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Arthropod communities dwelling in adjacent habitats are able to impact one another via shared natural enemies. In agricultural landscapes, drastic differences in resource availability between crop and non-crop habitats cause variation in insect herbivore densities over short distances, potentially driving inter-habitat effects. Moreover, the composition of the landscape in which the habitats are embedded likely affects realised attack rates from natural enemies via impacts on local arthropod community structure. Here, we examine indirect effects between herbivore species within and between habitat types by calculating the potential for apparent competition between multiple populations. Firstly, we aim to determine how disparities in resource availability impact the strength of the potential for apparent competition occurring between habitats, secondly to examine the impact of landscape composition upon these effects, and finally to couch these observations in reality by investigating the link between the potential for apparent competition and realised attack rates. We used DNA metabarcoding to characterise host-parasitoid interactions within two habitat types (with divergent nutrient inputs) at 11 locations with variable landscape composition within an agroecosystem context. We then used these interaction networks to estimate the potential for apparent competition between each host pair and to compare expected versus realised attack rates across the system. Shared natural enemies were found to structure host herbivore communities within and across habitat boundaries. The size of this effect was related to the resource availability of habitats, such that the habitat with high nutrient input exerted a stronger effect. The overall potential for apparent competition declined with increasing land-use intensity in the surrounding landscape and exhibited a discernible impact on realised attack rates upon herbivore species. Thus, our results suggest that increasing the proportion of perennial habitat in agroecosystems could increase the prevalence of indirect effects such as apparent competition among insect herbivore communities, potentially leading to enhanced population regulation via increased attack rates from natural enemies like parasitoid wasps.

Knowledge about herbivores and their parasitoids in forest canopies remains limited, despite their diversity and ecological importance. Thus, it is important to understand the factors that shape the herbivore–parasitoid community structure, particularly the effect of vertical gradient. We investigated a quantitative community dataset of exposed and semiconcealed leaf‐chewing larvae and their parasitoids along a vertical canopy gradient in a temperate forest. We sampled target insects using an elevated work platform in a 0.2 ha broadleaf deciduous forest plot in the Czech Republic. We analyzed the effect of vertical position among three canopy levels (first [lowest], second [middle], and third [highest]) and tree species on community descriptors (density, diversity, and parasitism rate) and food web structure. We also analyzed vertical patterns in density and parasitism rate between exposed and semiconcealed hosts, and the vertical preference of the most abundant parasitoid taxa in relation to their host specificity. Tree species was an important determinant of all community descriptors and food web structure. Insect density and diversity varied with the vertical gradient, but was only significant for hosts. Both host guilds were most abundant in the second level, but only the density of exposed hosts declined in the third level. Parasitism rate decreased from the first to third level. The overall parasitism rate did not differ between guilds, but semiconcealed hosts suffered lower parasitism in the third level. Less host‐specific taxa (Ichneumonidae, Braconidae) operated more frequently lower in the canopy, whereas more host‐specific Tachinidae followed their host distribution. The most host‐specific Chalcidoidea preferred the third level. Vertical stratification of insect density, diversity, and parasitism rate was most pronounced in the tallest tree species. Therefore, our study contradicts the general paradigm of weak arthropod stratification in temperate forest canopies. However, in the network structure, vertical variation might be superseded by variation among tree species.

The importance of competition in phytphagous insect communities is a highly controversial and unresolved issue. This debate was resolved, in part, by the recognition that species may compete indirectly as well as directly. I first test the importance of direct competition on the fitness of L. helianthi and C. platyptera with a series of laboratory experiments which manipulate the densities of each species. I focus on the leafminers Liriomyza helianthi and Calicomyza platyptera because these two leafminer species co-occur spatiotemporally, are closely related, and share a cryptic feeding niche, and, are therefore predicated to compete and provide a model system for testing the assumptions of competition theory. Liriomyza helianthi and C. platyptera along with their community of hymenopteran parasitoids provide a model system for studying apparent competition in the field. Apparent competition, one indirect pathway for competition, is defined as a negative effect of one species on the population growth rate or abundance of another species, mediated through the action of shared natural enemies. I surveyed populations of L. helianthi and C. platyptera, along with their community of parasitoids, over two years at seven sites in the Central Valley of California. Quantitative food webs for each year were used to visualize host-parasitoid interactions. I found the same dominant parasitoids were shared by both species of herbivore. Parasitism of C. platypera, was significantly positively correlated with L. helianthi leaf-miner density and parasitism levels. Indirect interactions can extend herbivore competition across space and time. Apparent competition has generated much recent attention, but studies that test the mechanisms that lead to apparent competition are still rare. I use experimental data to test hypotheses on the mechanisms leading to asymmetric apparent competition. Liriomyza helianthi populations emerge approximately one month (two parasitoid generations) before C. platyptera and have a higher population density throughout the season. Experimental removal of L. helianthi populations in the early summer leads to a 50% reduction in parasitism of C. platyptera. Because parasitoids are known to be important mortality agents in this system, I conclude that emergence asynchrony between the two herbivore species significantly impacts populations of C. platyptera. Land-use intensification has led to a mosaic landscape which juxtaposes human-managed and natural areas. In such human-dominated and heterogeneous landscapes, spillover across habitat types, especially in systems which differ in resource availability, may be an important ecological process structuring communities. While there is much evidence for spillover from natural habitats to managed areas, little attention has been given to flow in the opposite direction. Here we synthesize studies from five functionally important trophic groups, herbivores, pathogens, pollinators, predators, and seed dispersers, and discuss evidence for spillover from managed to natural systems in all five groups. I argue that this effect has been underestimated in the past. The most frequently documented examples of spillover from managed to natural systems report pathogen and vertebrate predator spillover, with a smaller number of examples reporting spillover of pollinators and herbivores.

Understanding how arthropod pests and their natural enemies interact in complex agroecosystems is essential for pest management programmes. Theory predicts that prey sharing a predator, such as a biological control agent, can indirectly reduce each other's density at equilibrium (apparent competition). From this premise, we (i) discuss the complexity of indirect interactions among pests in agroecosystems and highlight the importance of natural enemy-mediated indirect interactions other than apparent competition, (ii) outline factors that affect the nature of enemy-mediated indirect interactions in the field and (iii) identify the way to manipulate enemy-mediated interactions for biological control. We argue that there is a need to increase the link between community ecology theory and biological control to develop better agroecological methods of crop protection via conservation biological control. In conclusion, we identify (i) interventions to be chosen depending on agroecosystem characteristics and (ii) several lines of research that will improve the potential for enemy-mediated indirect interactions to be applied to biological control. © 2014 Society of Chemical Industry

Declining plant species richness in agro-ecosystems and thus reduced habitat quality can have cascading effects on ecosystem functioning, leading to reduced pollination and biological control. Here we test if plant diversity can affect arthropod diversity and abundance on a very small scale, manipulating plant species richness (2, 6, 12 and 20 sown species) in small adjacent subplots (63 9 m) in 10 wildflower strips in an agricultural landscape. We simultaneously analyzed the effect of plant species richness, vegetation structure, and plant composition on the species richness and abundance of cavity-nesting wild bees, wasps, their prey and natural enemies, and on the structure of their food webs. By separating the trap-nesting species into functional groups according to their prey, we aimed to understand the underlying patterns for the effects of plant diversity. Increasing plant species richness had a significant effect only on spider-predating wasps, the group of wasps trophically most distant from plants. In contrast, bees and food-web structure were unaffected by plant diversity. Spider-predating wasp abundance negatively correlated with the abundance of spiders, suggesting top-down control. Interestingly, the abundance of spiders was the only variable that was strongly affected by plant composition. The hypothesis that the effect of plant diversity decreases with increasing trophic level is not supported by our study, and the mobility of species appears to play a greater role at this small spatial scale

Understanding how arthropod pests and their natural enemies interact in complex agroecosystems is essential for pest management programmes. Theory predicts that prey sharing a predator, such as a biological control agent, can indirectly reduce each other's density at equilibrium (apparent competition). From this premise, we (i) discuss the complexity of indirect interactions among pests in agroecosystems and highlight the importance of natural enemy-mediated indirect interactions other than apparent competition, (ii) outline factors that affect the nature of enemy-mediated indirect interactions in the field and (iii) identify the way to manipulate enemy-mediated interactions for biological control. We argue that there is a need to increase the link between community ecology theory and biological control to develop better agroecological methods of crop protection via conservation biological control. In conclusion, we identify (i) interventions to be chosen depending on agroecosystem characteristics and (ii) several lines of research that will improve the potential for enemy-mediated indirect interactions to be applied to biological control. © 2014 Society of Chemical Industry

Understanding the environmental factors that structure biodiversity and food webs among communities is central to assess and mitigate the impact of landscape changes. Wildflower strips are ecological compensation areas established in farmland to increase pollination services and biological control of crop pests and to conserve insect diversity. They are arranged in networks in order to favour high species richness and abundance of the fauna. We describe results from experimental wildflower strips in a fragmented agricultural landscape, comparing the importance of landscape, of spatial arrangement and of vegetation on the diversity and abundance of trap-nesting bees, wasps and their enemies, and the structure of their food webs. The proportion of forest cover close to the wildflower strips and the landscape heterogeneity stood out as the most influential landscape elements, resulting in a more complex trap-nest community with higher abundance and richness of hosts, and with more links between species in the food webs and a higher diversity of interactions. We disentangled the underlying mechanisms for variation in these quantitative food web metrics. We conclude that in order to increase the diversity and abundance of pollinators and biological control agents and to favour a potentially stable community of cavity-nesting hymenoptera in wildflower strips, more investment is needed in the conservation and establishment of forest habitats within agro-ecosystems, as a reservoir of beneficial insect populations.