Parasitoid Assemblages Research Articles

Community structure in ichneumonid parasitoids at different spatial scales

The processes underlying parasitoid community structure are little known. Stochastic niche-apportionment models provide one route to underlying assemblage rules in this and other groups. Previous work has applied this approach to parasitoids found on single host species in single populations. However, parasitoid communities are known to extend across multiple hosts and scales. The patterns of relative abundances generated by five niche-apportionment models were compared to those observed in assemblages of two sub-families of the Ichneumonidae, the Diplazontinae and Pimplinae, at landscape and patch scales, Yorkshire, UK. Three of the five models produced patterns that were significantly different to the observed pattern for all taxonomic levels at both spatial scales. The Diplazontinae fit the random fraction (RF) model at the landscape scale in broadleaved woods. This suggests that hierarchical structuring and biotic interactions may play a role in the structuring of Diplazontinae assemblages at this scale. In contrast the Pimplinae fit the RF model only at the patch scale and only at one site. However, the Pimplini tribe (all chiefly parasitoids of Lepidoptera) fit the random assortment (RA) model at both the landscape and the patch scales, whilst the Ephialtini tribe (wide range of hosts) fit no model at either scale. The ecological interpretation of the RA model suggests that the Pimplini tribe is an unsaturated assemblage, where some of the total available resources are unused. Our results show, through the fit of mechanistic niche-apportionment models, that the processes that may structure ichneumonid parasitoid assemblages are not consistent across taxa and spatial scales.

Response of native parasitoids to a range‐expanding host

Abstract 1. As species shift their geographic distributions, new feeding interactions with natural enemies such as parasitoids, and resources such as host plants, may be established, and existing interactions may be severed.2. The leaf mining moth Phyllonorycter leucographella (Zeller, 1850) (Lep.: Gracillariidae) first colonised the southern United Kingdom in the mid 1980s associated with its ancestral host plant Pyracantha coccinea M. Roem. (Rosaceae), which is widely cultivated in the U.K. The moth has since spread northwards to central Scotland and has been recorded feeding on a novel host plant, Crataegus monogyna L.3. The combined effects of latitude and time since colonisation on parasitoid community responses to the arrival of this novel host were investigated across its U.K. range. The response of parasitoids to colonisation of C. monogyna was also investigated.4. Both the observed richness of parasitoid species associated with P. leucographella, and the proportion of P. leucographella parasitised declined with latitude and towards the current range margin. A combination of a latitudinal gradient in parasitoid and alternative host species richness is likely to lead to the trends in species richness and parasitism observed.5. Experimental host patches exposed to parasitism beyond the current range margin of P. leucographella experienced low levels of parasitism consistent with range‐margin populations, indicating an instantaneous response by native parasitoids to availability of the novel host. Parasitism levels and numbers of associated species in the U.K. were similar to those observed in the species’ native range in Turkey.6. The host plant switch to C. monogyna was not associated with an altered parasitoid assemblage, but rates of parasitism were significantly higher on the novel host plant.7. Alterations in the incidence and frequency of victim‐enemy interactions as species shift their geographic ranges may be key in determining rates of range expansion and the impact invading species have on ecological communities.

The effectiveness and optimal use of Malaise traps for monitoring parasitoid wasps

Abstract. Parasitoid wasps are species‐rich and likely to be sensitive indicators of environmental change. Malaise traps are widely used for sampling certain taxa of parasitic Hymenoptera, but little is known about how they should best be used to monitor the community at an individual site. To investigate the effects of sample duration, trap location and replication on the parasitoid assemblage, we sampled four ichneumonid subfamilies (Hymenoptera: Ichneumonidae) intensively using Malaise traps in two farm woods in the Vale of York, UK. Species accumulation curves showed that even with 16 Malaise traps per wood, the community is incompletely sampled. Despite this, we caught up to 28% of all UK species in a single wood, implying that local parasitoid diversity may be very high. Abundance and species richness of parasitoids differed significantly between sample periods (fortnights) and between traps, but did not differ overall between core and edge locations. Parasitoid community composition differed between core and edge traps, but differences were much stronger in one wood than the other. One subfamily, the Poemeniinae, was found predominantly in edge locations. Catch differences became greater with increasing distance between traps. The previous year, two traps in each of the same woods caught only half as many species, but species abundance was positively correlated between years. Our results suggest that a small number of traps can contain useful information about the parasitoid community but is likely to seriously underestimate total species richness. To achieve extensive species coverage, sampling should continue over several weeks, with widely separated traps sampling both core and edge locations. Our focal taxa should prove excellent for monitoring the effects of environmental change on biodiversity.

Structure and composition of the assemblage of parasitoids associated to Phyllocnistis citrella Pupae Stainton (Lepidoptera: Gracillariidae) in citrus orchards in Southern Brazil

The structure and composition of the assemblage of pupal parasitoids of Phyllocnistis citrella Stainton, the citrus leafminer, were studied in two citrus orchards (Citrus deliciosa Tenore cv. Montenegrina and Citrus sinensis (L.) Osbeck x Citrus reticulata Blanco hybrid Murcott), in Montenegro County (29 degrees 68'S and 51 degrees 46'W), southern Brazil. At fortnightly samplings, from July 2001 to June 2003, all the new shoots from 24 randomly selected trees were inspected. The species richness reached five native species in the Murcott orchard, and six in Montenegrina. In Murcott, the presence of Ageniaspis citricola (Hymenoptera: Encyrtidae), an exotic species, was detected in the first year of sampling, probably migrating from the nearby areas where it had been released for the miner control. In Montenegrina, its presence was only registered in the second year. A. citricola in both areas was dominant and changed the community structure of parasitoid complex of P. citrella in both orchards.

Seasonal phenology and stage‐specific parasitism of the apple ermine moth, Yponomeuta malinellus Zeller, in Korea

AbstractThe apple ermine moth, Yponomeuta malinellus Zeller (Lepidoptera: Yponomeutidae), is a tent caterpillar that feeds on Malus spp. in Korea. Populations of the moth in native areas appeared to be regulated by the assemblage of parasitoids. Phenological associations between host stages and parasitoids, susceptible stage(s) of the host for each parasitoid, and stage‐specific parasitism were studied. The egg larval parasitoid Ageniaspis fuscicollis (Dalman) had highest parasitism of first instar larvae (24%), with 14% parasitism of other larval stages. Dolichogenidea delecta (Haliday) was recovered from all larval instars with the highest parasitism rate of second instar larvae (20.1%), followed by 19.9% parasitism of mid‐larval hosts. Herpestomus brunicornis Gravenhorst was reared from second instar larvae through to pupal collection, and had the highest parasitism rate (29.9%) at the pupal stage. The larval pupal parasitoid Zenillia dolosa (Meigen) was recovered from mid‐larval to pupal stages with the highest parasitism rate (5.5%) occurring in third to fourth instar larvae. The host stages for developing A. fuscicollis completely overlap with those of D. delecta, and with those of H. brunicornis to some degree. A statistically significant negative correlation exists between A. fuscicollis and these dominant parasitoids, indicating competitive interaction within the host.

HYMENOPTERAN PARASITOIDS OF ANASTREPHA FRUIT FLIES (DIPTERA: TEPHRITIDAE) REARED FROM DIFFERENT HOSTS IN YUCATAN, MEXICO

In order to carry on the detection and species inventory of hymenopteran parasitoids associated with fruit flies, we examined various tropical fruits growing at the Southern region of Yucatan. During a yearly cycle (Jun 2000 to Jun 2001), 9 host fruit species (including some varieties) were collected by 2 different methods. The first method involved weekly collection of ripened fruits that were transported to the laboratory (“Fruit-Lab”); and the second method was collection of fruits placed on the ground below the tree canopy (“Fruit-Beds”), and which remained in the field for two weeks, after which they were transported to the laboratory. Fruits obtained were counted and weighed, and the recovered pupae were quantified for each sample. As a whole, we sampled 4,470 fruits (850.8 Kg) from the 9 host plant species and varieties, which were infested by 5 fruit fly species: Anastrepha ludens (Loew), A. obliqua (Macquart), A. serpentina (Wiedemann), A. striata Schiner, and A. fraterculus (Wiedemann). The average parasitism in all samples was 3.69% represented by 11 hymenopteran species as follows: Braconidae, Doryctobracon areolatus (Szépligeti), and Opius bellus (Gahan); Figitidae, Aganaspis pelleranoi (Brethes), Aganaspis sp., Odontosema anastrephae Borgmeier and Odontosema sp.; Diapriidae, Coptera haywardi (Oglobin); Chalcididae, Dirhinus sp.; Pteromalidae, Spalangia endius Walker; Eurytomidae, Sycophila sp.; and Perilampidae, Euperilampus sp. On the basis of results in differences among samples for parasitism rates, fruit fly parasitoid, and fruit fly host plant, parasitoid assemblages are analyzed and discussed.

Teratocytes growth pattern reflects host suitability in a host–parasitoid assemblage

Abstract In some parasitoid species, the serosa membrane breaks apart at hatching and produces teratocyte cells that assume various functions (immunossupression, secretion and nutrition) mediating host–parasitoid relationships. Teratocyte growth pattern may thus reflect the host suitability for a parasitoid. The teratocyte growth pattern (increase in size and number of teratocytes as a function of time) is studied and used as an indirect measure of fitness to compare the development of the endoparasitoid Dinocampus coccinellae in a marginal host, the coccinellid Harmonia axyridis, and in a suitable host, Coleomegilla maculata. Indirect measures of fitness recorded in both host species confirm that C. maculata is a suitable host for D. coccinellae contrary to the marginal host H. axyridis. According to regression analysis, teratocyte numbers decrease linearly whereas teratocyte size increases linearly with time in the suitable host C. maculata (larvae or adults). In the marginal host, parasitism occurs only in the larval stage where a delay in the parasitoid larval development is observed. Increase in teratocyte size is also highly variable. The teratocyte growth pattern of the parasitoid in the marginal host does not follow the linear model found in the suitable host. Teratocyte growth pattern may be a useful criterion to evaluate host‐suitability and host range of parasitoids.

Metapopulation extinction risk is increased by environmental stochasticity and assemblage complexity

Extinction risk is a key area of investigation for contemporary ecologists and conservation biologists. Practical conservation efforts for vulnerable species can be considerably enhanced by thoroughly understanding the ecological processes that interact to determine species persistence or extinction. Theory has highlighted the importance of both extrinsic environmental factors and intrinsic demographic processes. In laboratory microcosms, single-species single-habitat patch experimental designs have been widely used to validate the theoretical prediction that environmental heterogeneity can increase extinction risk. Here, we develop on this theme by testing the effects of fluctuating resource levels in experimental multispecies metapopulations. We compare a three-species host-parasitoid assemblage that exhibits apparent competition to the individual pairwise, host-parasitoid interactions. Existing theory is broadly supported for two-species assemblages: environmental stochasticity reduces trophic interaction persistence time, while metapopulation structure increases persistence time. However, with increasing assemblage complexity, the effects of trophic interactions mask environmental impacts and persistence time is further reduced, regardless of resource renewal regime. We relate our findings to recent theory, highlighting the importance of taking into account both intrinsic and extrinsic factors, over a range of spatial scales, in order to understand resource-consumer dynamics.

Conserving parasitoid assemblages of North American pest Lepidoptera: Does biological control by native parasitoids depend on landscape complexity?

The alternate host food–plant relationships of North American pest Lepidoptera were examined to better understand how the vegetational structure of agricultural landscapes may influence parasitoid populations and diversity and thus their potential in conservation biological control. Examined were whether the common pest Lepidoptera of the USA and Canada are generalists or specialists, the successional stage occupied by plants upon which alternative hosts of their hymenopteran parasitoids are found (i.e., what extra-field habitats might support a greater number and diversity of alternate hosts). Forty-two species of native North American pest Lepidoptera in eight families were examined. Lepidoptera were mainly polyphagous feeders. Most of the hymenopteran (53%) and tachinid (72%) parasitoids associated with the Lepidoptera were generalists. Food plant range was determined for 90% of the 649 alternate hosts identified. A majority fed either exclusively on trees (33.3%) or exclusively on ruderal species (29.9%). Half the alternate hosts (50.7%) feed at least sometimes on late successional species. Hymenopteran generalist parasitoids were associated strongly with lepidopteran alternate hosts (47%) that feed on trees and shrubs, whereas hymenopteran oligophagous and specialist parasitoids were associated strongly with lepidopteran alternate hosts (68 and 100% respectively) that feed on ruderals and shrubs. Tachinid generalist and oligophagous parasitoids were associated strongly with lepidopteran alternate hosts (48 and 71%, respectively) that feed exclusively on trees and shrubs. Effective conservation of native parasitoids will probably require the preservation and/or restoration of late successional habitats within the agricultural landscape.

Recruitment of Native Parasitoid Species by the Invading LeafminerPhyllocnistis citrella(Lepidoptera: Gracillariidae) on Citrus in Spain

The parasitoid assemblage associated with the citrus leafminer Phyllocnistis citrella Stainton (Lepidoptera: Gracillariidae) was studied in citrus orchards in eastern Spain over a 7-yr period (1995–2001) after the leafminer’s introduction in 1994. In total, 11,587 adult native parasitoids were collected. To evaluate parasitism, 93,846 live immature stages of the citrus leafminer were observed, of which 21,460 (22.9%) were found to be parasitized. The parasitoid complex recruited around P. citrella was typical for invader hosts: lower species richness, generalized habits, idiobiont strategy, and low to moderate rates of parasitism. Two of the 10 species reared from the citrus leafminer, Pnigalio sp. and Cirrospilus brevis Zhu, LaSalle and Huang, accounted for >90% of the parasitoids. Native parasitoids moved onto the invading host rapidly, except for C. brevis, which required 3 yr to become common and widespread. In other leafminer species from plants in the vicinity of citrus orchards, the proportion of P. citrella parasitoids was higher in woody (69.7%) than in herbaceous plants (22.2%). The high population levels reached by the new pest, associated with a negative density dependence response of the parasitoids at these high population levels, suggest that the native parasitoid assemblage exerted only a limited role in regulating the population of the new host.

Recruitment of Native Parasitoid Species by the Invading Leafminer <I>Phyllocnistis citrella</I> (Lepidoptera: Gracillariidae) on Citrus in Spain

Abstract The parasitoid assemblage associated with the citrus leafminer Phyllocnistis citrella Stainton (Lepidoptera: Gracillariidae) was studied in citrus orchards in eastern Spain over a 7-yr period (1995–2001) after the leafminer’s introduction in 1994. In total, 11,587 adult native parasitoids were collected. To evaluate parasitism, 93,846 live immature stages of the citrus leafminer were observed, of which 21,460 (22.9%) were found to be parasitized. The parasitoid complex recruited around P. citrella was typical for invader hosts: lower species richness, generalized habits, idiobiont strategy, and low to moderate rates of parasitism. Two of the 10 species reared from the citrus leafminer, Pnigalio sp. and Cirrospilus brevis Zhu, LaSalle and Huang, accounted for >90% of the parasitoids. Native parasitoids moved onto the invading host rapidly, except for C. brevis, which required 3 yr to become common and widespread. In other leafminer species from plants in the vicinity of citrus orchards, the propor...

Parasitoids (Hymenoptera, Ichneumonoidea) of Saproxylic beetles are affected by forest successional stage and dead wood characteristics in boreal spruce forest

The habitat requirements and effects of forest management on insects belonging to higher trophic levels are relatively unknown in forest ecosystems. We tested the effect of forest successional stage and dead wood characteristics on the saproxylic parasitoid ( Hymenoptera, Ichneumonoidea) assemblage in boreal spruce-dominated forests in northern Sweden. Within each of nine areas, we selected three sites with different management histories: (1) a clear-cut (2) a mature managed forest and (3) an old-growth forest. Parasitoids were collected in 2003 using eclector traps mounted on fresh logs, which were either untreated (control), burned, inoculated with fungi, or naturally shaded, and on artificially-created snags. Both forest type and dead wood characteristics had a significant effect on parasitoid assemblages. Grouped idiobionts and some species, such as Bracon obscurator and Ontsira antica, preferred clear-cuts, while others, such as Cosmophorus regius (Hym., Braconidae) and other koinobionts, were associated with older successional forest stages. No single dead wood substrate was sufficient to support the entire community of parasitoids in any forest type, even when the regular host was present. In particular, snags hosted a different assemblage of species from other types of dead wood, with parasitoids of Tetropium spp. such as Rhimphoctona spp. (Hym., Ichneumonidae) and Helconidea dentator (Hym., Braconidae) being abundant. These results indicate that a diversity of dead wood habitats is necessary to support complete assemblages of beetle-associated parasitoids from early successional stages of dead wood and that parasitoids may be more sensitive to habitat change than their hosts.

Coexistence of natural enemies in a multitrophic host–parasitoid system

Abstract. 1. This study explored the temporal and spatial aspects of coexistence over many generations in a multispecies host–parasitoid assemblage.2. The long‐term interaction between the cabbage root fly, Delia radicum (Diptera: Anthomyiidae), and two of its natural enemies, Trybliographa rapae (Hymenoptera: Fitigidae) and Aleochara bilineata (Coleoptera: Staphylinidae), in a cultivated field at Silwood Park over 19 years was explored.3. Although time series showed that the populations were regulated, the impact of the natural enemies was highly variable. Within‐year determinants showed that the spatial response of the specialist parasitoid, T. rapae, was predominantly independent of host density while A. bilineata acted simply as a randomly foraging generalist parasitoid.4. These findings are compared and contrasted with an earlier investigation of the same system when only the first 9 years of the time series were available. This study demonstrated the potential of long‐term field studies for exploring hypotheses on population regulation, persistence, and coexistence.

Relationships among abundance of galls, survivorship, and mortality factors in a cynipid wasp, Andricus moriokae (Hymenoptera: Cynipidae)

We describe the relationships among patterns of abundance of galls, survivorship, and mortality factors in the sexual generation of a cynipid wasp, Andricus moriokae, and detail the impacts of abundant parasitoid species on the survivorship of the cynipid in relation to the abundance patterns of galls. The mean number of galls per shoot, the frequency distribution of number of galls on a single shoot, and the mean number of galls per shoot in respective shoot size classes (the number of leaves per shoot) did not differ significantly between two sample trees. The relative impacts of the three mortality factors differed between trees, having different consequences for the survivorship of the cynipid. The rate of parasitoid attack, the main cause of mortality, was higher in the tree on which the cynipids survived better. On the other hand, the mortalities caused by gall predation and gall abortion were higher in the tree with the lower survival rate of the cynipid. Galls appeared more frequently on larger shoots on both trees. However, the survival rate and the mortality rates caused by three mortality factors did not differ among shoot size classes within each tree. The species composition of the parasitoid assemblage changed with shoot size, but the patterns of change differed between trees with similar abundance patterns of galls. Thus, the survivorship and mortality factors of the cynipid were probably not related to the abundance patterns of galls.

Host density influences parasitism of the armyworm Pseudaletia unipuncta in agricultural landscapes

Summary Several studies have shown positive responses of parasitism to either host density or landscape complexity. However, no experiments have manipulated host density in landscapes of differing complexity. Here we report the results of a field experiment conducted to determine how host density and agricultural landscape structure jointly affect parasitism and parasitoid diversity of Pseudaletia unipuncta (Haworth) (Lepidoptera: Noctuidae). Parasitism was assessed by experimentally adding P. unipuncta sentinel larvae at low (1 larvae/plant) and high (3 larvae/plant) densities to detect parasitism in commercial cornfields located in a complex and a simple agricultural landscape. The braconid wasps Glyptapanteles militaris (Walsh) and Meteorus spp. accounted for 98.4% of the observed parasitism. Landscape structure did not influence parasitism (80.2% on average) and contrary to expectations, showed a trend towards increased parasitoid richness and diversity in the simple landscape. Increasing host density revealed a trend of increasing parasitoid richness and diversity, and differentially affected parasitism at the parasitoid specific level. G. militaris parasitized a significantly greater proportion of hosts at low host density, while the opposite occurred for Meteorus spp. (primarily M. communis ). These offsetting responses of parasitoids resulted in the lack of an overall host density effect on parasitism. The differential response of these parasitoids to host density is discussed in relation to differences in morphological and life history characteristics. Our results suggest that the specific composition of parasitoid assemblages could significantly alter parasitism at different host densities independently of landscape structural complexity.

Community composition and species richness of parasitoids infesting Yponomeuta species in the Netherlands

Parasitoid assemblages infesting Yponomeuta species in the Netherlands were investigated. Parasitoid species richness and community composition were related to host species, habitat, temporal and spatial variation. Both community structure and species richness did not differ among habitats. There was no significant difference in species richness between years (1994 and 1995) but there was a significant difference in community composition. Community composition and species richness both differed among host species, although this latter result was solely due to the host species Y. evonymellus. There was no significant relationship between community similarity and distance. These results indicate that the parasitoids of the moth genus Yponomeuta in the Netherlands appear to form a spatially stable, but temporally variable community. Most of the variation in community structure was, however, related to the host species. The marked difference in parasitoid species richness and community composition of Y. evonymellus when compared to the other species warrants further study.

DETERMINANTS OF PARASITOID–HOST ASSOCIATIONS: INSIGHTS FROM A NATURAL TACHINID–LEPIDOPTERAN COMMUNITY

A major goal of insect community ecology is to understand how and why herbivorous insect species vary in the diversity of their parasitoid assemblages and the rates of parasitism that they experience. Most studies investigating these issues with Lepidoptera as hosts have relied on literature records of parasitoid–host associations that are often of limited quality and that do not necessarily reflect local interactions between hosts and parasitoids. We sampled externally feeding Lepidoptera in mesquite–oak savanna habitats of southeastern Arizona (USA) to assess the ecological and evolutionary determinants of parasitoid community structure. We focused on parasitoids in the family Tachinidae (Diptera) due to their dominance as larval parasitoids of macrolepidoptera at our site. Host abundance, morphology, coloration, gregariousness, and diet breadth of the host were all significantly correlated with tachinid species richness among hosts. Tachinid species richness also varied according to host taxonomy (family), but most of this variation appeared to be better explained by morphology and ecology than by phylogenetic position. Characteristics of host habitat and body size had no significant effect on tachinid species richness. Tachinid parasitism rates were higher for abundant, hairy, non-aposematic, and gregarious hosts. Hymenopteran parasitism rates were low and variable with only host family explaining a significant amount of variation. In general, we found that a substantial amount of the variation in tachinid species richness and parasitism rates among hosts can be explained by ecological attributes, and that interactions of host species with their host plants and predators may determine their suitability as hosts for parasitoids. Corresponding Editor: S. J. Simpson

Species richness and parasitism in an assemblage of parasitoids attacking maize stem borers in coastal Kenya

Abstract. 1. Parasitoids were reared from four species of lepidopteran stem borer collected in maize in southern coastal Kenya from 1992 to 1999. The stem borers included three native species, Sesamia calamistis Hampson, Busseola fusca Fuller, and Chilo orichalcociliellus (Strand), and one exotic borer, Chilo partellus (Swinhoe). A total of 174 663 caterpillars was collected, of which 12 645 were parasitised.2. Twenty‐six primary parasitoid species were reared from the exotic borer, C. partellus, indicating a rapid accumulation of native parasitoids on the alien borer.3. The three most abundant parasitoids were the larval parasitoids Cotesia sesamiae Cameron, Cotesia flavipes (Cameron), and the pupal parasitoid Pediobius furvus Gahan. The pupal parasitoid Dentichasmias busseolae Heinrich and the larval parasitoid Goniozus indicus Ashmead were also common. All used an ingress‐and‐sting method of attack.4. Cotesia flavipes, introduced into Kenya in 1993, was found in all seasons from 1997 onwards, and has become the most abundant stem borer larval parasitoid in the area. A native congener, Cotesia sesamiae, appeared in all seasons from 1992 to 1999. Together, these two parasitoids accounted for 83.3% of the parasitised borers.5. Thirty parasitoid species were recovered in Kilifi district, 27 in Kwale, and 15 in Taita Taveta. Parasitism was much greater in Taita Taveta district than in Kilifi or Kwale districts.

The influence of varying spatial heterogeneity on the refuge model for coexistence of specialist parasitoid assemblages

Models of host–parasitoid dynamics often assume constant levels of spatial heterogeneity in parasitoid attack rate, which tends to stabilize the interactions. Recently, authors have questioned this assumption and shown that outcomes of simple host–parasitoid models change if spatial heterogeneity is allowed to vary with parasitoid density. Here, we allow spatial heterogeneity to vary with either parasitoid density or percent parasitism in a model designed to explain specialist parasitoid coexistence on insect hosts with various levels of refuge. By examining this model we can evaluate the effect of varying spatial heterogeneity on a more complex model in which spatial heterogeneity is not considered the primary determinant of persistence. By modeling communities with one host and two parasitoid species, we show that the probability of species persistence for the competitively inferior parasitoid depends on the assumed relationship between spatial heterogeneity and both parasitoid density and percent parasitism. The probability of parasitoid coexistence is generally lower when spatial heterogeneity varies with parasitoid demographics. We conclude that the conditions for which host refuge promote specialist parasitoid coexistence are less common that proposed by the original model. Finally, we compared a model in which spatial heterogeneity varies with percent parasitism to data from laboratory trials and find a reasonable fit. We conclude that the change in spatial heterogeneity strongly influenced the outcome of the laboratory trials, and we suggest more research is necessary before researchers can assume constant spatial heterogeneity in future models.

Structure and Function of Parasitoid Assemblages Associated withPhyllonorycterLeafminers (Lepidoptera: Gracillariidae) on Deciduous Oaks in Japan

Parasitoid assemblages associated with Phyllonorycter spp. leafminers (Lepidoptera: Gracillariidae) were examined on six deciduous oak species (Quercus, Fagaceae) in Japan to understand how host leafminers, host food plants, and geographic location influence the structure and function of parasitoid assemblages. The parasitoid assemblages on five of the six oak species showed similar species richness (18–20 spp.), regardless of the number of host leafminer species and the abundance of hosts. Species composition was influenced by geographic location as well as host food oaks, and to a lesser extent by the leafminers. The assemblages varied in guild structure but showed nearly equal ratios of koinobiont/idiobiont species. The ratio was almost the same as that in Great Britain. These results suggest that the well-known argument that parasitoid communities are mostly influenced by host food plants is not universal, and that parasitoid assemblages may be organized with a balance between numbers of koinobiont and idiobiont species regardless of species richness, species composition, or geographic location. Total rate of parasitism did not correlate with the number of parasitoid species or the evenness of the assemblage. This implies that the host–parasitoid system is not under top-down control.