Aggregation of parasitism risk in an aphid-parasitoid system: Effects of plant patch size and aphid density

Few studies have linked density dependence of parasitism and the tritrophic environment within which a parasitoid forages. In the non-crop plant-aphid, Centaurea nigra–Uroleucon jaceae system, mixed patterns of density-dependent parasitism by the parasitoids Aphidius funebris and Trioxys centaureae were observed in a survey of a natural population. Breakdown of density-dependent parasitism revealed that density dependence was inverse in smaller colonies but direct in larger colonies (>20 aphids), suggesting there is a threshold effect in parasitoid response to aphid density. The CV² of searching parasitoids was estimated from parasitism data using a hierarchical generalized linear model, and CV²>1 for A. funebris between plant patches, while for T. centaureae CV²>1 within plant patches. In both cases, density independent heterogeneity was more important than density-dependent heterogeneity in parasitism. Parasitism by T. centaureae increased with increasing plant patch size. Manipulation of aphid colony size and plant patch size revealed that parasitism by A. funebris was directly density dependent at the range of colony sizes tested (50–200 initial aphids), and had a strong positive relationship with plant patch size. The effects of plant patch size detected for both species indicate that the tritrophic environment provides a source of host density independent heterogeneity in parasitism, and can modify density-dependent responses.     

Parasitism of the herbivore Pieris brassicae L. (Lep., Pieridae) by Cotesia glomerata L. (Hym., Braconidae) does not benefit the host plant by reduction of herbivory

Models of tritrophic interactions often imply that attraction of herbivore natural enemies by the plant constitutes a defence. Parasitoid attack on herbivores is assumed to result in a reduction in herbivory and/or an increase in plant fitness. Coupled with the active involvement of the plant in producing signals, this can be seen as indirect mediation of wound-induced defence. The assumption that parasitism of Pieris brassicae L. by the parasitoid Cotesia glomerata L. reduces leaf consumption of cabbage is tested. The effect of parasitism on the growth and feeding rates of P. brassicae larvae compared to unparasitized larvae. Either the defensive mechanism, if it exists, is more subtle, or the natural enemy is using an incidental chemical produced as a by-product of the herbivores feeding action as a cue for host-finding, with no increaction effects. This has implications for the study of tritrophic interactions, particularly in the context of agricultural systems and the assumption that in IPM strategies, parasitoids are a plant benefit.     

Genetic variation in resistance of Eucalyptus globulus to marsupial browsers

The evolution of plant defensive traits in response to selection pressures imposed by herbivores is central to co-evolutionary theory. To demonstrate the role of herbivores as selective agents on plant resistance there must be variability in plant resistance to herbivores within a plant population. This variability must be under genetic control, and the variability in plant resistant traits and consequently herbivore damage to plants must reflect variability in plant fitness. We used a common eucalypt species, Eucalyptus globulus, and two major mammalian herbivores, the common brushtail possum (Trichosurus vulpecula) and the red-bellied pademelon (Thylogale billardierii), as a system to investigate intraspecific variation in plant resistance to mammalian herbivores and to investigate if this variation has a genetic basis. We measured mammalian browsing damage on 2,302 individual trees of E. globulus, from 563 families derived from range-wide native stand seed collections of known pedigree and grown in a common environment field trial. Using a selection of trees from the field trial we then conducted a feeding trial with captive herbivores to assess if the genetic variation in plant resistance in the field was reflected in feeding preferences of captive animals, as measured by relative intake. Results from the field trial showed significant genetic variation in plant resistance amongst races, localities and amongst different families. These results were consolidated in the captive trial with similar trends in genetic variation among E. globulus localities. Dry matter intake of foliage by Trichosurus vulpecula was consistently greater than that by Thylogale billardierii; however, the intraspecific preferences of the two herbivores were significantly correlated.     

Ecological trade-offs between jasmonic acid-dependent direct and indirect plant defences in tritrophic interactions

Recent studies on plants genetically modified in jasmonic acid (JA) signalling support the hypothesis that the jasmonate family of oxylipins plays an important role in mediating direct and indirect plant defences. However, the interaction of two modes of defence in tritrophic systems is largely unknown. In this study, we examined the preference and performance of a herbivorous leafminer (Liriomyza huidobrensis) and its parasitic wasp (Opius dissitus) on three tomato genotypes: a wild-type (WT) plant, a JA biosynthesis (spr2) mutant, and a JA-overexpression 35S::prosys plant. Their proteinase inhibitor production and volatile emission were used as direct and indirect defence factors to evaluate the responses of leafminers and parasitoids. Here, we show that although spr2 mutant plants are compromised in direct defence against the larval leafminers and in attracting parasitoids, they are less attractive to adult flies compared with WT plants. Moreover, in comparison to other genotypes, the 35S::prosys plant displays greater direct and constitutive indirect defences, but reduced success of parasitism by parasitoids. Taken together, these results suggest that there are distinguished ecological trade-offs between JA-dependent direct and indirect defences in genetically modified plants whose fitness should be assessed in tritrophic systems and under natural conditions.     

Soil phosphorus heterogeneity and mycorrhizal symbiosis regulate plant intra-specific competition and size distribution

We investigated the interactive effects of soil phosphorus (P) heterogeneity, plant density and mycorrhizal symbiosis on plant growth and size variability of Trifolium subterraneum. We set up mesocosms (trays 49Ꮉ cm and 12 cm deep) with the same amount of available P, but distributed either homogeneously or heterogeneously, in randomly arranged cells (7ǻ cm each) with high or low available P. The trays were planted with either 1 or 4 seedlings of T. subterraneum per cell. Half of the trays were inoculated with spores of the mycorrhizal fungus Gigaspora margarita. We harvested the plants when leaves just started to overlap, 8 weeks after planting. Plants growing in high P cells had the lowest percentage infection, but the highest mean shoot and root biomass and root length. The mean size of the plants in each cell was determined mainly by local P concentration. However, in plants growing in high density, low P cells, ca. 20% of the variability in plant biomass was explained by the number of adjacent cells with high P. Patchy trays had the highest total shoot biomass, independently of mycorrhizal infection or plant density. Inoculated trays (M) had higher total shoot biomass and relative competition intensity (measured as reduction in plant biomass due to increased density) than non-inoculated trays (NM). Plant density reduced the plant response to mycorrhizal infection, and its effect was independent of P distribution. All populations growing in patchy trays, and low density mycorrhizal ones, had the highest plant-size inequality, presumably because patchy distribution of P and mycorrhizal infection increased competitive asymmetry. We conclude that mycorrhizal symbiosis has the potential to strongly influence plant population structure when soil nutrient distribution is heterogeneous because it promotes pre-emption of limiting resources.     

Host plant, host plant chemistry and the polyembryonic parasitoid Copidosoma sosares: indirect effects in a tritrophic interaction

Host plant identity and host plant chemistry have often been shown to influence host finding and acceptance by natural enemies but comparatively less attention has been paid to the tritrophic effects of host plant and host plant chemistry on other natural enemy fitness correlates, such as survivorship, clutch size, body size, and sex ratio. Such studies are central to understanding both the selective impact of plants on natural enemies as well as the potential for reciprocal selective impact of natural enemies on plant traits. We examined the effects of host plant and host plant chemistry in a tritrophic system consisting of three apiaceous plants (Pastinaca sativa, Heracleum sphondylium and H. mantegazzianum), the parsnip webworm (Depressaria pastinacella) and the polyembryonic parasitic wasp Copidosoma sosares. All of these plants produce furanocoumarins, known resistance factors for parsnip webworms. Furanocoumarin concentrations were correlated neither with the presence nor the number of webworms on a given plant. Concentrations of two furanocoumarins were negatively associated with C. sosares fitness correlates: isopimpinellin with the likelihood that a given webworm would be parasitized and xanthotoxin with both within‐brood survivorship (of all‐male and mixed‐sex broods) and clutch size. Brood sex ratio and body sizes of individual wasps were not correlated with furanocoumarin chemistry. Because additive genetic variation exists in P. sativa for furanocoumarin chemical traits, these are subject to selection by webworms through herbivory. Third trophic level selective impacts on furanocoumarin traits may include selection for reduced production of those chemicals that affect parasitoid survivorship yet do not influence host plant choice by the herbivore. That such might be the case is suggested by patterns of furanocoumarin production in populations of P. sativa with different histories of infestation; in the Netherlands, where parasitism rates of webworms by C. sosares are high, plants produce lower levels of all linear furanocoumarins and proportionately less isopimpinellin than do midwestern U.S. populations of P. sativa, where natural enemies of the webworm are effectively absent.     

Effect of leaf toughness on the susceptibility of the leaf-cutting ant Atta sexdens to attacks of a phorid parasitoid

Summary: Because the size of Atta spp. along foraging trails is partly determined by the characteristics of the plants harvested, and considering that parasitic phorid flies are attracted mostly to large individuals, we hypothesized that plant toughness affects the susceptibility of Atta spp. to these parasitoids. To test this hypothesis, we evaluated parasitism rates of the phorid Neodohrniphora sp. and its effect on Atta sexdens (L.) foragers in a laboratory colony. We manipulated forager size by alternating tough (Anthocephalus chinensis, Rubiaceae) and tender (Rosa chinensis, Rosaceae) plants given to the colony. Ants foraging on tough leaves were larger than ants foraging on soft leaves, and there was a significant reduction in forager size for both plants when the colony was exposed to Neodohrniphora sp. However, there were no relative differences on forager size between the two plants after the introduction of the parasitoid. The lack of response of Neodohrniphora sp. to the increase in ant size when the colony was given tough leaves may be attributed to the unusually large number of suitable hosts in a laboratory colony. However, large foragers are much less abundant in the field, in which case shifts in the size of the workforce triggered by different substrates could affect the incidence of parasitism.     

Parasitoids of grass-feeding chalcid wasps: a comparison of German and British communities

We compared the parasitoid communities associated with grass-feeding herbivores in Germany and Britain to examine geographical consistency in community composition and to test ecological characteristics of the plants and host insects that may explain variability in parasitoid community structure. The parasitoid communities of 16 chalcid wasps feeding on ten grass species were sampled between 1986 and 1989 at 4-11 sites per grass species in southwest Germany. The data were compared to published data from Great Britain, comprising 18 chalcid hosts on ten grass species sampled between 1980 and 1992 at 24 sites in Wales and England. Results showed that many conclusions drawn from patterns in Britain did not hold for Germany, emphasizing the need to repeat analyses in different geographical regions. The parasitoid communities of the Tetramesa hosts included on average 8.1 parasitoid species in Germany, while the British hosts supported only 4.1 parasitoids. The number of monophagous parasitoid species was similar in both areas (2.4 vs 3.2), but German host populations supported many more polyphagous species (5.1 vs 0.9). This difference reinforces the earlier conclusion that parasitoid communities in Britain are highly undersaturated. Increased numbers of parasitoid species in Germany did not result in increased parasitism rates, so the closer species packing was paralleled by reduced impact of each species. In Germany, percent parasitism (range: 5-74%) was closely correlated with log host density, explaining 90% of the variance, while in Great Britain, percent parasitism was less variable (range: 36-76%) and was not related to host density or other host or host plant characteristics. Gallers and non-gallers supported equal numbers of parasitoids in both Germany and Britain, offering support for neither the enemy hypothesis of the adaptive nature of plant galls nor for the finding that galls are often more susceptible to enemy attack than their non-galling relatives. Furthermore, gregarious Tetramesa hosts were not attacked by more parasitoid species than solitary hosts.     

Oviposition preferences of herbivores are affected by tritrophic interaction webs

Two tritrophic systems were experimentally coupled in the present study. One system consisted of a cabbage plant ( Brassica oleracea ), diamondback moth larvae ( Plutella xylostella ) and their parasitic wasp ( Cotesia plutellae ). The other system consisted of a cabbage plant, cabbage butterfly ( Pieris rapae ) larvae and their parasitic wasp ( Cotesia glomerata ). First, we demonstrated that parasitism by C. glomerata and C. plutellae increased and decreased, respectively, on plants infested by both herbivore species than on plants infested by their host larvae alone. We then demonstrated that adult Pl. xylostella oviposited preferentially on plants infested with Pi. rapae , whereas adult Pi. rapae revealed no significant preferences between uninfested plants or plants infested with Pl. xylostella . Based on the present results and those of our previous study, we discuss the oviposition preferences of herbivores in tritrophic contexts.     

Effects of organic and conventional fertilizer treatments on host selection by the aphid parasitoid Diaeretiella rapae

The type and quantity of fertilizer supplied to a crop will differ between organic and conventional farming practices. Altering the type of fertilizer a plant is provided with can influence a plant’s foliar nitrogen levels, as well as the composition and concentration of defence compounds, such as glucosinolates. Many natural enemies of insect herbivores can respond to headspace volatiles emitted by the herbivores’ host plant in response to herbivory. We propose that manipulating fertilizer type may also influence the headspace volatile profiles of plants, and as a result, the tritrophic interactions that occur between plants, their insect pests and those pests’ natural enemies. Here, we investigate a tritrophic system consisting of cabbage plants, Brassica oleracea, a parasitoid, Diaeretiella rapae, and one of its hosts, the specialist cabbage aphid Brevicoryne brassicae. Brassica oleracea plants were provided with either no additional fertilization or one of three types of fertilizer: Nitram (ammonium nitrate), John Innes base or organic chicken manure. We investigated whether these changes would alter the rate of parasitism of aphids on those plants and whether any differences in parasitism could be explained by differences in attractivity of the plants to D. rapae or attack rate of aphids by D. rapae. In free‐choice experiments, there were significant differences in the percentage of B. brassicae parasitized by D. rapae between B. oleracea plants grown in different fertilizer treatments. In a series of dual‐choice Y‐tube olfactometry experiments, D. rapae females discriminated between B. brassicae‐infested and undamaged plants, but parasitoids did not discriminate between similarly infested plants grown in different fertilizer treatments. Correspondingly, in attack rate experiments, there were no differences in the rate that D. rapae attacked B. brassicae on B. oleracea plants grown in different fertilizer treatments. These findings are of direct relevance to sustainable and conventional farming practices.     

Predicting the effects of climate change on natural enemies of agricultural pests

Climate change can have diverse effects on natural enemies of pest species. Here we review these effects and their likely impacts on pest control. The fitness of natural enemies can be altered in response to changes in herbivore quality and size induced by temperature and CO₂ effects on plants. The susceptibility of herbivores to predation and parasitism could be decreased through the production of additional plant foliage or altered timing of herbivore life cycles in response to plant phenological changes. The effectiveness of natural enemies in controlling pests will decrease if pest distributions shift into regions outside the distribution of their natural enemies, although a new community of enemies might then provide some level of control. As well as being affected by climate through host plants and associated herbivores, the abundance and activity of natural enemies will be altered through adaptive management strategies adopted by farmers to cope with climate change. These strategies may lead to a mismatch between pests and enemies in space and time, decreasing their effectiveness for biocontrol. Because of the diverse and often indirect effects of climate change on natural enemies, predictions will be difficult unless there is a good understanding of the way environmental effects impact on tritrophic interactions. In addition, evolutionary changes in both hosts and natural enemies might have unexpected consequences on levels of biocontrol exerted by enemies. We consider interactions between the pest light brown apple moth and its natural enemies to illustrate the type of data that needs to be collected to make useful predictions.     

Elevated volatile concentrations in high‐nutrient plants: do insect herbivores pay a high price for good food?

1. A tritrophic perspective is fundamental for understanding the drivers of insect–plant interactions. While host plant traits can directly affect insect herbivore performance by either inhibiting or altering the nutritional benefits of consumption, they can also have an indirect effect on herbivores by influencing rates of predation or parasitism. 2. Enhancing soil nutrients available to trees of the genus Eucalyptus consistently modifies plant traits, typically improving the nutritional quality of the foliage for insect herbivores. We hypothesised that resulting increases in volatile essential oils could have an indirect negative effect on eucalypt‐feeding herbivores by providing their natural enemies with stronger host/prey location cues. 3. Eucalyptus tereticornis Smith seedlings were grown under low‐ and high‐nutrient conditions and the consequences for the release of volatile cues from damaged plants were examined. The influence of 1,8‐cineole (the major volatile terpene in many Eucalyptus species) on rates of predation on model caterpillars in the field was then examined. 4. It was found that the emission of cineole increased significantly after damage (artificial or herbivore), but continued only when damage was sustained by herbivore feeding. Importantly, more cineole was emitted from high‐ than low‐nutrient seedlings given an equivalent amount of damage. In the field, predation was significantly greater on model caterpillars baited with cineole than on unbaited models. 5. These findings are consistent with the hypothesis that any performance benefits insect herbivores derive from feeding on high‐nutrient eucalypt foliage could be at least partially offset by an increased risk of predation or parasitism via increased emission of attractive volatiles.     

Effects of plant hybridization on herbivore-parasitoid interactions

We studied the effects of host plant hybridization on the survival and mortality of the leaf-mining moth Phyllonorycter salicifoliella on hybrid and parental willow plants in the field and in a common garden experiment. P. salicifoliella survival differed significantly among three willow taxa in the field in 1994 but not in the field in 1995 or in the common garden. Parasitism by eulophid wasps differed significantly among taxa in 1994 and appeared to account for the variation in their survival. In the field in 1995, host feeding predation varied significant among taxa. The theory of tritrophic interactions predicts that plant genotype can affect natural enemy impact, and this study supports this prediction. Significant variation in survival and eulophid parasitism was also found among genotypes within taxa in the field in both years and in the common garden experiment. The common garden results show that genetic differences in plants affect the herbivore-parasitoid interaction. Variation among years in the patterns of survival and causes of mortality among field plants suggest that genotype by environment interactions may be important. Received: 1 March 1996 / Accepted: 4 November 1996     

Beta and habitat diversity in marine systems: a new approach to measurement, scaling and interpretation

Specialist herbivores are known to alter their host's wound-induced responses but the beneficiaries of these alterations are unknown. Nicotiana attenuata plants release a burst of ethylene specifically in response to feeding by Manduca sexta larvae, which is known to suppress wound- and methyl jasmonate (MeJA)-inducible nicotine accumulation. The ethylene burst may be a mechanism by which M. sexta larvae feed "stealthily" on their host plants or, alternatively, it may allow the plant to optimize its defense response against this specialist herbivore by reducing costs of induction. We examined the impact of the ethylene burst on defense-related fitness costs that are readily observed when plants are treated with MeJA and grown in competition with untreated plants. We elicited nicotine induction (with MeJA), the ethylene burst (with the ethylene releasing compound, ethephon) and inhibited the plant's ability to perceive ethylene (with applications of an antagonist of ethylene receptors, 1-methylcyclopropene, 1-MCP). By simultaneously applying MeJA and ethephon we mimicked the plant's hormonal response to larval attack. We hypothesized that if the ethylene burst benefited the plant, the fitness costs of MeJA induction should be reduced by ethephon and restored if the plants were additionally treated with 1-MCP. In a second experiment, we applied larval oral secretion (OS) to elicit endogenous hormone production and predicted that the 1-MCP treatment should reduce the fitness of OS-treated plants. Our measures of plant fitness, namely the rate of stalk elongation and lifetime capsule production, supported these predictions. We conclude that the ethylene burst elicited by this specialist herbivore can reduce MeJA-induced fitness costs and increase the competitive strength of OS-treated plants. Suppressed nicotine production is likely to contribute to, but is not sufficient to explain, the observed fitness outcomes. The intensity of intra-specific competition and herbivore attack will likely determine the adaptive value of the M. sexta-elicited ethylene response.     

Temporal and geographic variation in parasitoid attack with no evidence for ant protection of the Melissa blue butterfly,Lycaeides melissa

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Artificial Defoliation Induces Trichome Production in the Tropical Shrub Cnidoscolus aconitifolius (Euphorbiaceae)1

Induction of plant defenses and their spatial variability are key subjects in the field of ecology and evolution of defensive traits in plants. Nevertheless, induction has been more commonly studied under controlled environments, ignoring other factors that might influence this process in natural settings. The main goal of this study was to determine if artificial defoliation induces trichome production in three natural populations of the tropical shrub Cnidoscolus aconitifolius. First, we performed trichome counts for each population before imposing artificial defoliation to assess differences in trichome loads between populations. Trichome densities (trichomes/cm²) were quantified for leaf blades, petioles, and flower stalks. To determine if defoliation induced trichome production, three defoliation treatments (0% leaves defoliated or controls, 50% of total leaves defoliated, and 100% defoliation) were applied once at the beginning of the reproductive season. Trichome counts were performed on each structure every ～20 d during a 3‐mo period after the application of treatments. Trichome counts showed significant differences in trichome densities between populations for all three structures. In turn, artificial defoliation increased trichome density. Significant differences among treatments were found for trichome densities on leaf blades and petioles. In both these structures, the 100 percent defoliation treatment differed significantly from control plants, presenting higher trichome densities. In addition, the treatment × population interaction was not significant for leaf blades and petioles, indicating that induction is a generalized response in this species, at least at the study sites. These results indicate that trichomes in C. aconitifolius are inducible due to defoliation.     

The influence of species identity and herbivore feeding mode on top-down and bottom-up effects in a salt marsh system

In this study we investigated the potential importance of species identity and herbivore feeding mode in determining the strengths of top-down and bottom-up effects on phytophagous insect densities. In 1998, we conducted two factorial field experiments in which we manipulated host plant quality and intensity of parasitoid attack on three salt marsh herbivores, the planthoppers Prokelisia marginata and Pissonotus quadripustulatus (Homoptera: Delphacidae), which feed only on Spartina alterniflora and Borrichia frutescens, respectively, and the gall fly Asphondylia borrichiae (Diptera: Cecidomyiidae), which feeds only on B. frutescens. We increased plant quality through addition of nitrogen fertilizer, and decreased parasitism by trapping hymenopteran parasitoids continuously throughout the study. Herbivore densities were censused biweekly. Increasing plant quality through fertilization increased the density of all three herbivores within 2 weeks of treatment application, and higher densities were maintained for the duration of the study. Reduction of top-down pressure had no effect on either planthopper species, possibly because of compensatory mortality affecting the two species. In contrast, reduction of parasitism significantly increased the density of A. borrichiae galls, perhaps because development within gall tissue reduces the sources of compensatory mortality affecting this species. The results of this study show that the bottom-up effects of plant quality were strong and consistent for all three species, but the strength of top-down effects differed between the two feeding guilds. Thus, even for herbivores feeding on the same host plant, conclusions drawn regarding the relative importance of top-down and bottom-up effects may vary depending upon the feeding mode of the herbivore.     

Three-way interactions among mutualistic mycorrhizal fungi, plants, and plant enemies: hypotheses and synthesis

A number of studies have shown that an association with mycorrhizal fungi can alter the outcome of interactions between plants and their enemies. While the directions of these effects vary, their strength suggests the need for greater attention to multispecies interactions among plant enemies, plants, and mycorrhizal fungi. We recognize that mycorrhizal fungi could effect plant enemies by improving plant nutrition, modifying plant tolerance, or modifying plant defenses. In addition, mycorrhizal fungi could directly interfere with pathogen infection, herbivory, or parasitism by occupying root space. We formalize these alternative outcomes of multispecies interactions and explore the long-term dynamics of the plant-enemy interactions based on these different scenarios using a general model of interactions between plants and plant enemies. We then review the literature in terms of the assumptions of the alternative mechanisms and the predictions of these models. Through this effort, we identify new directions in the study of tritrophic interactions between enemies, plants, and soil mutualists.     

寄主植物-蚜虫-天敌三重营养关系的化学生态学研究进展

综述了寄主植物－蚜虫－天敌三重营养关系的化学生态学研究,重点阐述了3个研究热点：①植物挥发性物质在蚜虫及其天敌选择寄主行为过程中的作用;②蚜虫信息素和蜜露对蚜虫天敌寄主选择行为的影响;③植物挥发性物质对蚜虫信息系作用的影响。对寄主植物－蚜虫－天敌三重营养关系的全面了解,将为蚜虫的综合治疗提供新思维。     

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.