Clutch size in a larval-pupal endoparasitoid

Clutch size decisions by Aphaereta minuta (Nees) (Hymenoptera: Braconidae), a polyphagous, gregarious, larval-pupal endoparasitoid, were studied under laboratory conditions. This parasitoid attacks larvae of Diptera inhabiting ephemeral microhabitats such as decaying plant and animal material. Females oviposit in young larval stages, but the eventual size of the host pupa determines host food availability for competing offspring. The size of the pupa can differ greatly between host species. We questioned how A. minuta females deal with this delay between the moment of oviposition and eventual host food availability, and whether they make clutch size decisions that benefit their fitness. It was shown that females indeed vary their clutch size considerably and in an adaptive way: (1) females lay larger clutches in larvae of host species that produce larger pupae, even when the larvae are the same size at the moment of oviposition, and (2) females lay larger clutches in larger larvae than in smaller larvae of the same host species. The latter seems functional as larvae parasitized at an older stage indeed developed into larger pupae compared to larvae parasitized at a younger stage. Furthermore, mortality of parasitized young host larvae was greater than that of both unparasitized larvae and parasitized older larvae. Under field conditions the risk of mortality of young host larvae is expected to be even higher due to the limited period of microhabitat (host food) availability, strong scramble type competition between the host larvae, and the longer period of being exposed to predation.     

The influence of competition between foragers on clutch size decisions in an insect parasitoid with scramble larval competition

The effect of competition between ovipositing females on theirclutch size decisions is studied in animals that lay their eggsin discrete units of larval food (hosts). In such species theeffect of competition depends on the form of the larval competitionwithin such units. In insect parasitoids, there might eitherbe contest (solitary parasitoids) or scramble competition (gregariousparasitoids) between larvae within a host For gregarious parasitoids,a decreasing clutch size with increasing competition betweenforagers is predicted. This prediction is tested in experimentsusing the parasitoid Aphaertta minuta. Parasitoids were eitherkept alone or in groups of four before the experiment, in whichthey were introduced singly in a patch containing unparasitizedhosts. Animals kept together laid on average clutches of 0.74eggs smaller than females kept alone (average clutch is 5.3),thereby confirming the prediction. Clutch size decreased withencounter number, which might be due to the adjustment of thefemale's estimate of the encounter rate with hosts. Finally,the results are compared with those reported for solitary parasitoids(that have scramble larval competition), for which it is predictedthat the clutch size will increase with increasing levels ofcompetition between females.     

Life-history, genotypic, and environmental correlates of clutch size in the Glanville fritillary butterfly

Abstract.  1. Glanville fritillary butterfly ( Melitaea cinxia ) females lay up to 10 clutches of 50–300 eggs in their lifetime. Clutch size is an important life-history trait as larval group size affects survival throughout larval development.
2. Two experiments were carried out in a large population cage in the field to investigate the life-history and environmental correlates of clutch size.
3. Clutch size decreased with the cumulative number of eggs laid previously, increased with both female body weight and the number of days between consecutive clutches.
4. Genotypic differences among females in the glycolytic enzyme phosphoglucose isomerase had a significant influence on clutch size, partly because females of particular genotypes were able to initiate oviposition earlier in the day and thereby take advantage of the most favourable environmental conditions for oviposition.
5. Factors influencing clutch size were partly different in two summers, indicating the modulating effect of prevailing environmental conditions on reproductive performance.     

Age-dependent clutch size in a koinobiont parasitoid

Abstract.  1. The Lack clutch size theory predicts how many eggs a female should lay to maximise her fitness gain per clutch. However, for parasitoids that lay multiple clutches it can overestimate optimal clutch size because it does not take into account the future reproductive success of the parasitoid.
2. From egg-limitation and time-limitation models, it is theoretically expected that (i) clutch size decreases with age if host encounter rate is constant, and (ii) clutch size should increase with host deprivation and hence with age in host-deprived individuals.
3. Clutch sizes produced by ageing females of the koinobiont gregarious parasitoid Microplitis tristis Nees (Hymenoptera: Braconidae) that were provided daily with hosts, and of females ageing with different periods of host deprivation were measured.
4. Contrary to expectations, during the first 2 weeks, clutch size did not change with the age of the female parasitoid, neither with nor without increasing host-deprivation time.
5. After the age of 2 weeks, clutch size decreased for parasitoids that parasitised hosts daily. The decrease was accompanied by a strong decrease in available eggs. However, a similar decrease occurred in host-deprived parasitoids that did not experience egg depletion, suggesting that egg limitation was not the only factor causing the decrease in clutch size.
6. For koinobiont parasitoids like M. tristis that have low natural host encounter rates and short oviposition times, the costs of reproduction due to egg limitation, time limitation, or other factors are relatively small, if the natural lifespan is relatively short.
7. Koinobiont parasitoid species that in natural situations experience little variation in host density and host quality might not have strongly evolved the ability to adjust clutch size.     

Clutch size manipulations in two seed beetles: consequences for progeny fitness

Seed beetles (Coleoptera: Bruchidae) lay their eggs on discrete resource patches, such that competition among larvae for food is an important component of their biology. Most seed beetles, including Stator limbatus, lay eggs singly on individual seeds and avoid superparasitism except when seeds are limiting. In contrast, S. beali, a closely related congener, lays eggs in clutches on a single seed. We tested the hypothesis that natural selection on larval life history characters favors small clutches (selection against large clutches) in S. limbatus, but that selection against large clutches is relaxed in S. beali because of the large size of its host's seeds. We manipulated clutch size and examined its relationship to offspring fitness. Clutch size affected the survivorship of S. limbatus larvae(r ²=0.14), but had no detectable effect on the survivorship of S. beali larvae (r ²=0.04). Also, clutch size had a large effect on development time and body weight of S. limbatus, but not of S. beali. We discuss the implications of this result for the evolution of clutch size in S. limbatus and S. beali.     

Resource-Dependent Clutch Size Decisions and Size-Fitness Relationships in a Gregarious Ectoparasitoid Wasp, <Emphasis Type="Italic">Bracon brevicornis</Emphasis>

Gregarious parasitic wasps, which lay more than one egg into or onto a host arthropod’s body, are usually assumed to lay an optimal number of eggs per host. If females would lay too few eggs, some resources may be wasted, but if females lay too many eggs, offspring may develop into substantially smaller-sized adults or may not develop successfully and die. The availability of hosts can further influence a female’s clutch size decision, as more eggs should be laid when hosts are scarce. Here, we analyzed clutch size decisions and the fitness consequences thereof in the ectoparasitic wasp Bracon brevicornis (Hymenoptera: Braconidae), a potential biocontrol agent against pest moth species. For experiments, larvae of the Mediterranean flower moth, Ephestia kuehniella (Lepidoptera: Pyralidae) were used. Using artificially created as well as naturally laid clutches of eggs, the effects of clutch size on fitness of first (F1) and second (F2) generation offspring were investigated. Our results revealed that the fitness consequences of large clutches included both increased mortality and smaller adult sizes of the emerging offspring (F1). Smaller F1 females matured fewer eggs during their lifetime and their offspring (F2) had reduced egg-to-adult survival probability. Naturally laid clutches varied with host size up to a maximum, which probably reflects egg limitation. Clutches remained smaller than the calculated optimal (Lack) clutch size and females responded to high host availability with a decreased number of eggs laid. We thus conclude that large clutches may result in significantly smaller offspring with reduced fitness, and that host size as well as host availability influence the clutch size decision made by B. brevicornis females.     

Influence of egg size differences within egg clutches on larval parameters in nine libellulid species (Odonata)

Abstract.  1. In libellulids, egg size differs between species and populations. There are also size differences within egg clutches of individual females.
2. Past experiments suggest that there are two different types of egg clutches in libellulids. Egg size decreases significantly during oviposition in species that perform non-contact guarding during oviposition. In contrast, in species ovipositing in tandem, egg size is randomly distributed.
3. This study deals with the possible consequences of egg size variation within the different egg clutch types. The study examined whether there is a correlation between egg development time, offspring sex or larval size and egg size.
4. The current experiments were conducted in Namibia and Germany. Five non-contact guarding and four tandem guarding libellulid species were used.
5. In some species larger eggs needed more time to develop, in some species no correlation between egg size and egg development time could be found, whereas in other species larger eggs developed faster.
6. The sex ratio was biased towards females in Leucorrhinia dubia and in Sympetrum striolatum and egg size was not associated with gender.
7. In both egg clutch types larger eggs resulted in larger larvae. In this study, evidence was found that the effects of egg size diminished with progressing larval development under good conditions. However, it is possible that the effects may have a greater influence under harsh circumstances.     

Maternal investment in reproduction and its consequences in leatherback turtles

Maternal investment in reproduction by oviparous non-avian reptiles is usually limited to pre-ovipositional allocations to the number and size of eggs and clutches, thus making these species good subjects for testing hypotheses of reproductive optimality models. Because leatherback turtles (Dermochelys coriacea) stand out among oviparous amniotes by having the highest clutch frequency and producing the largest mass of eggs per reproductive season, we quantified maternal investment of 146 female leatherbacks over four nesting seasons (2001–2004) and found high inter- and intra-female variation in several reproductive characteristics. Estimated clutch frequency [coefficient of variation (CV) = 31%] and clutch size (CV = 26%) varied more among females than did egg mass (CV = 9%) and hatchling mass (CV = 7%). Moreover, clutch size had an approximately threefold higher effect on clutch mass than did egg mass. These results generally support predictions of reproductive optimality models in which species that lay several, large clutches per reproductive season should exhibit low variation in egg size and instead maximize egg number (clutch frequency and/or size). The number of hatchlings emerging per nest was positively correlated with clutch size, but fraction of eggs in a clutch yielding hatchlings (emergence success) was not correlated with clutch size and varied highly among females. In addition, seasonal fecundity and seasonal hatchling production increased with the frequency and the size of clutches (in order of effect size). Our results demonstrate that female leatherbacks exhibit high phenotypic variation in reproductive traits, possibly in response to environmental variability and/or resulting from genotypic variability within the population. Furthermore, high seasonal and lifetime fecundity of leatherbacks probably reflect compensation for high and unpredictable mortality during early life history stages in this species.     

The problem of optimal clutch size in a tritrophic system: the oviposition strategy of the thistle gallfly Urophora cardui (Diptera,Tephritidae)

The problem of optimal clutch sizes is a central theme in life history theory. Optimal allocation of eggs is especially complicated for insects in tritrophic systems. In this study we analyze some of the processes determining clutch sizes of the thistle gallfly Urophora cardui, a monophagous tephritid fly associated with Cirsium arvense. U. cardui forms multilocular shoot galls, which vary broadly in their size and number of their gall cells. We investigate various fitness consequences of gall size. An analysis of the number of cells per gall (which is correlated with gall diameter and gall weight) showed that in U. cardui there is mutual facilitation rather than larval competition. Increasing numbers of larvae per gall led to a decreasing mortality and increasing larval weight. Larval weight in turn was positively correlated with the probability of survival to adulthood and with adult weight and fecundity. Thus, all fitness parameters measured favoured large galls. Clutch sizes in oviposition experiments were distinctly larger than the number of gall cells of field populations and in cage experiments, suggesting high mortality of eggs and/or early larval instars. There was a significant relationship between the internal structure (i.e., the size of the growing point) of the bud and clutch size, suggesting that U. cardui females are able to measure bud quality and adapt clutch sizes accordingly. Clutch size was positively correlated with the female's age at first oviposition and negatively with the number of previous ovipositions and previously laid eggs. Since the potential egg capacity per female is higher than the average number of larvae it is likely to produce during its short adult lifespan, U. cardui females tend to be time-limited rather than egglimited, which might favour large clutches once an appropriate oviposition site has been located. As the development of the gall and hence the fate of a clutch depends on a number of unpredictable factors, exclusive concentration of eggs in a few large clusters would involve risks which could be avoided by increasing the number of clutches. Therefore we interpret the high variation of clutch sizes in U. cardui as a mixed strategy of bet hedging and gambling.     

Cross‐generational costs of compensatory growth in nine‐spined sticklebacks

Compensatory growth (CG) is a form of phenotypic plasticity allowing individuals’ growth trajectories to rebound after a period of resource limitation, but little is known about the reproductive and cross‐generational costs of CG. We studied the potential costs of CG by exposing female nine‐spined sticklebacks Pungitius pungitius to 1) high (favourable), 2) low (stressful), and 3) recovery (initially stressful, subsequently favourable) feeding treatments, and quantified the effects of these treatments on female reproductive traits (clutch, egg and yolk size), and on the size of their offspring. The low feeding treatment reduced female size relative to the high and recovery feeding treatments, which produced equally large females. Hence, females from the recovery treatment demonstrated CG and full growth compensation. Feeding treatments had significant effects on clutch, yolk, egg and larval size, also when the effect of female size was controlled for. However, these effects came about mostly because females from the low feeding treatment produced small clutches with large eggs (containing little yolk) and larvae, whereas females from the recovery feeding treatment produced as large clutches, eggs (with similar amounts of yolk) and larvae as females from the high feeding treatment. Yet, structural equation modelling revealed that while a direct effect of female size on offspring size was positive in the low and high feeding treatments, it was negative in the recovery feeding treatment, independently of egg and clutch size. This indicates a cross‐generational tradeoff between female and offspring sizes in the recovery feeding treatment. Also the tradeoff between clutch and larval size was more pronounced in recovery than in low or high feeding treatments. Hence, apart from demonstrating that environmental influences experienced by females during their development have the potential to influence their size, fecundity and reproductive traits, the results also provide evidence for complex cross‐generational costs of recovery growth.     

Travel costs, oviposition behaviour and the dynamics of insect–plant systems

Insect attack can have major consequences for plant population dynamics. We used individually based simulation models to ask how insect oviposition behaviour influences persistence and potential stability of an herbivore–plant system. We emphasised effects on system dynamics of herbivore travel costs and of two kinds of behaviour that might evolve to mitigate travel costs: insect clutch size behaviour (whether eggs are laid singly or in groups) and female aggregation behaviour (whether females prefer or avoid plants already bearing eggs). Travel costs that increase as plant populations drop lead to inverse density dependence of plant reproduction under herbivore attack. Female clutch size and aggregation behaviours also strongly affect system dynamics. When females lay eggs in large clutches or aggregate their clutches, herbivore damage varies strongly among plants, providing probabilistic refuges that permit plant reproduction and persistence. However, the population dynamics depend strongly on whether insect behaviour is fixed or responds adaptively to plant population size: when (and only when) females increase clutch size or aggregation as plants become rare, refuges from herbivory weaken at high plant density, creating inverse density dependence in plant reproduction. Both herbivore travel costs themselves, and also insect behaviour that might evolve in response to travel costs, can thus create plant density dependence—a basic requirement for regulation of plant populations by their insect herbivores.     

The evolution of egg size in socially polyandrous shorebirds

In classical and multi-clutch polyandry, females lay multiple clutches during a breeding season for more than one mate. The production of multiple clutches may be energetically demanding. We used comparative analyses to investigate three possible ways of reducing such egg-laying costs in polyandrous shorebirds: (1) reduction in egg size, (2) reduction in clutch size, and (3) evolutionary increase in female size. Paired comparisons of polyandrous and non-polyandrous taxa showed that females of polyandrous shorebirds lay smaller eggs than females of closely related monogamous and polygynous species. Directional analyses corroborated this result by indicating a significant decrease in egg size after phylogenetically independent origins of polyandry. The comparative analyses uniformly rejected the two alternatives, i.e. neither clutch size nor female size is related to social mating pattern. We also tested and rejected three alternative explanations for reduced egg size in polyandrous taxa. First, we found no evidence that polyandrous females have evolved smaller egg sizes in response to selection to match smaller size of males, which provide the parental care in these species. Second, reduction in egg size was not related to longer breeding seasons (and hence more opportunity for re-nesting). Third, reduced egg sizes were also not related to rates of clutch predation (another potential correlate of multiple clutch production). Our results are thus consistent with the hypothesis that selection for reducing laying costs explains small egg size in socially polyandrous shorebirds.     

Brood conspicuousness and clutch viability in male‐caring assassin bugs (Rhinocoris tristis)

Abstract 1. Conspicuousness to mates can bring benefits to both males (increased mating success) and females (reduced search costs), but also brings costs (e.g. increased predation and parasitism). Assassin bugs, Rhinocoris tristis, lay egg clutches either on exposed stems or hidden under leaves. Males guard eggs against parasitoids. Guarding males are attractive to females who add subsequent clutches to the brood. This is an excellent opportunity to study the effects of conspicuousness on the fitness of males and females. 2. Using viable eggs in a multi‐clutch brood as a correlate of fitness, the present study examined whether laying eggs on stems affected (1) female fitness, through exposure to parasitism and cannibalism, and (2) male fitness, through attracting further females. 3. Stem broods were more parasitised. However, males on stems accumulated more mates and more eggs, a net benefit even accounting for parasitism. The eggs gained from being on a stem were cannibalised. By contrast, higher mortality on stems suggests that females should gain by ovipositing on leaves. To the extent that egg viability represents fitness, male and female interests may therefore differ. This suggests a potential for sexual conflict that may affect other species with male care. 4. Despite higher costs, females actually initiated more broods, and subsequently added bigger clutches to broods, on stems than under leaves. This suggests either that viable eggs do not reflect fitness, or that females laid in unfavourable locations. The key is now to address lifetime fitness, since unmeasured factors may affect offspring viability post‐hatching, and to investigate who controls the location of oviposition in R. tristis.     

Allocation of reproductive effort in female Arrenurus spp. water mites (Acari: Hydrachnidia; Arrenuridae)

Summary Twenty-seven species of water mites of the genus Arrenurus were compared with respect to allocation of reproductive effort. Clutch volume was positively correlated to female volume, female volume was positively correlated with clutch size and with larval volume, while clutch size and larval volume were negatively correlated. In threespace, corresponding to female volume, clutch size and larval volume, species were arranged along two trajectories representing separate reproductive strategies. The strategy characterized by small larvae, large clutches and large females corresponded to species of the subgenus Arrenurus, known to be larval parasites of adult Odonata. Members of three subgenera, known to be larval parasites of adult Diptera, followed the second strategy of small clutch size and alrge larvae. Of nine species of mites tested for intraspecific relationships, only one significant relationship was found, between female volume and clutch size for A. (Megaluracarus) bartonensis.     

Sex allocation and clutch size in parasitoid wasps that produce single-sex broods

The parasitoid wasp genus Achrysocharoides (Eulophidae) is unusual in that many of its species lay male and female eggs in single-sex clutches. The average clutch size of female broods is always greater than that of male broods, and in some species male clutch size is always one. We constructed models that predicted that severely egg-limited wasps should produce equal numbers of male and female eggs while severely host-limited wasps should produce equal numbers of male and female broods (and hence an overall female-biased sex ratio). Theory is developed to predict clutch size and sex ratio across the complete spectrum of host and egg limitation. A comparison of 19 surveys of clutch composition in seven species of Achrysocharoides showed a general pattern of equal numbers of male and female broods with a female-biased sex ratio (suggesting host limitation) although with considerable heterogeneity amongst collections and with a number of cases of unexpectedly low frequencies of male broods. Using a previous estimate of the relationship between fitness and size in the field, we predicted the maximally productive (Lack) clutch size for female broods of Achrysocharoides zwoelferi to be three. Of clutches observed in nature, 95% were equal to or smaller in size than the predicted Lack clutch size. When we manipulated local host density in the field, and as predicted by our models, clutch size and the proportion of female broods of A. zwoelferi decreased as hosts became more common, but the absolute frequency of male clutches was lower than expected. Copyright 1998 The Association for the Study of Animal Behaviour.     

Adaptive decision making or differential mortality: what causes offspring emergence in a gregarious parasitoid?

Hosts represent a limited resource for the developing offspring of parasitic insects laying eggs in or on spatially discrete resources like fruits, seeds, or other insects. The quality of hosts differs with respect to the value and amount of resources they provide for the feeding larvae. Accordingly, the size of a clutch of eggs laid on a given host should be a function of host quality, because severe competition between developing larvae can lead to increased mortality and/or decreased size of the offspring, both causing a fitness loss for the offspring and the mother. Therefore, females should be selected for the ability to estimate host quality and to adjust their clutch size accordingly. Using the parasitic wasp Nasonia vitripennis (Walker) (Hymenoptera: Pteromalidae) this study investigated the respective contribution of developmental mortality of offspring vs. the clutch size decision of the mother as a determinant of final offspring emergence per host. In addition, taking offspring size into account, the study examined the fitness consequences of female oviposition decisions. Developmental mortality was very low in all quality classes of hosts except previously frozen and thus dead host pupae. Females laid reduced clutch sizes on dead, previously parasitized, and smaller hosts. In contrast to offspring number, offspring size did not differ between host qualities. We conclude that females are able to sense the quality of a host and adjust the number of eggs they lay to mitigate larval competition.     

When should a female avoid adding eggs to the clutch of another female? A simultaneous oviposition and sex allocation game

Summary Avoidance of double oviposition (ADO) is the strategy not to oviposit on food patches where another female has oviposited before. If two females oviposit on the same patch, competitive and mating interactions within and between broods may lead to both a clutch size game and a sex allocation game between the two visitors. Though the two games interact, they are usually considered separately. Here, the ESS conditions for ADO are investigated in an analysis that combines the two games into one. The analysis strengthens the notion that it is really ADO that needs to be explained, because role-dependent net pay-off from an additional egg is most likely to favour double oviposition (DO). To a first female, the net payoff includes the effect on the eggs already present, whereas to a second female only the egg's gross pay-off matters. ADO is the evolutionary stable strategy (ESS) if there are enough patches still without eggs and either (1) the fitness of an additional egg is so low that the first female would not lay it even in the absence of detrimental effects on earlier offspring, so neither would a second female, or (2) differences in either the survival probability of the offspring or their reproductive success are sufficient to counterbalance the differential interest in the eggs already present. The first condition requires that eggs are relatively large, because then the decrease in pay-off between two successive eggs can be large. The second condition may be met when there is a time interval between ovipositions of subsequent females. The resulting developmental lag of the second clutch will (1) diminish its ability to compete for food and (2) lower its reproductive success when there is local mate competition and sons are too late to mate with daughters of the first female. If sons of first and second females compete on equal terms, however, ADO is unlikely. Male migration between patches reduces the influence of sex allocation strategies on clutch size decisions; the same holds for small clutch sizes. To illustrate the importance of considering sex allocation and clutch size decisions in an integrated way, oviposition strategies of plant-inhabiting predatory mites (Acari: Phytoseiidae) are discussed.     

Larval secretions and food odors affect orientation in femalePlodia interpunctella

Substrates contaminated by wandering fifth instar larvae ofPlodia interpunctella (Hübner) (Lepidoptera: Pyralidae) elicit oviposition by conspecific female moths, and larval rearing diet enhances oviposition and also induces upwind flight. Two-choice oviposition assays determined that four-day-old gravid femaleP. interpunctella preferred to lay eggs on dishes containing cornmeal-based rearing diet compared to empty dishes. Pieces of cheesecloth contaminated by fifth instar larvae elicited more oviposition than untreated cheesecloth or dishes with food. The combination of larval contamination and food was preferred over food only or larval contamination only in both two- and four-choice experiments. The factor(s) in larval contamination responsible for eliciting oviposition in female moths was extracted in hexane, confirming that organic semiochemicals are responsible for the effect. The oviposition-eliciting activity of larval contamination was retained on cheesecloth for up to 30 days following treatment with larvae, suggesting the active component(s) is stable and of low relative volatility. In two-choice windtunnel bioassays female moths initiated flight only when rearing food was present in one of the treatments, and they displayed the highest landing responses to a combination of larval contamination and food. Earlier work onP. interpunctella and related pyralid species found that larval contamination due to secretions from the mandibular glands acted as both a spacing pheromone for wandering larvae and as a kairomone for host-seeking parasitoid wasps. The present study suggests that the same or a similar secretion acts as an oviposition-eliciting pheromone for conspecific females.     

Clutch-size behavior and coexistence in ephemeral-patch competition models

Systems of patchy, ephemeral resources often support surprisingly diverse assemblages of consumer insects. Aggregation of consumer individuals over the landscape of patches has been suggested as one mechanism that can stabilize competition among consumer species. One mechanism for larval aggregation is the laying of eggs in clutches by females traveling among patches to distribute their total fecundity. We use simulation models to explore the consequences, for coexistence of competitors, of larval aggregation that arises from clutch laying. Contrary to some previous treatments, we find that clutch laying can be strongly stabilizing and under certain conditions can be sufficient to allow competitors to coexist stably. We extend these models by considering clutch size as a variable that responds to the abundance of resource patches. Such a relationship might be expected because females should lay their eggs in fewer but larger clutches when the cost of travel among patches is high (because patches are rare). When females adjust clutch size in response to resource abundance, coexistence can be easiest when resource patches are scarce and most difficult when resources are abundant.     

Prey Abundance, Intraguild Predators, Ants and the Optimal Egg-laying Strategy of a Furtive Predator

Larval performance can have a great influence on female oviposition choice, especially in insects where the newly hatched offspring are unable to move any great distance to find an appropriate food source. For furtive predators, like the predatory midge Aphidoletes aphidimyza which preys on aphids while simultaneously residing and remaining undetected within their colonies, oviposition behaviour is crucial because these slow moving offspring are restrained to their natal colony. Here we develop a new model for predicting the optimum number of eggs that a furtive predator should lay in an aphid colony, based on: (1) the number of available prey (aphids); (2) the protection from predation conferred by “hiding” in the colony and (3) the effects of interspecific and intraspecific competition. We also explore the effect of aphid attendance by ants on oviposition behavior. We compare model predictions with empirical field observations of the clutch sizes of A. aphidimyza in apple orchards. The simplest of the four models best fits the observed data and provides the first field evidence that a furtive predator adjusts its clutch size as a function of prey density. The slope of the relationship between clutch size and aphid number is quite close to that predicted by our models suggesting that intra-clutch competition is the main factor governing furtive aphid midge oviposition choice.