The impact of parasitoid emergence time on host–parasitoid population dynamics

We investigate the effect of parasitoid phenology on host–parasitoid population cycles. Recent experimental research has shown that parasitized hosts can continue to interact with their unparasitized counterparts through competition. Parasitoid phenology, in particular the timing of emergence from the host, determines the duration of this competition. We construct a discrete-time host–parasitoid model in which within-generation dynamics associated with parasitoid timing is explicitly incorporated. We found that late-emerging parasitoids induce less severe, but more frequent, host outbreaks, independent of the choice of competition model. The competition experienced by the parasitized host reduces the parasitoids’ numerical response to changes in host numbers, preventing the ‘boom-bust’ dynamics associated with more efficient parasitoids. We tested our findings against experimental data for the forest tent caterpillar (Malacosoma disstria Hübner) system, where a large number of consecutive years at a high host density is synonymous with severe forest damage.     

Indirect interactions in aphid–parasitoid communities

Indirect interactions between populations of different species can be important in structuring natural communities. Indirect effects are either mediated by changes in population densities (trophic or density-mediated effects) or by changes in the behavior of species that are not trophically connected (behavioral or trait-mediated effects). We reviewed the literature on aphids and their parasitoids to explore the various possible indirect interactions that can occur in such communities. The review was motivated by our study of a particular aphid–parasitoid community in a natural (i.e., nonagricultural) habitat, and by the wealth of information that exists about aphid–parasitoid systems in agricultural settings. We focused our review on aphid–parasitoid interactions, but considered how these were influenced by the other aphid natural enemies and also by aphid mutualists and host plants. We conclude that indirect effects are likely to have a major effect in structuring aphid–parasitoid communities, and that the latter are a valuable model system for testing ideas about community interactions. Received: December 20, 1998 / Accepted: January 12, 1999     

Does composition of tropical agricultural landscape affect parasitoid diversity and their host–parasitoid interactions?

相似文献   

Host–parasitoid interaction as affected by interkingdom competition

Although still underrepresented in ecological research, competitive interactions between distantly related organisms (so-called “interkingdom competition”) are expected to be widespread in various ecosystems, with yet unknown consequences for, e.g. trophic interactions. In the model host–parasitoid system Drosophila melanogaster–Asobara tabida, toxic filamentous fungi have been shown to be serious competitors that critically affect the density-dependent survival of host Drosophila larvae. This study investigates the extent to which the competing mould Aspergillus niger affects key properties of the well-studied Drosophila–parasitoid system and how the host–parasitoid interaction influences the microbial competitor. In contrast to slightly positive density-dependent host mortality under mould-free conditions, competing A. niger mediated a strong Allee effect for parasitised larvae, i.e. mortality decreased with increasing larval density. It was found that the common toxic fungal metabolite kojic acid is not responsible for higher death rates in parasitised larvae. Single parasitised Drosophila larvae were less harmful to fungal reproduction than unparasitised larvae, but this effect vanished with an increase in larval density. As predicted from the negative effect of fungi on host survival and thus on parasitoid fitness at low larval densities, A. tabida females spent less time foraging in fungus-infested patches. Interestingly, even though high host larval densities increased host survival, parasitoids still reduced their search efforts in fungus-infested patches, indicating a benefit for host larvae from feeding in the presence of noxious mould. Thus, this experimental study provides evidence of the potentially important role of interkingdom competition in determining trophic interactions in saprophagous animal communities and the dynamics of both host–parasitoid and microbial populations.     

Coevolution of dispersal in a parasitoid–host system

Interspecific interactions and the evolution of dispersal are both of interest when considering the potential impact of habitat fragmentation on community ecology, but the interaction between these processes is not well studied. We address this by considering the coevolution of dispersal strategies in a host–parasitoid system. An individual-based host–parasitoid metapopulation model was constructed for a patchy environment, allowing for evolution in dispersal rates of both species. Highly rarefied environments with few suitable patches selected against dispersal in both species, as did relatively static environments. Provided that parasitoids persist, all the variables studied led to stable equilibria in dispersal rates for both species. There was a tendency toward higher dispersal rates in parasitoids because of the asymmetric relationships of the two species to the patches: vacant patches are most valuable for hosts, but unsuitable for parasitoids, which require an established host population to reproduce. High host dispersal rate was favoured by high host population growth rate, and in the parasitoid by high growth rates in both species. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Is one parasitoid enough? A test comparing one with a pair of parasitoid species in the biological control of arrowhead scales

In classic biological control using natural enemies, the question of whether a single species or multiple species should be introduced has been a matter of debate. The introduction of two parasitoids, Aphytis yanonensis and Coccobius fulvus (Hymenoptera: Aphelinidae), to control the arrowhead scale, Unaspis yanonensis (Hemoptera: Diaspididae), which is a serious pest in Japanese citrus orchards, has been one of the most successful biological control projects in Japan. The success of this program may be explained by two alternative hypotheses: (1) the parasitoid species work complementarily, or (2) only one of them plays a major role. To test which hypothesis is applicable to this host-parasitoid system, we conducted caging experiments and observed temporal changes in the proportion of the parasitisms and the densities of arrowhead scales enclosed with one of the following combinations of parasitoids: (1) A. yanonensis and C. fulvus together, (2) A. yanonensis alone, (3) C. fulvus alone, or (4) neither parasitoid. Parasitisms in the cohorts with A. yanonensis and C. fulvus together and C. fulvus alone rapidly increased to approximately 70%; parasitism with A. yanonensis alone also increased slightly, although it remained consistently lower that those with A. yanonensis and C. fulvus together and C. fulvus. At the end of the experiment, parasitisms with A. yanonensis and C. fulvus together and C. fulvus alone were significantly higher than that with A. yanonensis alone. Parasitism by C. fulvus constituted most of (74%) the parasitism in the cohort with A. yanonensis and C. fulvus together. Further, only C. fulvus suppressed the population growth rates of scales significantly. These results suggest that C. fulvus alone successfully suppresses scale populations as efficiently as both species together do.     

Do parasitoid preferences for different host species match virulence?

Abstract.  Leptopilina boulardi is a parasitoid wasp specialist of Drosophila larvae of the melanogaster subgroup. In Mediterranean areas, natural populations are highly virulent against their main host Drosophila melanogaster . In Congo, populations are less virulent against D. melanogaster but are able to develop successfully inside the tropical African species Drosophila yakuba . Host preferences are compared between two laboratory isofemale lines of L. boulardi , obtained from populations of Congo and Tunisia, respectively, and differing in virulence levels against D. melanogaster and D. yakuba . Host selection is studied by offering female parasitoids a choice between larvae of the two host species. In agreement with optimal foraging models, the line highly virulent against D. melanogaster shows a clear preference for this host species. The other line, less virulent against D. melanogaster but more virulent against D. yakuba , prefers to oviposit on D. yakuba . Such preferences can be observed after a period of host-patch exploitation only, suggesting that experience plays an important role in the host-selection process. These results evidence the existence of intraspecific variability in preference between two host species in L. boulardi , a major requisite in theoretical models of parasite specialization by the host. They also sustain the hypothesis that intraspecific variation in parasitoid preferences between host species might mirror intraspecific variation in virulence.     

Persistent host–parasitoid interaction caused by host maturation variability

The heterogeneity of parasitism risk among host individuals is a key factor for stabilizing or sustaining host–parasitoid interactions. Host maturation variability, or the variation in the maturation times among host individuals, is the simplest source of such heterogeneity, but it has often been neglected in previous theoretical studies. We developed a configuration individual-based model (cIBM) of host–parasitoid interaction to investigate to what degree of host maturation variability promotes the persistence of host–parasitoid interactions. We ran simulations with various degrees of host maturation variability for different lengths of unsusceptible period. The result showed that low host maturation variability could sustain host–parasitoid dynamics when the host-unsusceptible period was short. Conversely, high levels of variability could sustain host–parasitoid dynamics when the host-unsusceptible period was about half of the total larval period. This suggests that the balance between variability and unsusceptible period is important for the persistence of host–parasitoid interaction. We conclude that maturation variability is a factor that can contribute to the sustainment of host–parasitoid interactions.     

Excised or intact inflorescences? Methodological effects on parasitoid wasp longevity

The development of accurate and repeatable experimental techniques is a cornerstone of any research program. Indeed, the first stage in developing a conservation biological control program typically involves ranking the suitability of various plant species as food resources for the target species of natural enemy in the laboratory or glasshouse. Herein the choice of flower presentation method is a highly relevant consideration. It is unclear whether excised flowers with their peduncles submerged in water will generate similar effects on the life history traits of a natural enemy compared with those using flowers remaining intact on a rooted plant. Either method has been used in 86 previous studies, yet none has quantified this effect. It is possible that both plant nectar content and production are altered as a result of changes in the physiological condition of the excised flowers. A laboratory test was designed to assess the influence of flower presentation method (excised or intact inflorescences) and different types of nectar (artificial and natural) on the longevity of the wasp Aphidius ervi, an important parasitoid of aphids. Distinct differences were revealed in the suitability of the nine flower species and three control treatments on parasitoid wasp longevity, with buckwheat being the most suitable plant. However, apart from coriander, flower presentation method and wasp gender generally did not affect parasitoid longevity for the set of species tested. As there was little evidence that parasitoid wasp longevity would be altered on excised flowers, and because of reasons pertaining to improved logistical and experimental requirements, the use of excised flowers is cautiously recommended to researchers for further laboratory evaluations of the effects of nectar provision on parasitoid fitness.     

Effects of habitat configuration on host–parasitoid food web structure

The dispersal of organisms among patches affects community structure in spatially heterogeneous habitats. The enhancement of dispersal frequency among patches can be expected to increase potential interaction between organisms in food webs. However, it has been difficult to fairly evaluate the effects of dispersal on the food web structure because the quantification of actual dispersal is difficult. In this study, in order to manipulate the dispersal frequency, two oak plantations (each with 100 oak trees) were established as high-patch connectivity (1-m interval) and low-patch connectivity (3-m interval) plots. Quantitative food webs of herbivores and their parasitoids were constructed for the high- and low-connectivity plots, and quantitative measures of food web metrics as indices of structure were calculated for both webs to examine dispersal effects on food web complexity. In the entire web, 86 herbivore species (Lepidoptera and Coleoptera) were attacked by 50 parasitoid species (Hymenoptera and Diptera). As a result, although we found no significant difference in herbivore abundance between high- and low-connectivity plots, a higher parasitism rate and greater complexity in web structure were observed in many food web metrics for the high-connectivity plot. Furthermore, the parasitoid overlap diagram showed a higher potential for indirect interactions among herbivore species in the high-connectivity plot. These results imply that the increase in dispersal frequency among habitat patches facilitates food web complexity, and the role of dispersal as a determinant of food web structure should be considered in food web ecology.     

Do laboratory studies predict parasitoid interaction outcomes in the field?

Studying competitive interactions among natural enemies is important to elucidate the success and non-target impact of candidate biological control agents. Increased regulation of new introductions requires that studies on non-target species be carried out in confined conditions. Hypotheses about potential impacts of biological control agents in the field are based on data from Petri dish or small cage experiments conducted in the laboratory. This study compared the performance of the diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae), parasitoids Diadegma insulare (Cresson) (Hymenoptera: Ichneumonidae) and Microplitis plutellae (Muesebeck) (Hymenoptera: Braconidae) in experiments conducted in small cages in the laboratory and in large cages in the field. Results showed no significant differences between laboratory and field outcomes for D. insulare alone and when D. insulare and M. plutellae were combined. For M. plutellae alone, parasitism in the laboratory cages was significantly less than in the field cages. These results demonstrate that laboratory studies may be useful to develop hypotheses on competitive interactions of candidate parasitoid biological control agents.     

Effect of Bt maize on the plant-aphid–parasitoid tritrophic relationships

The effect of Bt maize on aphid parasitism and the aphid–parasitoid complex was measured in field conditions on three transgenic varieties, two derived from Event MON810 and one from Bt176, and their near-isogenics in a two-year study. No differences in aphid abundance were found between Bt maize varieties and their near-isogenics. Differences within Bt and within near-isogenic varieties were found, but only in one year. Differences in aphid abundance were probably better accounted for the variety background and year conditions than by the transgenesis or Event. Lysiphlebus testaceipes (Cresson), Lipolexis gracilis Förster (Hymenoptera, Braconidae, Aphidiinae) and Aphelinus sp. (Hymenoptera, Aphelinidae) were the prevalent parasitoids. Bt maize did not alter the aphid–parasitoid associations and had no effect on the aphid parasitism and hyperparasitism rates. The results suggest that Bt maize has no negative impact on second, third and fourth levels of the trophic relationships studied.     

Delayed feedback and multiple attractors in a host–parasitoid system

 Continuous-time, age structured, host–parasitoid models exhibit three types of cyclic dynamics: Lotka–Volterra-like consumer-resource cycles, discrete generation cycles, and “delayed feedback cycles” that occur if the gain to the parasitoid population (defined by the number of new female parasitoid offspring produced per host attacked) increases with the age of the host attacked. The delayed feedback comes about in the following way: an increase in the instantaneous density of searching female parasitoids increases the mortality rate on younger hosts, which reduces the density of future older and more productive hosts, and hence reduces the future per head recruitment rate of searching female parasitoids. Delayed feedback cycles have previously been found in studies that assume a step-function for the gain function. Here, we formulate a general host–parasitoid model with an arbitrary gain function, and show that stable, delayed feedback cycles are a general phenomenon, occurring with a wide range of gain functions, and strongest when the gain is an accelerating function of host age. We show by examples that locally stable, delayed feedback cycles commonly occur with parameter values that also yield a single, locally stable equilibrium, and hence their occurrence depends on initial conditions. A simplified model reveals that the mechanism responsible for the delayed feedback cycles in our host–parasitoid models is similar to that producing cycles and initial-condition-dependent dynamics in a single species model with age-dependent cannibalism. Received: 24 October 1997 / Revised version: 13 June 1998     

Refuge evolution and the population dynamics of coupled host—parasitoid associations

Summary We have investigated the theoretical consequences of character evolution for the population dynamics of a host—parasitoid interaction, assuming a monophagous parasitoid. In the purely ecological model it is assumed that hosts can escape parasitism by being in absolute refuges. A striking property of this model is a threshold effect in control of the host by the parasitoid, when host density dependence is weak. The approximate criteria for the parasitoid to regulate the host to low densities are (1) that the parasitoid's maximum population growth rate should exceed the host's and (2) that the maximum growth rate of the host in the refuge should be less than unity. We then use this ecological framework as a basis for a model which considers evolutionary changes in quantitative characters influencing the size of the absolute refuge. For each species, an increase in its refuge-determining character comes at a cost to maximum population growth rate. We show that refuge evolution can substantially alter the population dynamics of the purely ecological model, resulting in a number of emergent and sometimes counter-intuitive properties. In general, when the host has a high carrying capacity, systems are polarized either with low or minor refuge and top-down control of the host by the parasitoid or with a refuge and bottom-up control of the host by a combination of its own density dependence and the parasitoid. A particularly tantalizing result is that co-evolutionary dynamics can modify ecologically unstable systems into ones which are either stable or quasi-stable (with bouts of unstable dynamics, punctuating long-term periods of quasi-stable behaviour). We present five quantitative criteria which must all be met for the parasitoid to be the agent responsible for control of the host at a co-evolutionary equilibrium. The apparent stringency of this full set of requirements supports the empirically-based suggestion that monophagous parasitoid-driven systems should be less common in nature than those driven by multiple forms of density dependence. Further, we apply our theory to the question of whether exploiters may harvest their victims at maximum sustainable yields and to the evolutionary stability of biological control. Finally, we present a series of testable predictions of our theory and methods useful for testing them.     

Evolution of contest competition and its effect on host–parasitoid dynamics

In experimental populations of the cowpea bean weevil Callosobruchus maculatus (Coleoptera: Bruchidae) and a parasitic wasp Heterospilus prosopidis (Hymenoptera: Braconidae), large changes in the abundances and the fluctuations of both species occurred after approximately 20 generations. In this paper, we examine the hypothesis that this observed change in the dynamics may have been caused by an evolutionary shift in the mode of competition among the bean weevils. A Nicholson-Bailey type model is developed using parameters measured from the experiments. The host larvae can differ in the type of competitive behaviour that they exhibit, which can be either of a contest type or of a scramble type. If a bean contains one or more larvae of the contest type, only one of these will survive and any scramble-type larvae in the bean will be killed. If no contest-type larvae are present within a bean, multiple individuals of the scramble type can emerge from a single bea n. The model assumes many genotypes, differing in the fraction of offspring of the two types. If a high per capita resource availability is maintained, then the scramble type is selected for, but if resources are limited, then the contest type is selected for. The host population at the start of the experiment, taken from a stock culture, was composed mostly of the scramble type. The model is successful in explaining the initial quick increase in the host's abundance, followed by the evolutionary increase in the fraction of the contest type among hosts, resulting in the more stable population dynamics of the host–parasitoid system, as observed in the experiments. However, it predicts a parasitoid abundance much higher than that observed. We discuss alternative hypotheses to explain the observed evolutionary shift in the population dynamics. We also examine the effect of the difference in size of the beans in the stock culture and those used in the experiments.     

Differential effects of flower feeding in an insect host–parasitoid system

In many insect host–parasitoid systems, both the host and its parasitoids forage on shared floral resources. As a result of insect behaviour, morphology and physiology, flower species may act selectively at different levels of such systems, e.g., between the trophic levels of hosts and parasitoids, between species within a guild, between sexes or individuals within a species or between life history traits within an individual. We asked if effects of selectivity are consistent across levels in the horse chestnut leafminer, Cameraria ohridella, and its parasitoid complex. Insects were exposed singly in no-choice feeding trials to twelve common flower species and their survival and reproduction were recorded. Only one of twelve flower species (Ranunculus acris) tended to selectively favour the longevity of leafminers, but not of parasitoids. No flower species were found to favour parasitoids only. Both trophic levels profited from feeding on Anthriscus sylvestris, however, parasitoids benefited up to eight times more than their hosts. No differences were found among the species of the parasitoid guild, but females lived significantly longer than males, and single individuals within species were able to exploit generally unfavourable flower species. Out of the seven flower species that increased the longevity of leafminer females, only Chaerophyllum hirsutum significantly enhanced the number of eggs laid. Fecundity was generally positively correlated with longevity of leafminer females, but two flower species (C. hirsutum, Taraxacum officinale) had an additional positive effect on fecundity. In conclusion, we demonstrated that flowers act differently on life history traits in a host–parasitoid system at a multitude of biological levels and that these effects are not always consistent across levels. Selective plant-derived resources can therefore modify herbivore–natural enemy interactions in ways that are more complex than currently appreciated.     

Plant–insect interactions: gentians,seed predators and parasitoid wasps

While studying breeding systems and pollination ecology of nine Gentiana species (G. lutea, G. punctata, G. asclepiadea, G. pneumonanthe, G. cruciata, G. pyrenaica, G. verna, G. utriculosa, and G. nivalis) in the Bulgarian mountains, we recorded number of insects that feed on their maturing seeds. In addition, parasitoid wasps in connection to these seed predators were detected. Insects are identified and the impact on the seed set of afore mentioned Gentiana species is estimated. Fruit capsules of Gentiana spp. from different populations in the mountains in Bulgaria were investigated for the presence or absence of damage by larvae during the period of 16 years. The seed destruction varies among the nine investigated Gentiana species. The insects whose larvae damaged the seed/fruit set belonged mainly to Coleoptera and Diptera. The larvae of lycaenid butterflies, Maculinea spp. (Lepidoptera), were recorded only in seeds of G. asclepiadea, G. pneumonanthe and G. cruciata. Parasitoid wasps from the families Ichneumonidae, Braconidae, and Pteromalidae were identified, some of them new for the fauna of Bulgaria.     

Host-size-dependent sex ratios among parasitoid wasps: does host growth matter?

Summary Waage's (1982) hypothesis that host-size-dependent sex ratios will occur in parasitoids of nongrowing hosts and not in parasitoids of growing hosts is examined using published data on parasitoid wasps. Waage's hypothesis is supported as a general, but not absolute, rule: among solitary parasitoid wasps, a significantly greater proportion of parasitoids of nongrowing than of growing hosts show some evidence of host-size-dependent sex ratios (85% versus 42%, G=6.54, P< 0.05). The premise of Waage's hypothesis-that for parasitoids which develop in a growing stage, host size at oviposition is not a good predictor of the amount of resources available to the developing parasitoid-is also examined. It is suggested that across host species Waage's premise will hold for some, but not all, parasitoids of growing hosts. Likely exceptions to Waage's premise, and thus his prediction, are discussed. Parasitoids of growing hosts which are expected to have evolved hostsize-dependent sex ratios include parasitoids which utilize a narrow size range of host species, parasitoids which can distinguish among host species by some criterion other than size, and parasitoids which utilize host species whose susceptible instars do not overlap in size.