggii) host plant. Our results are consistent with the hypothesis. S. limbatus larval performance was affected by egg size only when developing on the poor-quality host (C. floridum); larvae from large eggs survived better on C. floridum than those from small eggs, while there was no evidence of an effect of egg size on progeny development time, body weight, or survivorship when larvae developed on A. greggii. These results indicate intense selection for large eggs within C. floridum-associated populations, but not in A. greggii-associated populations, so that egg size is predicted to vary among populations associated with different hosts. Our results also support this hypothesis; females from a C. floridum-associated population (Scottsdale) laid larger eggs than females from an A. greggii-associated population (Black Canyon City).     

Performance of a spring-feeding moth in relation to time of oviposition and bud-burst phenology of different host species

Abstract. 1. As a spring-feeding moth committed to immature foliage, the autumnal moth Epirrita autumnata (Lepidoptera, Geometridae) must have egg hatch synchronised with the bud-burst of its host plants. Due to large individual variation in the length of the pupal period, however, E. autumnata populations exhibit a prolonged period of flight and oviposition. Because the timing of oviposition in autumn is associated with the timing of egg hatch in the following spring, the time window for egg hatch expands and more potential hosts may become attainable. This suggestion was evaluated under field conditions by rearing E. autumnata eggs and larvae on four different hosts.
2. The performance of E. autumnata was measured by using estimates for fecundity (pupal mass) as well as survivorship of eggs and larvae. Based on the availability of foliage and phenological synchrony between larval and leaf development, early-laid eggs and the larvae originating from them were predicted to perform better on the hosts that have early-flushing leaves. On the late-flushing hosts, the larvae that hatched later were predicted to perform better than the larvae that hatched earlier. Half of the trials were exposed to predators and parasitoids, while the rest were conducted inside mesh-bags preventing larval dispersal and mortality due to natural enemies.
3. The results of the experiment did not support the simple predictions. In particular, host-plant quality and natural enemies appeared to operate discordantly between early- and late-laid eggs. Larvae from the late-laid eggs had rapid development during the larval stages and pupated at the same time and with the same pupal mass as the larvae hatched from the early-laid eggs.
4. The results indicate an occurrence of several, unknown selective forces in E. autumnata populations maintaining variation in the length of the pupal period, timing of oviposition, and timing of egg hatch.     

Egg size variation and its relationship with larval performance in the Lepidoptera: the case of the European grapevine moth Lobesia botrana

Life history theory attempts to define the pattern of resource distribution among offspring and predicts that egg size should be positively correlated with offspring fitness. In this paper we investigated the effect of an array of ecological and reproductive factors on the size of eggs laid by Lobesia botrana, and the ecological significance of egg size by experimentally testing whether or not egg size increased larval performance in this moth. Egg size was significantly affected by female age at mating, water availability, pupal weight loss, the phenological stage of the vine in which larvae developed, female body weight, and oviposition day, but was unaffected by the size of the spermatophore received. The greater the size of the first eggs produced, the smaller the size of the last eggs laid, as predicted by the resource depletion hypothesis. As expected, the larger the size of eggs the larger the size of larvae emerging from them. Large larvae displayed a better ability to endure starvation than small ones. When small and large larvae were allowed to develop on grape clusters, an adverse environment, large larvae performed better, settling and survival being significantly enhanced, almost tripled. Instead, when the same experiment was carried out on an artificial diet in the laboratory, a much more favourable milieu, survival in both sized-larval groups was not significantly different. We discuss our results in the context of current ideas relating egg size, larval performance and fitness in the Lepidoptera.     

Pollen dispersal in Yucca filamentosa (Agavaceae): the paradox of self-pollination behavior by Tegeticula yuccasella (Prodoxidae)

We investigated pollen dispersal in an obligate pollination mutualism between Yucca filamentosa and Tegeticula yuccasella. Yucca moths are the only documented pollinator of yuccas, and moth larvae feed solely on developing yucca seeds. The quality of pollination by a female moth affects larval survival because flowers receiving small amounts of pollen or self-pollen have a high abscission probability, and larvae die in abscised flowers. We tested the prediction that yucca moths primarily perform outcross pollinations by using fluorescent dye to track pollen dispersal in five populations of Y. filamentosa. Dye transfers within plants were common in all populations (mean ± 1 SE, 55 ± 3.0%), indicating that moths frequently deposit self-pollen. Distance of dye transfers ranged from 0 to 50 m, and the mean number of flowering plants between the pollen donor and recipient was 5 (median = 0), suggesting that most pollen was transferred among near neighbors. A multilocus genetic estimate of outcrossing based on seedlings matured from open-pollinated fruits at one site was 94 ± 6% (mean ± 1 SD). We discuss why moths frequently deposit self-pollen to the detriment of their offspring and compare the yucca-yucca moth interaction with other obligate pollinator mutualisms in which neither pollinator nor plant benefit from self-pollination.     

Preference and performance linkage of a leaf-mining moth on different Salicaceae species

The relationship between oviposition prefer-ence and offspring performance of a leaf-mining moth (Paraleucoptera sinuella) on four Salicaceae species was investigated in 1997 and 1998. We observed the egg distribution pattern on different plant species in the field and carried out oviposition experiments in the laboratory to determine the preference of ovipositing females. We also examined larval survival, pupal mass, and developmental time to compare larval performance on each plant species. Egg density in the field differed significantly among plant species. However, egg density was not correlated exactly with demonstrated oviposition preference. No larvae could develop on two Salix species. This finding indicated that larval survival is the most critical index of larval performance. Larval performance on each plant species was correlated well with oviposition preference that was revealed by a no-choice experiment in the laboratory. However, this correlation was not found in the field. These results indicate that the preference–performance linkage that was observed under laboratory conditions, was not always maintained in the field. Received: September 25, 2000 / Accepted: April 27, 2001     

Butterfly oviposition preference is not related to larval performance on a polyploid herb

The preference–performance hypothesis predicts that female insects maximize their fitness by utilizing host plants which are associated with high larval performance. Still, studies with several insect species have failed to find a positive correlation between oviposition preference and larval performance. In the present study, we experimentally investigated the relationship between oviposition preferences and larval performance in the butterfly Anthocharis cardamines. Preferences were assessed using both cage experiments and field data on the proportion of host plant individuals utilized in natural populations. Larval performance was experimentally investigated using larvae descending from 419 oviposition events by 21 females on plants from 51 populations of two ploidy types of the perennial herb Cardamine pratensis. Neither ploidy type nor population identity influenced egg survival or larval development, but increased plant inflorescence size resulted in a larger final larval size. There was no correlation between female oviposition preference and egg survival or larval development under controlled conditions. Moreover, variation in larval performance among populations under controlled conditions was not correlated with the proportion of host plants utilized in the field. Lastly, first instar larvae added to plants rejected for oviposition by butterfly females during the preference experiment performed equally well as larvae growing on plants chosen for oviposition. The lack of a correlation between larval performance and oviposition preference for A. cardamines under both experimental and natural settings suggests that female host choice does not maximize the fitness of the individual offspring.     

The evolution of egg clustering in butterflies: A test of the egg desiccation hypothesis

Females of many insect species cluster their eggs. Egg clustering by lepidopteran species usually results in aggregation of larvae that are more often conspicuously coloured and apparently distasteful or unpalatable than larvae of solitary species. While the costs and benefits of aggregation in terms of larval survival and growth are well documented, the evolutionary ecology of egg clustering has been long debated and is still unresolved. We tested the egg desiccation hypothesis, first proposed by Stamp (1980), which to our knowledge has never been examined experimentally. The egg desiccation hypothesis proposes that egg clustering is adaptive per se (i.e. increases fitness of females) by reducing egg mortality via desiccation.We tested this hypothesis for the Nymphalid butterfly, Chlosyne lacinia, an egg-clustering species on its sunflower host plant, Helianthus annuus. We first documented natural variation in batch size for this butterfly. We then tested experimentally hatch success of varying batch sizes and egg-layering arrangements under controlled humidity levels. Hatch success was positively related to relative humidity. Eggs in larger groups with greater number of layers had greater hatch success than smaller, monolayered egg batches, especially when relative humidity was low. Our results indicate that, not only number of eggs, but also the arrangement (i.e. layering and density), increase batch survival by protecting eggs from desiccation. However, despite increased hatch success in dense, multilayered clusters, we found wide variation in layering and density in natural populations of C. lacinia. This variation is probably maintained by trade-offs in egg survival, such as increased cannibalism of eggs by siblings, in dense clusters. Nevertheless, protection from egg desiccation provides an alternative explanation for the origin and maintenance of egg clustering in lepidopterans and possibly other insects. The pattern of egg deposition in the Nymphalidae supports this hypothesis, since most North American species cluster their eggs tightly, whereas most species in tropical regions lay eggs singly or in loose monolayers.     

Lasting effects of maternal behaviour on the distribution of a dispersive stream insect

1. Predicting population dynamics at large spatial scales requires integrating information about spatial distribution patterns, inter-patch movement rates and within-patch processes. Advective dispersal of aquatic species by water movement is considered paramount to understanding their population dynamics. Rivers are model advective systems, and the larvae of baetid mayflies are considered quintessential dispersers. Egg laying of baetids along channels is patchy and reflects the distribution of oviposition sites, but larvae are assumed to drift frequently and far, thereby erasing patterns created during oviposition. Dispersal kernels are often overestimated, however, and empirical tests of such assumptions are warranted because of the pivotal role distribution patterns can have on populations. 2. We tested empirically whether the egg distribution patterns arising from oviposition behaviours persisted and were reflected in the distribution patterns of larval Baetis rhodani. In field surveys, we tested for associations between egg mass and larval densities over 1 km lengths of four streams. A control species, the mayfly Ephemerella ignita, was employed to test for covarying environmental factors. We estimated drift rates directly to test whether larvae dispersed between riffles (patches of high egg mass density) and whether drift rates were density-dependent or density-related - expected outcomes if drift erases patterns established by maternal behaviours. 3. Positive associations between egg masses and larval benthic densities were found for neonate and mid-stage larvae of Baetis, but not the control species, suggesting persistence of the patchy distribution patterns established at oviposition. Drift rates were high, and riffles were net exporters of neonate and mid-stage larvae, but drift rates were unrelated to benthic densities and few drifters reached the next riffle. Riffles were sinks for large larvae, suggesting ontogenetic shifts in habitat use, but little long-distance dispersal. 4. Overall, the results suggest that most neonate and mid-stage larvae of B. rhodani remain close to the natal riffle, and late-stage larvae disperse shorter distances than routinely assumed. The persistence of maternal effects on distribution patterns well into juvenile life of an allegedly iconic disperser suggests that traditional models of how dispersal influences the population dynamics of many lotic invertebrates may be incorrect.     

Distribution ofChilo partellus (Lepidoptera: Pyralidae) egg batches on maize

Distribution ofChilo partellus egg masses was studied in field, greenhouse, and laboratory experiments. The eggs were laid in batches mainly on the lower side and the lower leaves of the plant. The egg batch size ranged from 1 to 169 eggs, with a median of 33.5 eggs per batch (average, 40.5). Oviposition ofC. partellus is described at two levels. The first level, choice of oviposition plants, followed a random distribution. The second level, number of egg batches per plant, followed an aggregated distribution in the field, where more than one egg batch was deposited on the same plant by the same female, which was found on 25% of the oviposition plants. A mechanism for egg-layingC. partellus females to perceive preceding oviposition or injured plants could not be detected. Oviposition site choice seemed to be mediated by tactile stimuli.     

Diet, fecundity and egg size in some polyphagous predatory carabid beetles

Egg size was measured at different rates of egg laying in three polyphagous carabid species, known to be useful predators of cereal aphids; the small Bembidion lampros Herbst and the medium-sized Pterostichus cupreus L. and P. melanarius Illiger. Variations in fecundity, as well as the ability of the medium-sized species to also build up fat reserves, were obtained when beetles were subjected to different dietary regimes consisting of aphids, or foods with a lower or higher protein content. Egg size was found to be dependent on the rate of egg laying within a species. A diet of cereal aphids appeared to be adequate for egg production in these polyphagous carabids, but female P. cupreus were unable to build up fat reserves when they ingested aphids contaminated with the aphicide pirimicarb. Beetles were able to devote resources to more and larger eggs (B. lampros), or to larger eggs and/or fat reserves (P. melanarius/P. cupreus) when given access to a carbohydrate-rich food with low protein content. The highest rate of egg laying was obtained when female P. cupreus and P. melanarius were given a more varied diet at frequent intervals; including regular shifts between unsprayed aphids, carbohydrate-rich food and protein-rich maggots. Within the varied diet treatment a negative relationship was obtained between egg size and egg number among similar-sized individuals of P. cupreus and P. melanarius; females producing the largest number also laid the smallest eggs. Egg size affected larval survival, since first instars hatching from large eggs were found to survive longer than those hatching from small eggs. The influence of differences in food intake on reproduction, maintenance metabolism, and survival of fieldinhabiting carabids is discussed.     

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.