Consequences for a host–parasitoid interaction of host-plant aggregation, isolation, and phenology

Abstract.  1. Spatial habitat structure can influence the likelihood of patch colonisation by dispersing individuals, and this likelihood may differ according to trophic position, potentially leading to a refuge from parasitism for hosts.
2. Whether habitat patch size, isolation, and host-plant heterogeneity differentially affected host and parasitoid abundance, and parasitism rates was tested using a tri-trophic thistle–herbivore–parasitoid system.
3.  Cirsium palustre thistles ( n = 240) were transplanted in 24 blocks replicated in two sites, creating a range of habitat patch sizes at increasing distance from a pre-existing source population. Plant architecture and phenological stage were measured for each plant and the numbers of the herbivore Tephritis conura and parasitoid Pteromalus elevatus recorded.
4. Mean herbivore numbers per plant increased with host-plant density per patch, but parasitoid numbers and parasitism rates were unaffected. Patch distance from the source population did not influence insect abundance or parasitism rates. Parasitoid abundance was positively correlated with host insect number, and parasitism rates were negatively density dependent. Host-plant phenological stage was positively correlated with herbivore and parasitoid abundance, and parasitism rates at both patch and host-plant scales.
5. The differential response between herbivore and parasitoid to host-plant density did not lead to a spatial refuge but may have contributed to the observed parasitism rates being negatively density dependent. Heterogeneity in patch quality, mediated by variation in host-plant phenology, was more important than spatial habitat structure for both the herbivore and parasitoid populations, and for parasitism rates.     

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.     

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.     

A plant pathogen reduces the enemy-free space of an insect herbivore on a shared host plant

An important mechanism in stabilizing tightly linked host-parasitoid and prey-predator interactions is the presence of refuges that protect organisms from their natural enemies. However, the presence and quality of refuges can be strongly affected by the environment. We show that infection of the host plant Silene latifolia by its specialist fungal plant pathogen Microbotryum violaceum dramatically alters the enemy-free space of a herbivore, the specialist noctuid seed predator Hadena bicruris, on their shared host plant. The pathogen arrests the development of seed capsules that serve as refuges for the herbivore's offspring against the specialist parasitoid Microplitis tristis, a major source of mortality of H. bicruris in the field. Pathogen infection resulted both in lower host-plant food quality, causing reduced adult emergence, and in twofold higher rates of parasitism of the herbivore. We interpret the strong oviposition preference of H. bicruris for uninfected plants in the field as an adaptive response, positioning offspring on refuge-rich, high-quality hosts. To our knowledge, this is the first demonstration that plant-inhabiting micro-organisms can affect higher trophic interactions through alteration of host refuge quality. We speculate that such interference can potentially destabilize tightly linked multitrophic interactions.     

Plant Resistance Affects the Olfactory Response and Parasitism Success of Cotesia vestalis

Understanding the factors influencing host-selection behavior of parasitoids is essential in studies on host-parasitoid ecology and evolution, and in combining sustainable strategies of pest management, such as host-plant resistance and biological control. The effects of host-plant resistance on the olfactory response and parasitism success by Cotesia vestalis, a parasitoid of diamondback moth (Plutella xylostella) larvae were examined. Here, it was demonstrated that host-plant resistance can strongly influence foraging behavior and parasitism success of the parasitoid. In olfactometer experiments, C. vestalis did not differentiate between crucifer plant types with similar levels of susceptibility or resistance to P. xylostella but showed a strong preference for susceptible compared with partially-resistant host plants. The influence of previous oviposition activity varied with the host-plant type experienced by the parasitoid. In cage experiments, C. vestalis preferred to parasitize P. xylostella larvae on a susceptible plant compared with larvae on a partially resistant host plant when exposed to hosts for 24 h. However, this preference appeared to be transitory, and was not found after 96 h exposure. The present study suggests that combining partial host-plant resistance with biological control by C. vestalis for the control of P. xylostella may in some circumstances be antagonistic and negatively affect parasitism success.     

Host-parasitoid extinction and colonization in a fragmented prairie landscape

Few field studies of natural populations have examined the factors influencing local extinctions and colonization of empty habitat patches for a prey species and its predator. In this study, I carried out a census of planthopper (Prokelisia crocea; Hemiptera: Delphacidae) and egg parasitoid (Anagrus columbi; Hymenoptera: Mymaridae) incidence and densities in 147 host-plant patches (Spartina pectinata; Poaceae) over seven planthopper generations in a tall-grass prairie landscape. For both species, the likelihood of going extinct in a patch was related to a number of patch-specific variables: density, temporal variability in density, proportion of hosts parasitized (planthopper only), host-plant density, patch size, patch isolation, and composition of the surrounding matrix. Colonization likelihood was related only to the physical attributes of the patch. There was high patch turnover in this prairie landscape. On average, planthoppers went extinct in 23% of the patches and A. columbi went extinct in 51% of the patches in each generation. For the planthopper, extinction likelihood increased with a decrease in patch size and the proportion of the matrix composed of mudflat. Parasitism of eggs had no effect on the extinction likelihood of local P. crocea populations, suggesting that A. columbi may not play a major role in the patch dynamics of its host. The likelihood of extinction for A. columbi was dependent on factors that spanned three trophic levels. An increase in plant density, decrease in host density and decrease in parasitoid density all increased the likelihood of A. columbi extinction within a patch. The dependency on multiple trophic levels may explain the higher extinction risk for the parasitoid than its host. A. columbi extinction was also affected by the matrix habitat surrounding the patch—the effect was the opposite of that for P. crocea. Finally, vacant patches were colonized at rates of 53% and 34% per generation for the planthopper and parasitoid, respectively. For both species, colonization probabilities decreased with an increase in patch isolation. High host densities in a patch also favored high rates of colonization by A. columbi. I discuss how anthropogenic changes to the prairie landscape can affect the metapopulation dynamics and persistence time of this host-parasitoid interaction.     

Context‐dependency and anthropogenic effects on individual plant–frugivore networks

Anthropogenic activities, such as grazing by domestic animals, are considered drivers of environmental changes that may influence the structure of interaction networks. The study of individual‐based networks allows testing how species‐level interaction patterns emerge from the pooled interaction modes of individuals within populations. Exponential random graph models (ERGMs) examine the global structure of networks by allowing the inclusion of specific node (i.e. interacting partners) properties as explanatory covariates. Here we assessed the structure of individual plant–frugivore interaction networks and the ecological variables that influence the mode of interactions under different land‐use (grazed versus ungrazed protected areas). We quantified the number of visits, the number of fruits removed per visit and the interaction strength of mammal frugivore species at each individual tree. Additionally we quantified ecological variables at the individual, microhabitat, neighborhood and habitat scales that generated interaction network structure under the different land uses. Individual plant–frugivore networks were significantly modular in both land uses but the number of modules was higher in the grazed areas. We found interaction networks for grazed and ungrazed lands were structured by phenotypic traits of individual trees, by the microhabitat beneath the tree canopy and were affected by habitat modifications of anthropogenic origin. The neighborhood surrounding each individual plant influenced plant–frugivore interactions only at the grazed‐land trees. We conclude that anthropogenic land uses influence the topological patterns of plant–frugivore networks and the frugivore visitation to trees through modification of both habitat complexity and the ecological traits underlying interactions between individual plants and frugivore species.     

森林放牧对东北虎豹国家公园东部有蹄类动物灌草层食物资源的影响

森林放牧是中国东北虎豹国家公园内影响最广泛且强度最大的人类干扰之一。研究放牧对有蹄类动物食物资源的影响,是估算当前状态下东北虎和东北豹主要猎物承载力的关键,可以为国家公园的管理提供有效的科学依据。本研究于2016年在中国东北虎豹国家公园东部的牧场和非牧场区域分层抽取50个林下样地设置围栏对照实验,于2017年生长季进行灌草层植被调查 (每个样地的围栏与对照各随机调查3个1m×1m的样方,共调查300个植物样方) ,并应用红外相机技术获取对照样地内有蹄类动物丰富度和活动情况,研究放牧对研究区灌草层植物及动物的影响。研究结果显示：在生长季内,森林放牧显著降低林下灌草层植物生物量 (减少约24%),牧场样地的嫩枝叶显著降低。除禾草外,牧场样地中其他类别植物的氮含量均显著高于非牧场样地 (平均超出非牧场样地25 %)。非牧场样地梅花鹿的相 对丰富度指数 (RAI) 显著高于牧场样地,而狍与野猪的相对丰富度指数 (RAI) 在这两类样地之间没有显著差别。研究结果表明,东北虎豹国家公园东部森林放牧,减少了有蹄类动物灌草层食物资源,降低了有蹄类动物的多度。建议停止森林放牧,恢复有蹄类动物栖息地。     

Changes in composition and structure of a tropical dry forest following intermittent cattle grazing

In northwestern Costa Rica, cattle are being used as a "management tool" to reduce the amount of combustible material, mainly dominated by Hyparrhenia rufa, an African grass. This project is being developed within Parque Nacional Palo Verde and Reserva Biológica Lomas Barbudal, both of which form part of the only remaining tropical dry forests in Mesoamerica. To determine the short-term effects of cattle grazing on the natural vegetation, we compared the floristic composition within Palo Verde in an area under intermittent cattle grazing with an area that has not been grazed. There were significantly fewer plant species in the area with intermittent cattle grazing compared to the area with no grazing. Floristic composition of these two habitats was different as reflected by both Fisher's alpha values and the Shannon index of diversity, both of which were significantly higher in the ungrazed site. The ungrazed area contained more plant species and was more similar to mature forest. The structure of the vegetation was significantly different between the intermittently grazed and ungrazed sites with more small stems (1-5 cm dbh) and fewer large stems (> 5 cm dbh) in the intermittently grazed habitat. These results indicate that cattle grazing has an impact on the dry forest by reducing the relative abundance and density of larger tree species and by changing the species composition and structure of the community. The current management plan implemented in Palo Verde and Lomas Barbudal is not appropriate because of the impact that cattle have on the structure of the natural vegetation and should not be considered a viable alternative in other protected areas of dry forest in the Neotropics. We suggest that alternative fire prevention measures be evaluated including hand-cutting H. rufa, the creation of more frequent and larger fire breaks, and the development of green breaks.     

A hierarchical multi‐scale analysis of the spatial relationship between parasitism and host density in urban habitats

Studies on spatial density dependence in parasitism have paid scarce attention to how changes in host density at different hierarchical scales could influence parasitism in an herbivore at a particular scale. Here, we evaluated if rates of parasitism per leaf (by the whole parasitic complex and by dominant species) of the specialist leaf miner Liriomyza commelinae (Diptera: Agromyzidae) respond to variations in host density at the leaf, plant patch and site levels in an urban setting. We used multi‐level Bayesian models that incorporate the spatial hierarchy occurring in this system, as well as habitat factors previously found to have an effect on the L. commelinae parasitoid community in an urban context (patch size, patch isolation and urbanization level). According to the fitted model, overall parasitism rates decreased with increasing number of mines per leaf, being independent of host‐density variations at patch and site level. Patch structure was found to have a strong effect on parasitism rates per leaf. The analysis of parasitism by parasitoid species separately showed consistent results with the response at community level. These results suggest that parasitism of the parasitoid community here studied would be sensitive to hierarchical cues related to the host at the leaf level and to the host habitat at the patch level.     

Cattle grazing in CRP grasslands during the nesting season: effects on avian reproduction

Bird populations in grasslands have experienced declines coinciding with loss and fragmentation of prairies. The United States Department of Agriculture (USDA)-administered Conservation Reserve Program (CRP) is the most extensive grassland restoration program in North America and it has especially benefitted grassland birds. Grazing by domestic cattle has been restricted in CRP during avian nesting seasons despite the potential improvements in structuring habitat for a greater diversity of grassland bird species. Potential negative consequences of grazing in CRP grasslands include trampling of nests by cattle, reductions in nest concealment from predators, and attraction of brood-parasitic brown-headed cowbirds (Molothrus ater). We designed an experiment to test for effects of cattle grazing in CRP fields during the nesting season on nest survival and brood parasitism of 5 bird species that commonly nest in CRP grasslands: mourning dove (Zenaida macroura), grasshopper sparrow (Ammodramus savannarum), dickcissel (Spiza americana), and eastern (Sturnella magna) and western (S. neglecta) meadowlarks. Grazing was implemented during summers 2017 and 2018 on 17 of 36 fields followed by a year of rest on all fields in 2019. Of the 879 nests on grazed fields, only 4 were likely trampled by cattle (vs. 54% of all nests estimated as failing because of depredation). Experimental grazing (grazed vs. ungrazed fields) had variable effects on nest survival and cowbird parasitism among the bird species analyzed. Negative effects of grazing on daily nest survival of dickcissel and meadowlarks were apparent, at least in some years. We found no direct effects of grazing on nest survival of mourning dove or grasshopper sparrow. Probability and intensity (cowbird offspring/nest) of cowbird parasitism in dickcissel nests was higher on grazed versus ungrazed sites but only in conservation practice (CP) CP2 (vs. CP25 fields). Parasitism probability of grasshopper sparrow nests by cowbirds was higher on grazed fields in the 2 years after introduction of cattle in 2017. Greater vegetative concealment around nest sites was associated with reduced cowbird parasitism of meadowlark and grasshopper sparrow nests and higher nest survival for grasshopper sparrows. Reductions in vegetative height caused by longer-term or high-intensity grazing might therefore have negative consequences for some grassland birds by increasing nest site visibility and exposure to cowbird parasitism. Our results indicate that cattle grazing in CRP fields during the nesting season might have some negative effects on reproductive success of some grassland bird species, at least in the short term; however, the potential improvements of structuring habitat to accommodate more grassland bird species and increasing landowner participation in the CRP are considerable.     

Multitrophic influences on egg distribution in a specialized leaf beetle at multiple spatial scales

Egg distribution in herbivorous beetles can be affected by bottom-up (host plant), and by top-down factors (parasitoids and predators), as well as by other habitat parameters. The importance of bottom-up and top-down effects may change with spatial scale.

In this study, we investigated the influence of host plant factors and habitat structure on egg distribution in the leaf beetle Cassida canaliculata Laich. (Coleoptera: Chrysomelidae), a monophagous herbivore on Salvia pratensis L. (Lamiales: Lamiaceae), on four spatial scales: individual host plant, microhabitat, macrohabitat, and landscape. At the individual host plant scale we studied the correlation between egg clutch incidence and plant size and quality. On all other scales we analyzed the relationship between the egg clutch incidence of C. canaliculata and host plant percentage cover, host plant density, and the surrounding vegetation structure. Vegetation structure was examined as herbivores might escape egg parasitism by depositing their eggs on sites with vegetation factors unfavorable for host searching parasitoids.

The probability that egg clutches of C. canaliculata were present increased with an increasing size, percentage cover, and density of the host plant on three of the four spatial scales: individual host plant, microhabitat, and macrohabitat. There was no correlation between vegetation structure and egg clutch occurrence or parasitism on any spatial scale. A high percentage of egg clutches (38–56%) was parasitized by Foersterella reptans Nees (Hymenoptera: Tetracampidae), the only egg parasitoid, but there was no relationship between egg parasitism and the spatial distribution of egg clutches of C. canaliculata on any of the spatial scales investigated. However, we also discuss results from a further study, which revealed top-down effects on the larval stage.     

Spatial ecology of host–parasitoid interactions: a threatened butterfly and its specialised parasitoid

Habitat conservation for threatened temperate insect species is often guided by one of two paradigms: a metapopulation approach focusing on patch area, isolation and number; or a habitat approach focusing on maintaining high quality habitat for the focal species. Recent research has identified the additive and interacting importance of both approaches for maintaining populations of threatened butterflies. For specialised host-parasitoid interactions, understanding the consequences of habitat characteristics for the interacting species is important, because (1) specialised parasitoids are particularly vulnerable to the consequences of fragmentation, and (2) altered interaction frequencies resulting from changes to habitat management or the spatial configuration of habitat are likely to have consequences for host dynamics. The spatial ecology of Cotesia bignellii, a specialist parasitoid of the threatened butterfly Euphydryas aurinia, was investigated at two spatial scales: within habitat patches (at the scale of individual aggregations of larvae, or ‘webs’) and among habitat patches (the scale of local populations). Parasitism rates were investigated in relation to larval web size, vegetation sward height and host density. Within patches, the probability of a larval webs being parasitized increased significantly with increasing number of larvae in the web, and parasitism rates increased significantly with increasing web isolation. The proportion of webs parasitized was significantly and negatively correlated with cluster density. Among habitat patches the proportion of parasitized webs decreased as cluster density increased. Clusters with a high proportion of larval webs parasitized tended to have lower parasitism rates per larval web. These results support the call for relatively large and continuous habitat patches to maintain stable parasitoid and host populations. Conservation efforts directed towards maintenance of high host plant density could allow E. aurinia to reduce parasitism risk, while providing C. bignellii with sufficient larval webs to allow population persistence.     

Host shift to peas in the diamondback moth Plutella xylostella (Lepidoptera: Plutellidae) and response of its parasitoid Diadegma mollipla (Hymenoptera: Ichneumonidae)

Host shifts in herbivorous insects are thought to sometimes provide enemy-free space on the novel host plant. A population of the diamondback moth Plutella xylostella (Linnaeus), an oligophagous pest on crucifers, recently shifted to sugar snap- and snowpeas (Pisum sativum) in Kenya, resulting in heavy damage to these crops. The impact of this host shift on the interaction with Diadegma mollipla (Holmgren), one of the most frequent parasitoid species attacking P. xylostella in this area, was investigated. Parasitism rates and development of two strains of D. mollipla, one reared from a cabbage-feeding strain of P. xylostella and the second from the new pea-feeding strain, changed based on the host-plant that P. xylostella fed upon, with both parasitoid strains more effective on the novel host plant. Parasitism by the cabbage-D. mollipla strain on P. xylostella infesting peas was four times higher than on P. xylostella infesting cabbage when a single plant species was present. However, when both crops were offered together, the level of parasitism dropped to the level seen when cabbage was offered alone. Diadegma mollipla developed on both hosts, but cabbage-D. mollipla had a longer total development time. Pupae of cabbage-feeding P. xylostella were significantly heavier than pupae of pea-feeding P. xylostella and parasitism had no influence on these differences. Diadegma mollipla preferred to parasitize the pea-feeding P. xylostella. Thus, the host shift by P. xylostella to a novel host plant did not necessarily provide enemy-free space, with the parasitoid species tested. The implications of these findings for the host-parasitoid relationship are discussed.     

Herbivore phenology can predict response to changes in plant quality by livestock grazing

Livestock grazing can have a strong impact on herbivore abundance, distribution and community. However, not all species of herbivores respond the same way to livestock grazing, and we still have a poor understanding of the underlying mechanisms driving these differential responses. Here, we investigate the effect of light intensity cattle grazing on the abundance of two grasshoppers (Euchorthippus cheui and E. unicolor) that co-occur in the same grasslands and feed on the same food plant (the dominant grass Leymus chinensis). The two grasshopper species differ in phenology so that their peak abundances are separated into early- and late-growing seasons. We used an exclosure experiment to monitor grasshopper abundance and food quality in the field under grazed and ungrazed conditions, and performed feeding trials to examine grasshopper preference for grazed or ungrazed food plants in the laboratory. We found that the nitrogen content of L. chinensis leaves continuously declined in the ungrazed areas, but was significantly enhanced by cattle grazing over the growing season. Cattle grazing facilitated the early-season grasshopper E. cheui, whereas it suppressed the late-season grasshopper E. unicolor. Moreover, feeding trials showed that E. cheui preferred L. chinensis from grazed plots, while E. unicolor preferred the leaves from ungrazed plots. We conclude that livestock grazing has opposite effects on the two grasshopper species, and that these effects may be driven by grazing-induced changes in plant nutrient content and the unique nutritional niches of the grasshoppers. These results suggest that insects that belong to the same guild can have opposite nutrient requirements, related to their distinct phenologies, and that this can ultimately affect their response to cattle grazing. Our results show that phenology may link insect physiological needs to local resource availabilities, and should be given more attention in future work on interactions between large herbivores and insects.     

Livestock exclusion increases the spatial heterogeneity of vegetation in Colorado shortgrass steppe

Abstract. Spatial heterogeneity, an important characteristic in semi‐arid grassland vegetation, may be altered through grazing by large herbivores. We used Moran's I, a measure of autocorrelation, to test the effect of livestock grazing on the fine scale spatial heterogeneity of dominant plant species in the shortgrass steppe of northeastern Colorado. Autocorrelation in ungrazed plots was significantly higher than in grazed plots for the cover of the dominant species Bouteloua gracilis, litter cover and density of other bunchgrasses. No species had higher autocorrelation in grazed compared to ungrazed sites. B. gracilis cover was significantly auto‐correlated in seven of eight 60‐yr ungrazed exclosures, four of six 8‐yr exclosures, and only three of eight grazed sites. Autocorrelograms showed that B. gracilis cover in ungrazed sites was frequently and positively spatially correlated at lag distances less than 5 m. B. gracilis cover was rarely autocorrelated at any sampled lag distance in grazed sites. The greater spatial heterogeneity in ungrazed sites appeared linked to patches characterized by uniformly low cover of B. gracilis and high cover of C₃ grasses. This interpretation was supported by simple simulations that modified data from grazed sites by reducing the cover of B. gracilis in patches of ca. 8 m diameter and produced patterns quite similar to those observed in ungrazed sites. In the one exclosure where we intensively sampled soil texture, autocorrelation coefficients for sand content and B. gracilis cover were similar at lag distances up to 12 m. We suggest that the negative effect of sand content on B. gracilis generates spatial heterogeneity, but only in the absence of grazing. An additional source of heterogeneity in ungrazed sites may be the negative interaction between livestock exclusion and B. gracilis recovery following patchy disturbance.     

Herbivore regulation in urban agroecosystems: Direct and indirect effects

Urban agroecosystems can provide habitat for biodiversity and can benefit human communities through urban food provisioning. Moreover, urban agroecosystems could be managed so as to optimize ecosystem services like natural pest control provided by trophic interactions between natural enemies and herbivores. As in other ecosystems, predation and parasitism regulate herbivores in urban settings, but less is known about the relative importance of direct and indirect effects at local and landscape scales in highly managed urban agroecosystems. We collected data on herbivore (cabbage aphid) density and parasitism ratios (proportion of parasitized aphid “mummies”) in 25 community gardens in three counties in the California central coast, USA. We used structural equation modeling to examine the effects of direct factors (host plant characteristics and parasitism) and indirect factors (soil, garden, and landscape characteristics) on herbivore density changes at two time points in the growing season (June and August). Aphid density, but not parasitism, varied across counties over the season, and there was a strong negative relationship between aphid density and parasitism. Direct effects were strong drivers of aphid density but not parasitism. In June, aphid density increased with host plant volume but decreased with greater floral density, while parasitism was only influenced by aphid density. In August, host plant volume similarly positively affected aphid density, and soil water holding capacity increased host plant volume. In addition, host plant density had a strong negative effect on parasitism. Urban gardeners may be able to reduce aphid pest densities by increasing floral resource density and strategically spatially distributing host plants throughout garden beds, though these processes depend on the season. The indirect effects of soil water holding capacity on aphid densities further suggest a critical role of human management on pest populations and pest control services through soil amendments and irrigation.     

Effects of patch scale on density-dependence and species-dependence in two host-parasitoid systems

Summary Data from two host-parasitoid communities were analyzed to ascertain whether patch scale affected the kinds of correlations existing between 1) spatial differences in host density and the intensity of parasitism (density-dependence) and 2) number of species of parasitoids and the intensity of parasitism (species-dependence). We concluded that parasitization rates are usually independent of both host density and number of parasitoid species present regardless of patch scale. Therefore, the responses of parasitoids to host density and the addition of parasitoid species to a community are equally unpredictable in outcome.     

The effects of plant dispersion and prey density on parasitism rates in a naturally patchy habitat

Despite extensive research on parasitoid-prey interactions and especially the effects of heterogeneity in parasitism on stability, sources of heterogeneity other than prey density have been little investigated. This research examines parasitism rates by three parasitoid species in relationship to prey density and habitat spatial pattern. The herbivore Itame andersoni (Geometridae) inhabits a subdivided habitat created by patches of its host plant, Dryas drummondii, in the Wrangell Mountains of Alaska. Dryas colonizes glacial moraines and spreads clonally to form distinct patches. Habitat subdivision occurs both on the patch scale and on the larger spatial scale of sites due to patchy successional patterns. Itame is attacked by three parasitoids: an ichneumonid wasp (Campoletis sp.), a braconid wasp (Aleiodes n. sp.), and the tachinid fly (Phyrxe pecosensis). I performed a large survey study at five distinct sites and censused Itame density and parasitism rates in 206 plant patches for 1–3 years. Parasitism rates varied with both plant patch size and isolation and also between sites, and the highest rates of overall parasitism were in the smallest patches. However, the effects of both small- and large-scale heterogeneity on parasitism differed for the three parasitoid species. There was weak evidence that Itame density was positively correlated with parasitism for the braconid and tachinid at the patch scale, but density effects differed for different patch sizes, patch isolations, and sites. At the site scale, there was no evidence of positive, but some indication of negative density-dependent parasitism. These patterns do not appear to be driven by negative interactions between the three parasitoid species, but reflect, rather, individual differences in habitat use and response to prey density. Finally, there was no evidence that parasitism strongly impacted the population dynamics of Itame. These results demonstrate the importance of considering habitat pattern when examining spatial heterogeneity of parasitism and the impacts of parasitoids. Received: 3 June 1999 / Accepted: 4 October 1999