The spatial patterns of parasitism of eumenid wasps,Anterhynchium flavomarginatum andOrancistrocerus drewseni by the miltogrammine flyAmobia distorta

1. Spatial patterns of parasitism of eumenid wasps Anterhynchium flavomarginatum and Orancistrocerus drewseni by the miltogrammine fly Amobia distorta were studied in Kyoto, Japan during 1980–1984.
2. In generations of low (<5%) and medium (5–20%) parasitism, percent parasitism per shed (the habitat of the hosts) increased as a function of host density. Conversely, in generations of high (>20%) parasitism, percent parasitism was rather constant over different host densities.
3. The spatial distributions of adult miltogrammine flies among sheds were censused in generations of low and medium parasitism. The frequency of observations of adult miltogrammine flies was higher at sheds of higher host density (aggregative behavioral response), but on the other hand, the adult miltogrammine flies distributed in an underdispersed (or regular) manner in relation to other conspecifics.
4. The spatially density independent relationship between host density and percent parasitism in generations of high parasitism was explained in relation to parasitoid dispersal from patches of high parasitoid density.

     

Heterogeneity and density dependence in a field study of a tephritid-parasitoid interaction

Abstract.

• 1 The spatial distributions of two tephritid flies (Urophora stylata (Fabricius) and Terellia serratulae L.) attacking thistle flower heads and the levels of parasitism from their associated parasitoid guilds were studied over a 7-year period.
• 2 Using these data it is possible to seek both temporal, density dependent relationships between average levels of parasitism and host density per generation, and also any spatial patterns of parasitism contributing to stability that may be operating within the same field system.
• 3 Parasitism by the two most important generalist parasitoids of T.serratulae is a direct function of average T.serratulae density per year. There is little evidence of any stabilizing heterogeneity arising from the spatial distribution of parasitism within generations.
• 4 Temporal density dependence of Urophora stylata cannot be confirmed from the 7 years of study but there is evidence of spatial heterogeneity which may have an important effect on the dynamics of the host population.

     

Spatio‐temporal variation in Helicoverpa egg parasitism by Trichogramma in a tropical Bt‐transgenic cotton landscape

• 1 Understanding the spatio‐temporal dynamics of insects in agroecosystems is crucial when developing effective management strategies that emphasize the biological control of pests.
• 2 Wild populations of Trichogramma Westwood egg parasitoids are utilized for the biological suppression of the potentially resistant pest species Helicoverpa armigera (Hübner) in Bt‐transgenic cotton Gossypium hirsutum L. crops in the Ord River Irrigation Area (ORIA), Western Australia, Australia.
• 3 Extensive, spatially‐stratified sampling during a season of relatively high Trichogramma abundance found that spatial patterns of pest egg parasitism in the ORIA tend toward heterogeneity, and do not necessarily coincide with host spatio‐temporal dynamics. Both patterns of host egg density and mean rates of parasitism are not good indicators of parasitoid spatio‐temporal dynamics in ORIA cotton crops.
• 4 Parasitism rates can be significantly higher within the middle strata of the cotton plant canopy before complete canopy closure, despite a similar number of host eggs being available elsewhere in the plant.
• 5 Spatial variation in egg parasitism by Trichogramma in Bt‐transgenic cotton is evident at the between‐field, within‐field and within‐plant scale, and is not solely driven by host spatial dynamics. These factors should be considered when estimating Trichogramma impact on pest species during biological control and spatio‐temporal studies of host‐parasitoid interactions in general.

     

Do distances among host patches and host density affect the distribution of a specialist parasitoid?

The effect of spatial habitat structure and patchiness may differ among species within a multi-trophic system. Theoretical models predict that species at higher trophic levels are more negatively affected by fragmentation than are their hosts or preys. The absence or presence of the higher trophic level, in turn, can affect the population dynamics of lower levels and even the stability of the trophic system as a whole. The present study examines different effects of spatial habitat structure with two field experiments, using as model system the parasitoid Cotesia popularis which is a specialist larval parasitoid of the herbivore Tyria jacobaeae. One experiment examines the colonisation rate of the parasitoid and the percentage parasitism at distances occurring on a natural scale; the other experiment examines the dispersal rate and the percentage parasitism in relation to the density of the herbivore and its host plant. C. popularis was able to reach artificial host populations at distances up to the largest distance created (at least 80 m from the nearest source population). Also, the percentage parasitism did not differ among the distances. The density experiment showed that the total number of herbivores parasitised was higher in patches with a high density of hosts, regardless of the density of the host plant. The percentage parasitism, however, was not related to the density of the host. The density of the host plant did have a (marginally) significant effect on the percentage parasitism, probably indicating that the parasitoid uses the host plant of the herbivore as a cue to find the herbivore itself. In conclusion, the parasitoid was not affected by the spatial habitat structure on spatial scales that are typical of local patches.     

The effect of multi‐species host density on superparasitism and sex ratio in a gregarious parasitoid

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Response of the egg parasitoids of the pine processionary moth to host density and forest cover at the southern edge of the range

1. Thaumetopoea pityocampa is the most important pine defoliator in the Mediterranean basin. Despite being attacked by a number of natural enemies, populations occur frequently at high density in several areas.
2. Egg parasitism was studied in 27 pine and cedar forests in Algeria, in relation to the host density (tents per tree) and the proportion of forest cover in the landscape.
3. Egg parasitism varied from 2% to 25%, accounted by two parasitoid species, the specialist Baryscapus servadeii and the generalist Ooencyrtus pityocampae.
4. Tent density was negatively correlated with parasitism by B. servadeii but not with that of O. pityocampae. Conversely, parasitism by O. pityocampae increased with the proportion of forest and agricultural cover, but not in the case of B. servadeii.
5. Maximum summer temperature showed no correlation with parasitism rates. Still, temperature frequently exceeded 40 °C during the period of adult parasitoid activity.
6. The low performance of the egg parasitoids at the southern edge of the host range could be explained by the reduced fecundity of the host, climate effects, and phenological mismatching between the parasitoids and the egg development. These and other factors potentially involved the need to be further explored with a long-term study of population dynamics.

     

Aggregation in field parasitoid populations: foraging time allocation by a population of Diadegma (Hymenoptera, Ichneumonidae)

ABSTRACT.

• 1 A field study was made of foraging time allocation by a population of parasitic wasps, Diadegma spp. (Ichneumonidae), to plants containing different densities of their hosts, the caterpillars of Plutella xylostella (L.).
• 2 The parasitoid population exhibited a clear aggregative response, spending more total time on higher density patches, which probably resulted from wasps making more and longer visits to these densities.
• 3 Despite this aggregation, positive density dependent parasitism was not found. The functional response of the Diadegma population exhibited an upper asymptote at high host densities, probably due to an increase in the proportion of time spent handling hosts, which countered the effect of aggregation.
• 4 While Diadegma may select and forage preferentially on plants with higher host density, they do not exhibit the tendency, predicted by some optional foraging models, to exploit progressively less profitable plants during a foraging bout. Some factors affecting patterns of parasitoid foraging in the field are discussed.

     

European field collections and Canadian releases ofCeranthia samarensis (Dipt.: Tachinidae), a parasitoid of the gypsy moth

A programme to collect, import and release into Canada the gypsy moth parasitoid,Ceranthia samarensis (Diptera: Tachinidae) is described. The parasitoid's potential for biological control in Canada is also discussed. The parasitoid was collected in Europe by exposing experimental gypsy moth larvae in areas where local gypsy moth populations were at low densities. Following field exposure, the host larvae were returned to the laboratory and parasitoids reared from them. This technique has shown thatC. samarensis is the suffers 7–16% hyperparasitism. From 83–90% of theC. samarensis typically enter diapause as pharate adults within the puparia. Laboratory tests of post-exposure host rearing conditions indicate that constant temperatures disrupt the normal parasitoid diapause and that this effect can not be offset by use of either static long or short photoperiods or natural daylengths. Shipping and cold-storage procedures for puparia are described. Post-storage time to emergence of adultC. samarensis decreased with longer cold storage periods and with higher post-storage incubation temperatures. Emergence requires 112 degree-days above a threshold of 8°C after a period of at least 8 months cold storage. Releases of adultC. samarensis into field cages at four locations in southern Ontario are documented. While dissection of host larvae from the field cages has failed so far to demonstrate evidence of parasitism, we remain hopeful that some establishment of the parasitoid has occurred.      

Parasitization by Edovum puttleri (Hymenoptera: Eulophidae) in relation to host density in the field

Abstract.

• 1 The relationship between parasitization by Edovum puttleri Grissell and density of eggs of the Colorado potato beetle, Leptinotarsa decemlineata Say (Coleoptera: Chrysomelidae), was studied on two spatial scales (eggs mass and 6 m² cage).
• 2 For both scales, rates of parasitism were generally inversely related to host density for periods ranging from 2 to 8 days after parasitoid release. Thereafter, parasitism became independent of host density.
• 3 The initial inverse-density relationship and subsequent shift to density independence may result from several factors: (1) ambient temperatures, (2) the parasitoid's limited egg production, (3) differential times of exposure of egg masses to parasitoids, and/or (4) the parasitoid's patterns of host feeding and oviposition.
• 4 Although overall levels of parasitism were relatively low, total mortality of L.decemlineata eggs (including nonviable and cannibalized eggs, and those killed by parasitoid feeding) in parasitized egg masses was consistently high (?70–90%).

     

Role of a parasitoid with a low fecundity,Praestochrysis shanghaiensis (Hymenoptera: Chrysididae), in the population dynamics of its host

1. Analysis of life tables of the oriental moth, Monema flavescens, obtained for 8 generations over 4 years, disclosed that the cocoon parasitoid, Praestochrysis shanghaiensis, acted as a density-disruptive factor.
2. The density of the host cocoon remained stable (max./min.=3.2), whereas that of the host adult varied (max./min.=14.3) although both showed similar fluctation patterns.
3. Stability of the host population was associated with the density-dependence in the ratio of first generation cocoons to overwintered generation moths, which was the key factor for the rate of change throughout the year. Chrysidid parasitism among the first generation cocoons ranged from 37.7 to 70.1%, and that among the second generation cocoons from 16.7 to 63.2%, each showing an inverse density-dependence and acting as the main determinant (key-factor) of the between-year variation in the density of the adult moths.
4. The density-dependence of the rate of change from overwintered generation adults to first generation cocoons was so strong that the parasitism on the second generation hosts had not effect on the cocoon density of the first generation. On the other hand, the density-dependence of the rate of change from first generation adults to second generation cocoons was weak, and the parasitism on the first generation hosts became the key factor for the between-year variation of the second generation cocoons.
5. It is suggested that the stability of the parasitoid-host system will be disrupted without three parasitism-restricting factors: asynchrony in the parasitoid attack on the second generation hosts, high mortality among parasitoid larvae of the second generation, and the high proportion of those first generation parasitoids that enter diapause. These factors are considered to be effective only in cooler parts of the distribution of the parasitoid.

     

The relationship between variable host grouping and functional responses among parasitoids of Antispila nysaefoliella (Lepidoptera: Heliozelidae)

Our study investigated the importance of variability in the parasitoid community as a source of selection on host group size using a field population of the tupelo leafminer, Antispila nysaefoliella Clemens, which specializes on tupelo, Nyssa sylvatica Marsh. Larvae were collected from leaves with variable numbers of larvae and screened for parasitism using polymerase chain reaction of mitochondrial cytochrome oxidase I using markers designed specifically for amplifying parasitoid DNA while excluding host DNA. This method of selective PCR was effective for detecting the presence and identifying species of immature stages of three hymenopteran superfamilies: Chalcidoidea, Ichneumonoidea and Platygastroidea, which represented 83.4%, 16.0% and 0.6% of the total detectable parasitism, respectively. Our resulting sequences were then calibrated with sequences from identified adult parasitoids that had been either reared or field‐captured. A cluster analysis revealed 10 distinct clades that showed differences in attack patterns with respect to host traits and season. Total parasitism followed an inverse density‐dependent or density‐independent pattern with respect to host density (number per leaf). However, when parasitoid taxa were considered separately, one clade, which could be a cryptic species of Pnigalio maculipes Crawford (Chalcidoidea: Eulophidae), was found to increase its per leaf attack rate with host density. Our results suggest that parasitoid community composition and differences among species in their attack strategies can play a large role in determining the adaptive advantage of host grouping.     

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.     

Spatial patterns of Trybliographa rapae parasitism of Delia radicum larvae in oilseed rape and cauliflower

Trybliographa rapae (Westwood) is an important parasitoid of Delia radicum (L.). Parasitism of D. radicum larvae by T. rapae in relation to host density on canola (oilseed rape) and cauliflower roots was examined at 10 field sites in Germany and Switzerland. For roots with host larvae, the proportion of roots with one or more parasitized hosts increased with increasing host density. However, for these infested roots, the parasitism of individual larvae was not consistently related to host density. When considering only roots on which there were parasitized larvae and the opportunity for multiple attacks, the proportion of larvae that were parasitized decreased with increasing host density in the field locations, and in a cage study under controlled conditions. A model of patch‐finding and number of attacks by female parasitoids suggests that patch‐finding is density‐dependent, but that low attack rate and interference effects limit numbers of attacks to three or less per visit to a host patch; the reduced number of attacks per visit leads to the inverse relationship of larval parasitism with host density in the host patches visited. The interplay of the density‐dependent and inversely density‐dependent processes appears to be responsible for the inconsistency of density dependence of overall larval parasitism in this and previous studies. In the laboratory, adult female T. rapae parasitized hosts at ≤4 cm deep in soil, but not at 6 cm deep. From the depth distribution of larval feeding sites in the field, we infer that between 4% and 20% of Delia larvae may be in a physical refuge from T. rapae parasitism, which may have a stabilizing influence on the host–parasitoid interaction.     

Optimal use of patches by parasitoids with a limited fecundity

1. a mathematical model is presented which predicts the expected optimal-patch-use strategy for solitary parasitoids with a limited fecundity.
2. The model predicts that the quality of the patches is determined by the proportion of unparasitized hosts and not by the density of those hosts, and that throughout the searching period the parasitoids should maintain the level of parasitism equal in all the patches irrespective of the host density per patch.
3. The spatial pattern of parasitism among field patches by a parasitoid with a low fecundity, Praestochrysis shanghaiensis, was in agreement with the prediction of the model, i.e., a similar level of parasitism in different patches was observed when the ratio of female parasitoids to hosts in the whole study area exceeded 0.07. When the ratio was less than 0.05, however, the level of parasitism per patch showed an inverse relation to the host density, and was positively correlated with the female parasitoid-host ratio.
4. The model assumes that the parasitoids move between patches without cost and have perfect information about patch quality. Consideration of the cost of moving and sampling bridges the gap between the observed and predicted rates of parasitism found when the female parasitoid-host ratio in the whole study area was low

     

Are galling insects better protected against parasitoids than exposed feeders?: a test using tenthredinid sawflies

ABSTRACT.

• 1 Data mostly from the published literature were used to assess the effect of galling on the number of parasitoid species per host species in the phylogeny of nematine sawflies from free external feeders (colonial and solitary) to leaf gallers and shoot gallers.
• 2 The strongest effects of galling were the total elimination of the species-rich cocoon-attacking guild of parasitoids, and eonymphal parasitoids, from the parasitoid community on shoot gallers, all of which are in the genus Euura.
• 3 All tachinid larval parasitoids were also eliminated by the galling habit.
• 4 The cumulative effects of these exclusions resulted in a decline in mean number of parasitoid species per host species from almost sixteen species on external colonial feeders to 4.0 species on shoot gallers.
• 5 General patterns in per cent parasitism by non-tachinid and tachinid larval parasitoids, eonymphal and cocoon parasitoids, on exposed feeders to shoot gallers, showed declines in non-tachinid attack and elimination of tachinid, eonymphal and cocoon parasitoids. But leaf gallers tended to be attacked more than exposed feeders by non-tachinid larval parasitoids.
• 6 The galling habit had a long-term impact by reducing the number of parasitoid species attacking nematine sawfly gallers and per cent mortality inflicted, so that natural enemies may have been important as a selective factor in the evolution of galling nematine sawflies.

     

Invertebrate predation of 15N-marked prey in semi-field wheat enclosures

One of the most famous examples of successful, classical biological control in Japan is the introduction of the parasitoids Coccobius fulvus and Aphytis yanonensis against the citrus pest arrowhead scale Unaspis yanonensis. Together, they comprise a host‐parasitoid system that has been demonstrated to be stable. To test the conventional theory that successful biological control of pests occurs through the establishment of a low stable equilibrium, brought about by the density‐dependent responses of natural enemies to the pest species, sampling was carried out at five sites in the field during 2000 and 2001 to examine the relationship between the rate of parasitism by C. fulvus and the density of its host. The data were analysed using three statistical techniques at nine spatial scales. Contrary to conventional theoretical predictions, each method of analysis detected very little density‐dependence at any spatial level in this study. Parasitoid aggregations independent of host density were not sufficient to stabilise host–parasitoid interactions. Our results suggest that neither spatial density‐dependent nor density‐independent parasitism is necessary for successful biological control, or for the stability of the host–parasitoid system. We propose an alternative mechanism: a spatial refuge induced by parasitoid introduction may stabilise a system.     

Dynamics of parasitism in the organ-pipe wasp, Trypoxylon politurn: effects of spatial scale on parasitoid functional response

Abstract.

• 1 Life tables and rates of parasitism were tabulated from mud nests built by Trypoxylon politum (Hymenoptera: Sphecidae) at nine different nesting sites from Missouri and Mississippi.
• 2 Most developmental mortality occurred either during the first two instars of development, or during the inactive prepupal phase. The majority (76%) of deaths were caused by insect parasitoids and cleptoparasites. Levels of parasitism and survivorship varied among nesting sites, and among locations within the two sites surveyed at a fine spatial scale.
• 3 Total developmental mortality, K, was positively associated with the number of hosts (immature T.politum) per site. Within one of two sites sampled at a fine spatial scale, K was negatively associated with the local density of hosts. Levels of total parasitism were positively associated with host population size, and negatively associated with local host density within one of the two sites sampled at a fine spatial scale.
• 4 Levels of parasitism by Melittobia (Hymenoptera: Eulophidae) were positively associated with the number of hosts per site, but negatively associated with the local density of hosts within sites. Melittobia parasitism was also negatively associated with the local density of old nesting material within sites.
• 5 Parasitism by Melittobia was a function of both the numbers of nests per quadrat and the mean nest size per quadrat at one of the two sites surveyed at a fine scale. At the other site, parasitism by Melittobia was a function of mean nest size per quadrat.
• 6 The life cycle and nesting behaviour of T.politum, in relation to the regulation of its numbers, is discussed.

     

The reproductive strategy of the gregarious parasitoid,Pteromalus puparum (Hymenoptera: Pteromalidae)

Summary Pteromalus puparum is a gregarious parasitoid of lepidopterous pupae. To determine in which phase of a host outbreak superparasitism occurs, field investigations were carried out on seasonal prevalence of the host, Papilio xuhtus, and parasitism by P. puparum in a citrus grove in Fukuoka, Japan in 1974. Host pupation occurred from May to November and the parasitoid attacked the host throughout this period. Pupal density increased rapidly after mid-August and the percentage parasitism decreased during this period. A high level of parasitism was attained after one or more parasitoid generations in mid-September. The superparasitism was observed after mid-September when the parasitoid attained extremely high density, and resulted in an increase in the proportion of males, high mortality, and a decrease in the size of the progeny.     

Aggregation of parasitoids and density-independence of parasitism in field populations of the wasp Aphytis melinus and its host, the red scale Aonidiella aurantii

ABSTRACT.

• 1 Repeated counts were made of the number of adult Aphytis melinus (DeBach) wasps per fruit on Valencia oranges in an orchard over two successive periods. Resulting rates of parasitism per fruit were measured at the end of each period.
• 2 For both periods, corresponding to high and low mean numbers of adult parasites, there was a significant positive regresssion of adult wasps per fruit on the number of available hosts per fruit. However, there was a high level of variability about the regression, and the overall aggregative response appears to be weak.
• 3 For both periods, rates of parasitism per fruit were independent of host density per fruit and they showed a high level of variability at all densities. Similar patterns were found in another, commercial, orchard over a wide range of mean host densities.
• 4 There was no evidence for aggregation of parasites or density dependence of parasitism at a patch size corresponding to the whole tree.
• 5 Suggestions based on some host-parasitoid models, that aggregations of parasite attack in areas of high host density are necessary for effective biological control, are rejected. However, the model of Hassell (1982), incorporating aggregation of parasites and limitations on the effectiveness of the parasite, seems to fit the data quite well.