Dispersal and foraging behaviour of Platygaster californica: hosts can't run, but they can hide

Abstract.  1. Host–parasitoid models often identify foraging behaviour and dispersal distance as important for system persistence.
2. Laboratory observations and field trials were used to characterise foraging behaviour and dispersal capability of Platygaster californica Ashmead (Platygasteridae), a parasitoid of the gall midge Rhopalomyia californica Felt (Cecidomyiidae).
3. Although foraging parasitoids meticulously searched plants in laboratory observations, none of the laboratory trials resulted in 100% parasitism, and the proportion of parasitism declined as midge egg density increased.
4. The field trials showed that the distribution of parasitism over distance from a central release point was hump-shaped, as predicted by a simple diffusion model. Mean parasitoid dispersal distance was 4.5 m, considerably farther than the 1.7 m mean midge dispersal found in previous work.
5. Although the parasitoid appears to search thoroughly for midge eggs and to disperse farther than the midge, the results of this study show how this host–parasitoid system may persist due to spatially variable incomplete parasitism.     

Risk of dispersal in western spruce budworm

1 Western spruce budworm Choristoneura occidentalis Free. larvae emerge in the spring before buds have expanded and spend a variable period of time foraging on branches and mining needles.
2 Losses of dispersing budworms during this needle-mining period are related directly to the severity of defoliation in previous years and inversely related to foliage biomass in the study plot and to temperature and rainfall during the needle-mining period.
3 Losses can be interpreted in terms of risk of dispersal, which is the product of the propensity of early-stage budworms to disperse in search of resources and the consequences of this behaviour for survival under variable ecological conditions.
4 A comparison of the species-specific nature of risk of dispersal in three conifer-feeding budworm systems of North America may elucidate the common nature but variable features of their respective population dynamics.     

Density-dependent dispersal in host-parasitoid assemblages

Most spatial population models assume constant rates of dispersal. However, in a given community, dispersal may not only depend on the density of conspecifics, i.e. density‐dependent dispersal, but also on the density of other species, a phenomenon we term ‘community‐dependent dispersal’. We co‐vary the densities of both the beetle host Callosobruchus chinensis and its parasitoid wasp, Anisopteromalus calandrae, in a laboratory study and record the proportions of each species that disperse within a two‐hour period. The parasitoid in these systems exhibits community‐dependent dispersal – dispersing more frequently when parasitoid density is high and larval host density is low. This supported our prediction that individuals should disperse according to competition for available resources. However, in this study the host's dispersal was independent of density. We suggest that this may be due to less intense selection acting on host dispersal strategies than on the parasitoid. We consider some possible consequences of community‐dependent dispersal for a number of spatial population processes. A well‐known host‐parasitoid metapopulation model is expanded so that it includes a greater range of dispersal functions. When the model is parameterised with the parasitoid community‐dependent dispersal function observed in the empirical study, similar population dynamics are obtained as when fixed‐rate dispersal functions are applied. The importance of dispersal functions for invasions of both competitive and host‐parasitoid systems is also considered. The model results demonstrate that understanding how individuals disperse in response to different species’ population densities is important in determining the rate of spread of an invasion. We suggest that more empirical studies are needed to establish what determines dispersal rate and distance in a range of species, combined with theoretical studies investigating the role of the dispersal function in determining spatial population processes.     

Factors affecting host selection in an insect host–parasitoid interaction

1. Many parasitoids can develop successfully in different stages of the same host but the costs of parasitism may vary between the stages. The stage of host attacked has generally been determined when there is no choice, giving a misleading impression of host selection or preference.
2. The rate of parasitism by a solitary endoparasitoid, Venturia canescens , of each larval stage of the Indian meal moth, Plodia interpunctella , was examined with and without a host refuge from parasitism. In addition, when given a choice of host stages, with and without a refuge, the influence of parasitoid age on host selection was examined.
3. Wasps were able to parasitize all except the first instar, but second instars experienced significantly reduced parasitism, in both refuge treatments, compared with third to fifth instars. Whilst parasitoid emergence was always significantly less when all host stages had a refuge, the reduction was only marginally significant when second instars were attacked.
4. When given a choice of second- and fifth-instar larvae, wasps consistently parasitized more fifth instars, both with and without a refuge. Moreover, significantly fewer second-instar larvae were parasitized in the presence of fifth instars than when presented alone to the wasps. This pattern of parasitism was unaffected by the increasing age of the parasitoids.
5. Host selection by V. canescens is discussed in terms of host–parasitoid population dynamics and structure.     

Intraspecific competition between healthy and parasitised hosts in a host–parasitoid system: consequences for life-history traits

Abstract  1. In two different treatments, groups of healthy hosts ( Ephestia kuehniella ) or hosts parasitised by Venturia canescens competed for a limited amount of food. The larva to adult survival in each group, as a function of the initial number of hosts and treatment, was fitted to the generalised Beverton and Holt and generalised Ricker survival functions, and a number of life-history traits of the parasitoids was measured.
2. Intraspecific competition was scramble-like, and the parasitised hosts were less susceptible to competition than were their healthy counterparts.
3. For both the healthy and the parasitised hosts, the number of larvae surviving to adulthood gave a good fit to both the generalised Beverton and Holt and generalised Ricker models, but the values of all the parameters differed between the two treatments.
4. Parasitoid size, egg load, and adult survival time decreased significantly with the initial host number.
5. Previous theoretical work suggests that both lower susceptibility to competition by parasitised hosts and scramble competition contribute to the dynamical instability of host–parasitoid systems. Changes registered in life-history traits may also affect host–parasitoid dynamics. These changes have not yet been incorporated into host–parasitoid models.     

A field study of size–fitness relationships in the parasitoid Asobara tabida

1. Many evolutionary models of parasitoid behaviour assume a positive correlation between size and fitness. In this paper we study the size–fitness relationship in the laboratory and in the field using females of the solitary parasitoid Asobara tabida (Hymenoptera: Braconidae).
2. In the laboratory, fecundity, fat reserves and longevity without food were positively correlated with size.
3. Release–recapture experiments in the field showed that dispersal diminishes fat reserves. Dispersal ability is size-dependent: larger females, with larger fat reserves, disperse over larger distances than smaller females.
4. The form of the relationship between size and fitness in the field was estimated in two ways: one based on a comparison of the size distribution of released and recaptured females; the other based on the egg load and fat reserves of wild-caught females. Both showed an accelerating increase of fitness with size.
5. The majority of females appeared to be time-limited. Therefore, the increase in fitness with size is predominantly due to a larger dispersal ability and not to a higher egg load.     

Patch departure mechanisms and optimal host exploitation in an insect parasitoid

1. Patch-leaving decisions are of utmost importance in determining parasitoid foraging success. Parasitoids are known to use both marks left by hosts (chemical or otherwise) and ovipositions to assess host availability and to decide when to leave a host patch.
2. Previous studies have shown that, depending on the species, ovipositions either increase (an incremental mechanism) or decrease (a decremental mechanism) the patch residence times of parasitoids. Reports in the literature conflict on which mechanism is used by Venturia canescens , a parasitoid of pyralid moth larvae.
3. We hypothesize that, as a consequence of saturation in the capacity of the parasitoid to discriminate between host densities at high host numbers, V. canescens uses a decremental mechanism at low host numbers and an incremental one at high host numbers. We call this a 'switching mechanism'.
4. Our experiments show that even if discrimination capacity saturates, V. canescens uses a decremental mechanism over a wide range of host densities.
5. The distribution of hosts in different fruits species under field conditions suggests a switching mechanism would not evolve in natural situations.
6. A model of patch departure in V. canescens is constructed and tested using an independent set of experiments. The model suggests that the patch leaving mechanism in V. canescens is a stochastic decremental one. As might be expected from Weber's Law, the initial leaving tendency is a convex decreasing function of kairomone concentration. The leaving tendency increases exponentially with the time spent in the patch without ovipositing. Ovipositions cause a sudden increase in leaving tendency.
7. Simulations suggest that a decremental mechanism would be out-competed by either one indifferent to ovipositions or an incremental one, only when travel times are much larger than those that are likely to occur in the field.     

Variation in dispersal mortality and dispersal propensity among individuals: the effects of age, sex and resource availability

1.  Dispersal of individuals between habitat patches depends on both the propensity to emigrate from a patch and the ability to survive inter-patch movement. Environmental factors and individual characteristics have been shown to influence dispersal rates but separating the effects of emigration and dispersal mortality on dispersal can often be difficult. In this study, we use a soil mite laboratory system to investigate factors affecting emigration and dispersal mortality.
2.  We tested the movement of different age groups in two-patch systems with different inter-patch distances. Differences in immigration among age groups were primarily driven by differences in emigration but dispersal mortality was greater for some groups. Immigration declined with increasing inter-patch distance, which was due to increasing dispersal mortality and decreasing emigration.
3.  In a second experiment, we compared the dispersal of recently matured males and females and tested the impact of food availability during the developmental period on their dispersal. Dispersal was found to be male biased but there was no significant sex bias in dispersal mortality. There was some evidence that food availability could affect emigration and dispersal mortality.
4.  These results demonstrate that both emigration and dispersal mortality can be affected by factors such as individual age and resource availability. Understanding these effects is likely to be important for predicting the fitness costs and population consequences of dispersal.     

Consequences of large-scale processes for the conservation of bird populations

1.  Detailed studies of population ecology are usually carried out in relatively restricted areas in which emigration and immigration play a role. We used a modelling approach to explore the population consequences of such dispersal and applied ideas from our simulations to the conservation of wild birds.
2.  Our spatial model incorporates empirically derived variation in breeding output between habitats, density dependence and dispersal. The outputs indicate that dispersal can have considerable consequences for population abundance and distribution. The abundance of a species within a patch can be markedly affected by the surrounding habitat matrix.
3.  Dispersal between habitats may result in lower population densities at the edge of good quality habitat blocks and could partially explain why some species are restricted to large habitat fragments.
4.  Habitat deterioration may not only lead to population declines within that habitat but also in adjacent habitats of good quality. This may confound studies attempting to diagnose population declines.
5.  Although mobile species have the advantages of colonizing sites within metapopulations, dispersal into poorer quality territories may markedly reduce total populations.
6.  There are two main approaches to conservation: one is to concentrate on establishing and maintaining protected areas, while the other involves conservation of the wider countryside. If dispersal is an important process then protecting only isolated areas may be insufficient to maintain the populations within them.     

Movement and dispersal patterns in the bush cricket Pholidoptera griseoaptera: the role of developmental stage and sex

Abstract.  1. To persist in a highly fragmented environment, such as the intensively used agricultural landscapes of Europe, species may need to successfully disperse between sub-populations. This paper describes a capture – mark – resight experiment to investigate movement and dispersal patterns of the flightless bush cricket species Pholidoptera griseoaptera .
2. Movement parameters showed little difference between juveniles and imagos as well as between males and females. Both developmental stages and sexes moved considerable distances, were cap\able of crossing habitat boundaries, and showed successful inter-patch dispersal through the agricultural matrix.
3. The results suggest that P. griseoaptera exhibits movement behaviours that compensate for its inability to fly. It was concluded that the good dispersal ability exhibited by animals of both sexes and developmental stage is an important factor allowing P. griseoaptera to persist in fragmented agricultural landscapes.     

Dispersal abilities of adult click beetles in arable land revealed by analysis of carbon stable isotopes

1 The dispersal abilities of agrioted beetles, serious pests on a variety of crops, are poorly known under natural conditions. This hampers their control. We used, for the first time, a stable isotope approach to assess dispersal of adult Agriotes obscurus in arable land.
2 After a diet switch from a C3- to a C4-plant, carbon isotope ratios of A. obscurus larvae significantly changed towards the isotopic signature of the new diet. Moreover, the larval δ¹³C signatures were transferred to the wing covers of the adult beetles with little distortion.
3 To assess the dispersal abilities under natural conditions, pheromone traps, lured for Agriotes sp., were installed at two study sites in Western Austria. Each site comprised a maize field (= C4-plant) and adjacent C3-grasslands with traps established along a transect of increasing distance to the maize.
4 δ¹³C signatures of wing covers revealed that adult male A. obscurus were able to migrate at least 80 m, which was the maximum distance that dispersal could be traced in the present study. The dispersal behaviour might have been influenced by site-specific factors.
5 The results obtained demonstrate a higher potential of adult male Agriotes to disperse than previously assumed. Moreover, the combination of pheromone trapping and stable isotope analysis proved to be an effective approach to study insect movement and dispersal in arable systems harbouring C3- and C4-crops.     

Avoidance of the host immune response by a generalist parasitoid, Compsilura concinnata Meigen

Abstract.  1. Ecological interactions between parasitoids and their hosts are extremely strong as parasitoid offspring rely entirely on an individual host to complete development. The ability of a parasitoid to use a host is influenced directly by the degree to which the parasitoid can overcome host defences and grow within the host.
2. Hymenopteran parasitoids have evolved different host-specific strategies to defeat the host immune system, such as the use of venom, endosymbiont virus, or mimicking the host tissue. Dipteran parasitoids from the Tachinidae family do not use these subterfuges and rely mainly on avoiding the host immune system by hiding in specific tissues.
3. Little is known of the effect of this strategy on the host immune system, the absorption of nutrients by the parasitoid larvae, or the implications for parasitoid host range.
4. In this study, the impact of a polyphagous tachinid parasitoid Compsilura concinnata Meigen on a pest lepidopteran Trichoplusia ni Hübner are assessed. Phenoloxidase levels and haemolymph proteins were measured in parasitised T. ni as a function of host immune response.
5. Haemolymph phenoloxidase in the host did not vary with parasitisation but was triggered when a piece of monofilament was implanted in the haemocoel. Haemolymph proteins were depleted in heavily parasitised T. ni .
6. These results indicate that C. concinnata has a strategy that avoids the host immune system, and accesses the necessary nutrients for larval growth. This strategy could explain the success of this tachinid and its wide host range.     

Effect of biotic and abiotic factors on reproductive parameters of the synovigenic endoparasitoid Venturia canescens

Abstract.  The main effects and interactions of adult age, access to food and host deprivation, on the egg load of Venturia canescens (Gravenhorst) (Hymenoptera: Ichneumonidae), a larval parasitoid of Ephestia kuehniella (Lepidoptera: Pyralidae) and other pyralids, were studied in the laboratory. Intraspecific variation in the number of ovarioles was also studied. There was a positive and significant correlation between wasp size and both egg load and ovariole number, with the reproductive system of large wasps containing significantly more ovarioles and mature eggs than small wasps. Newly emerged adult wasps contained 27 ± 2.4 ovulated (mature) eggs in their lateral oviducts. Access to food and host deprivation were the only conditions under which egg load increased with parasitoid age. Wasps that had access to hosts immediately after emergence showed a significant decline in their egg complement, irrespective of food presence. Under conditions of both host and food deprivation, there was practically no alteration of egg load with parasitoid age. A three-way analysis of variance revealed that egg load varies significantly with food or host access but not with parasitoid age. All interactions among the three factors were significant. It is confirmed that there is no egg resorption in V. canescens and that egg production stops in the absence of food.     

Bottom-up cascading effects in a tritrophic system: interactions between plant quality and host-parasitoid immune responses

Abstract.  1. Little is known about underlying mechanisms by which plants indirectly affect parasitism success in hymenopteran endoparasitoids. The hypothesis that host-plant effects can challenge the innate immune system of an insect host was experimentally tested in this study using a model tritrophic, crucifer – lepidopteran [ Plutella xylostella (L.)] – parasitoid [ Cotesia plutellae (Kurdjumov)], system.
2. The effects of host-plant suitability on herbivore performance and parasitism were examined. The bottom-up effect of plant suitability on host-parasitoid immune responses was then evaluated using measures of cellular and humoral effectors.
3. Host-plant quality showed a significant effect on the encapsulation response of P. xylostella to first instar but not to second instar parasitoid larvae. Encapsulation was never sufficient to prevent parasitoid emergence.
4. Poor host-plant suitability suppressed phenoloxidase activity in the absence of the parasitoid. The suppressive effect of C. plutellae on phenoloxidase activity was much greater and no plant effects were detectable after insects had been parasitized.
5. Despite strong plant effects on parasitism, those on immune effectors of the host were transitory or overwhelmed by the effect of the parasitoid.
6. These results demonstrated that plant-mediated variation in parasitism success by C. plutellae were not as a result of plant nutritional status or other attributes affecting the immune function of P. xylostella , nor to host-plant effects on superparasitism.
7. In these experiments, P. xylostella was a fully permissive host to C. plutellae and host-plant-mediated effects on the innate immune response appeared to play no part in parasitoid survival within hosts.     

Bottom-up and top-down effects in a tritrophic system: the population dynamics of Plutella xylostella (L.)–Cotesia plutellae (Kurdjumov) on different host plants

Abstract.  1. The effects of host-plant resistance on the population dynamics of the Diamondback moth, Plutella xylostella L., and its solitary parasitoid, Cotesia plutellae (Kurdjumov), were studied in replicated time-series experiments.
2. Host-plant resistance did not affect the equilibrial abundance of the Diamondback moth, but it affected the dynamics of Diamondback moth populations.
3. The mean population size of Diamondback moth showed no significant difference between Brassica rapa (a susceptible host plant) and Brassica napus (a partially resistant host plant) either in the presence or absence of the parasitoid.
4. Time-series analysis suggests that the dynamics of Diamondback moth on B. rapa were underpinned by delayed density-dependent processes. In contrast, the dynamics of the moth on B. napus were influenced by a direct density-dependent process.
5. Although measures of short-term parasitism showed a significantly higher rate of parasitism by C. plutellae on Diamondback moth feeding on B. napus compared with B. rapa , this individual performance does not translate into differences in the population dynamics. Analysis shows no significant difference in the persistence time of the population-level interaction between the host and parasitoid on the two different host plants.     

Dispersal of aquatic organisms by waterbirds: a review of past research and priorities for future studies

1. Inland wetlands constitute ecological islands of aquatic habitat often isolated by huge areas of non-suitable terrestrial habitats. Although most aquatic organisms lack the capacity to disperse by themselves to neighbouring catchments, many species present widespread distributions consistent with frequent dispersal by migratory waterbirds.
2. A literature review indicates that bird-mediated passive transport of propagules of aquatic invertebrates and plants is a frequent process in the field, at least at a local scale. Both endozoochory (internal transport) and ectozoochory (external transport) are important processes.
3. The characteristics of the dispersed and the disperser species that facilitate such transport remain largely uninvestigated, but a small propagule size tends to favour dispersal by both internal and external transport.
4. We review the information currently available on the processes of waterbird-mediated dispersal, establishing the limits of current knowledge and highlighting problems with research methods used in previous studies. We also identify studies required in the future to further our understanding of the role of such dispersal in aquatic ecology.     

Ant mutualists alter the composition and attack rate of the parasitoid community for the gall wasp Disholcaspis eldoradensis (Cynipidae)

Abstract.  1. The strength or density dependence of pairwise species interactions can depend on the presence or absence of other species, especially potential mutualists.
2. The gall wasp Disholcaspis eldoradensis induces plant galls that secrete a sweet honeydew from their top surfaces while the wasp larvae are active. These galls are actively tended by Argentine ants, which collect the honeydew and drive off parasitoids attempting to attack the gall wasp.
3. When ants were excluded, the total rate of parasitism by seven species of parasitoids increased by 36%, and the rate of gall-wasp emergence decreased by 54%.
4. The total percentage parasitism was affected by gall density when ants were excluded but not when ants were unmanipulated, suggesting a change in parasitoid functional responses due to ant tending.
5. In addition, excluding ants significantly altered the proportions of different parasitoid species that emerged from galls; one parasitoid species increased from 1% to 34%, and another decreased from 46% to 19%.
6. The invasive Argentine ants studied are capable of maintaining the mutualism with the gall wasps that evolved in the presence of different ant species and also act as a selective filter for the local community of generalist parasitoids trying to attack this gall species.     

Apparent competition between two species of aphid via the fungal pathogen Erynia neoaphidis and its interaction with the aphid parasitoid Aphidius ervi

Abstract 1. Motivated by a community study on aphids and their fungal pathogens, three hypotheses were tested experimentally to investigate the influence of the fungal pathogen, Erynia neoaphidis Remaudière and Hennebert, on aphid population and community ecology.
2. Field experiments were performed in 2 years to test whether two susceptible aphid species on different host plants might interact through the shared fungal pathogen. No strong pathogen-mediated indirect interactions (apparent competition) between populations of pea aphid Acyrthosiphon pisum Harris and nettle aphid Microlophium carnosum Buckton were detected.
3. In the first of the field experiments, pea aphids exposed to the fungus showed a weak tendency to produce more winged dispersal morphs than control populations not exposed to the fungus. In a laboratory test, however, no support was found for the hypothesis that the presence of volatiles from fungus-infected cadavers promotes production of winged offspring.
4. The response of the pea aphid parasitoid Aphidius ervi Halliday to colonies containing hosts infected 1 and 3 days previously was assessed. Wasps initiated fewer attacks on 1-day-old infected colonies than on healthy colonies, with the numbers on 3-day-old fungus-infected colonies intermediate.     

Flight activity of adult stoneflies in relation to weather

Abstract. 1. Dispersal of adult aquatic insects between streams may have important consequences for local and regional population dynamics, but little is known about how dispersal is affected by weather conditions.
2. The influence of meteorological variables on flight activity of adult stoneflies (Plecoptera: Leuctridae, Nemouridae, and Chloroperlidae) was investigated using Malaise traps adjacent to three upland streams in the Plynlimon area of mid Wales, U.K.
3. Numbers of adult stoneflies captured weekly in the traps were related positively to air temperature and related negatively to wind speed. Meteorological conditions during daylight showed stronger relationships with flight activity than did conditions at night.
4. There was inter-site variation in the strength of weather effects on stonefly flight. Wind speed was significant at only one site, which had higher average wind speed than the other sites.
5. Annual variation in weather conditions during adult flight periods may result in varying extent of dispersal between sites, influencing community dynamics over a wide area.     

Spatial dynamics in a host–multiparasitoid community

1. The harlequin bug, a herbivore on bladderpod, is attacked by two specialist egg parasitoids Trissolcus murgantiae and Ooencyrtus johnsonii . Ooencyrtus can out-compete Trissolcus in the laboratory, but coexistence is the norm in field populations. Despite the heavy mortality inflicted by the two parasitoids, the host–parasitoid interaction is persistent in all sites that have been studied in southern California.
2. I manipulated inter-patch distances in a field experiment to determine whether spatial processes drive parasitoid coexistence and/or host–parasitoid dynamics. I first tested the hypothesis that the parasitoids coexist via a dispersal–competition trade-off. Both parasitoid species took significantly longer to colonize isolated patches than well-connected patches, suggesting that they have comparable dispersal abilities. Ooencyrtus did not exclude Trissolcus even when inter-patch distances were reduced to 25–30% of those observed in natural populations. These data suggest that parasitoid coexistence can occur in the absence of a dispersal advantage to the inferior competitor.
3. Since the treatments with isolated vs. well-connected patches did not differ in parasitoid composition, I next asked whether isolation would destabilize, or drive extinct, the host–multiparasitoid interaction. No local extinctions of bugs or parasitoids were observed during the 18-month study period. Bug populations in the isolated patches were no more variable than those in the well-connected patches. In fact, temporal variability in the experimentally isolated patches was comparable to that observed in highly isolated natural populations.
4. These data argue against a strong effect of spatial processes on host–parasitoid dynamics. Local processes may mediate both parasitoid coexistence as well as the host–parasitoid interaction.