Does insect herbivory on oak depend on the diversity of tree stands?

Studies on the effects of plant diversity on insect herbivory have produced conflicting results. Plant diversity has been reported to cause positive and negative responses of herbivores. Explanations for these conflicting responses include not only various population-level processes but also changes in plant quality that lead to changes in herbivore performance. In a tree diversity experiment, we investigated the effects of tree diversity on insect herbivory on oak in general and whether the effects of tree diversity on herbivore damage are reflected by the performance (leaf consumption, growth) of the generalist herbivore Lymantria dispar. Our study showed that the feeding damage caused by naturally occurring herbivores on oak trees decreased with increasing diversity of tree stands. The performance of L. dispar on oak leaves was not affected by tree diversity, neither in field nor laboratory experiments. Our results can be explained by the various processes behind the hypothesis of associational resistance.     

Influence of surrounding vegetation on insect herbivory: A matter of spatial scale and herbivore specialisation

The diversity of surrounding vegetation is thought to modify the interactions between a focal plant and its herbivores, disrupting (associational resistance) or enhancing (associational susceptibility) host plant location and colonisation. We compared the effects of host plant concentration on herbivory by generalist and specialist insects feeding on oak seedlings by increasing local concentration of seedlings. We also assessed the effects of the composition and structure of surrounding vegetation, both at stand and local levels. The damage caused by generalist leaf-feeding insects depended on the structure of plant communities at stand level, and increased with tree cover. By contrast, infestation by specialist leaf miners was affected by local understorey vegetation surrounding oak seedlings, and decreased with increasing shrub cover and stratification diversity. Leaf mine abundance was higher at higher oak seedling density, giving support to the host concentration hypothesis. However, the abundance of these specialist herbivores was also negatively correlated with damage caused by the generalist external leaf-feeders, suggesting competitive interactions.     

Comparison of tree genotypic diversity and species diversity effects on different guilds of insect herbivores

Although the effects of plant diversity on herbivores are contingent upon herbivore traits and the source of plant diversity (e.g. intra‐ and interspecific), most studies have analyzed these effects separately. We compared the effects of genotypic diversity of big‐leaf mahogany Swietenia macrophylla with that of tree species diversity on two specialist caterpillars (Hypsipyla grandella stem borers and Phyllocnistis meliacella leaf miners) and three generalist leafhoppers (Cicadellidae) feeding on mahogany in a large‐scale (7.2 ha) forest diversity experiment in southern Mexico. The experiment consisted of fifty‐nine 21 × 21‐m plots, with 64 tree saplings each (3‐m spacing between plants). Plots were either mahogany monocultures or species polycultures of four species (including mahogany) and – within each of these two plot types – mahogany was represented by either one or four genotypes. Throughout a five‐month period, beginning six months after planting, we measured mahogany growth and monitored herbivore and predator (spider) abundance. We found no effect of mahogany genotypic diversity on either specialist caterpillars or generalist leafhoppers, and this result was consistent across levels of tree species diversity. In contrast, species diversity had significant effects on both specialists but neither of the generalist herbivores. Specifically, species diversity lowered H. grandella attack at the middle of the sampling season, but increased attack at the end of the season, whereas P. meliacella abundance was consistently reduced. Such effects were not mediated by effects of species diversity on plant growth (of which there were none), but rather through resource heterogeneity. Diversity did not influence spider abundance. This study is one of few to directly compare sources of plant diversity, and uniquely compares such effects among herbivores with contrasting life histories (e.g. diet breadths). Overall, we demonstrate that plant species diversity effects outweigh those of genotypes, and our results suggest that such effects are stronger on specialist than generalist herbivores.     

Tree diversity drives associational resistance to herbivory at both forest edge and interior

Tree diversity is increasingly acknowledged as an important driver of insect herbivory. However, there is still a debate about the direction of associational effects that can range from associational resistance (i.e., less damage in mixed stands than in monocultures) to the opposite, associational susceptibility. Discrepancies among published studies may be due to the overlooked effect of spatially dependent processes such as tree location within forests. We addressed this issue by measuring crown defoliation and leaf damage made by different guilds of insect herbivores on oaks growing among conspecific versus heterospecific neighbors at forest edges versus interior, in two closed sites in SW France forests. Overall, oaks were significantly less defoliated among heterospecific neighbors (i.e., associational resistance), at both forest edge and interior. At the leaf level, guild diversity and leaf miner herbivory significantly increased with tree diversity regardless of oak location within stands. Other guilds showed no clear response to tree diversity or oak location. We showed that herbivore response to tree diversity varied among insect feeding guilds but not between forest edges and interior, with inconsistent patterns between sites. Importantly, we show that oaks were more defoliated in pure oak plots than in mixed plots at both edge and forest interior and that, on average, defoliation decreased with increasing tree diversity from one to seven species. We conclude that edge conditions could be interacting with tree diversity to regulate insect defoliation, but future investigations are needed to integrate them into the management of temperate forests, notably by better understanding the role of the landscape context.     

Selection Mosaic Exerted by Specialist and Generalist Herbivores on Chemical and Physical Defense of Datura stramonium

Selection exerted by herbivores is a major force driving the evolution of plant defensive characters such as leaf trichomes or secondary metabolites. However, plant defense expression is highly variable among populations and identifying the sources of this variation remains a major challenge. Plant populations are often distributed across broad geographic ranges and are exposed to different herbivore communities, ranging from generalists (that feed on diverse plant species) to specialists (that feed on a restricted group of plants). We studied eight populations of the plant Datura stramonium usually eaten by specialist or generalist herbivores, in order to examine whether the pattern of phenotypic selection on secondary compounds (atropine and scopolamine) and a physical defense (trichome density) can explain geographic variation in these traits. Following co-evolutionary theory, we evaluated whether a more derived alkaloid (scopolamine) confers higher fitness benefits than its precursor (atropine), and whether this effect differs between specialist and generalist herbivores. Our results showed consistent directional selection in almost all populations and herbivores to reduce the concentration of atropine. The most derived alkaloid (scopolamine) was favored in only one of the populations, which is dominated by a generalist herbivore. In general, the patterns of selection support the existence of a selection mosaic and accounts for the positive correlation observed between atropine concentration and plant damage by herbivores recorded in previous studies.     

Strong but opposing effects of associational resistance and susceptibility on defense phenotype in an African savanna plant

The susceptibility of plants to herbivores can be strongly influenced by the identity, morphology and palatability of neighboring plants. While the defensive traits of neighbors often determine the mechanism and strength of associational resistance and susceptibility, the effect of neighbors on plant defense phenotype remains poorly understood. We used field surveys and a prickle‐removal experiment in a semi‐arid Kenyan savanna to evaluate the efficacy of physical defenses against large mammalian herbivores in a common understory plant, Solanum campylacanthum. We then quantified the respective effects of spinescent Acacia trees and short‐statured grasses on browsing damage and prickle density in S. campylacanthum. We paired measurements of prickle density beneath and outside tree canopies with long‐term herbivore‐exclusion experiments to evaluate whether associational resistance reduced defense investment by decreasing browsing damage. Likewise, we compared defense phenotype within and outside pre‐existing and experimentally created clearings to determine whether grass neighbors increased defense investment via associational susceptibility. Removing prickles increased the frequency of browsing by ~25%, and surveys of herbivory damage on defended leaves suggested that herbivores tended to avoid prickles. As predicted, associational resistance and susceptibility had opposing effects on plant phenotype: individuals growing beneath Acacia canopies (or, analogously, within large‐herbivore exclosures) had a significantly lower proportion of their leaves browsed and produced ~ 70–80% fewer prickles than those outside refuges, whereas plants in grass‐dominated clearings were more heavily browsed and produced nearly twice as many prickles as plants outside clearings. Our results demonstrate that associational resistance and susceptibility have strong, but opposing, effects on plant defense phenotype, and that variable herbivore damage is a major source of intraspecific variation in defense phenotype in this system.     

Tree Diversity Limits the Impact of an Invasive Forest Pest

The impact of invasive herbivore species may be lower in more diverse plant communities due to mechanisms of associational resistance. According to the “resource concentration hypothesis” the amount and accessibility of host plants is reduced in diverse plant communities, thus limiting the exploitation of resources by consumers. In addition, the “natural enemy hypothesis” suggests that richer plant assemblages provide natural enemies with more complementary resources and habitats, thus promoting top down regulation of herbivores. We tested these two hypotheses by comparing crown damage by the invasive Asian chestnut gall wasp (Dryocosmus kuriphilus) on chestnut trees (Castanea sativa) in pure and mixed stands in Italy. We estimated the defoliation on 70 chestnut trees in 15 mature stands sampled in the same region along a gradient of tree species richness ranging from one species (chestnut monocultures) to four species (mixtures of chestnut and three broadleaved species). Chestnut defoliation was significantly lower in stands with higher tree diversity. Damage on individual chestnut trees decreased with increasing height of neighboring, heterospecific trees. These results suggest that conservation biological control method based on tree species mixtures might help to reduce the impact of the Asian chestnut gall.     

Going undercover: increasing canopy cover around a host tree drives associational resistance to an insect pest

Neighbouring heterospecific plants are often observed to reduce the probability of herbivore attack on a given focal plant. While this pattern of associational resistance is frequently reported, experimental evidence for underlying mechanisms is rare particularly for potential plant species diversity effects on focal host plants and their physical environment. Here, we used an established forest diversity experiment to determine whether tree diversity effects on an important insect pest are driven by concomitant changes in host tree growth or the light environment. We examined the effects of tree species richness, canopy cover and tree growth on the probability of occurrence, the abundance, and volume of galls caused by the pineapple gall adelgid Adelges abietis on Norway spruce. Although tree diversity had no effect on gall abundance, we observed that both the probability of gall presence and gall volume (an indicator of maternal fecundity) decreased with tree species richness and canopy cover around host spruce trees. Structural equation models revealed that effects of tree species richness on gall presence and volume were mediated by concurrent increases in canopy cover rather than changes in tree growth or host tree density. As canopy cover did not influence tree or shoot growth, patterns of associational resistance appear to be driven by improved host tree quality or more favourable microclimatic conditions in monocultures compared to mixed‐stands. Our study therefore demonstrates that changes in forest structure may be critical to understanding the responses of herbivores to plant diversity and may underpin associational effects in forest ecosystems.     

Individual tree traits shape insect and disease damage on oak in a climate‐matching tree diversity experiment

Diversifying planted forests by increasing genetic and species diversity is often promoted as a method to improve forest resilience to climate change and reduce pest and pathogen damage. In this study, we used a young tree diversity experiment replicated at two sites in the UK to study the impacts of tree diversity and tree provenance (geographic origin) on the oak (Quercus robur) insect herbivore community and a specialist biotrophic pathogen, oak powdery mildew. Local UK, French, and Italian provenances were planted in monocultures, provenance mixtures, and species mixes, allowing us to test whether: (a) local and nonlocal provenances differ in their insect herbivore and pathogen communities, and (b) admixing trees leads to associational effects on insect herbivore and pathogen damage. Tree diversity had variable impacts on foliar organisms across sites and years, suggesting that diversity effects can be highly dependent on environmental context. Provenance identity impacted upon both herbivores and powdery mildew, but we did not find consistent support for the local adaptation hypothesis for any group of organisms studied. Independent of provenance, we found tree vigor traits (shoot length, tree height) and tree apparency (the height of focal trees relative to their surroundings) were consistent positive predictors of powdery mildew and insect herbivory. Synthesis. Our results have implications for understanding the complex interplay between tree identity and diversity in determining pest damage, and show that tree traits, partially influenced by tree genotype, can be important drivers of tree pest and pathogen loads.     

Diversity and composition of arboreal beetle assemblages in tropical pasture afforestations: effects of planting schemes and tree species identity

Timber tree plantations are considered for rehabilitating forest biodiversity in the tropics, but knowledge on determinants of faunal diversity patterns in such human-modified forest landscapes is scarce. We quantified the composition of beetle assemblages on three native timber species (Anacardium excelsum, Cedrela odorata and Tabebuia rosea) planted on former pasture to assess effects of tree species identity, tree species diversity, and insecticide treatment on a speciose group of animals in tropical plantations. The beetle assemblage parameters ‘abundance’, ‘species richness’, ‘Chao1 estimated species richness’ and ‘Shannon diversity’ were significantly reduced by insecticide treatment for each tree species. Shannon diversity increased with stand diversification for T. rosea but not for A. excelsum and C. odorata. Species similarity was highest (lowest species turnover) between beetle assemblages on T. rosea, and it was lowest (highest species turnover) for assemblages on insecticide-treated trees of all timber species. Considering trophic guilds, herbivorous beetles dominated on all tree species and in all planting schemes. Herbivores were significantly more dominant on T. rosea and C. odorata than on A. excelsum, suggesting that tree species identity affects beetle guild structure on plantation trees. Insecticide-treated stands harbored less herbivores than untreated stands, but exhibited a high abundance of predator beetle species. Our study revealed that even young pasture-afforestations can host diverse beetle assemblages and thus contribute to biodiversity conservation in the tropics. The magnitude of this contribution, however, may strongly depend on management measures and on the selected tree species.     

Tree Species Composition and Harvest Intensity Affect Herbivore Density and Leaf Damage on Beech,Fagus sylvatica,in Different Landscape Contexts

Most forests are exposed to anthropogenic management activities that affect tree species composition and natural ecosystem processes. Changes in ecosystem processes such as herbivory depend on management intensity, and on regional environmental conditions and species pools. Whereas influences of specific forest management measures have already been addressed for different herbivore taxa on a local scale, studies considering effects of different aspects of forest management across different regions are rare. We assessed the influence of tree species composition and intensity of harvesting activities on arthropod herbivores and herbivore-related damage to beech trees, Fagus sylvatica, in 48 forest plots in three regions of Germany. We found that herbivore abundance and damage to beech trees differed between regions and that – despite the regional differences - density of tree-associated arthropod taxa and herbivore damage were consistently affected by tree species composition and harvest intensity. Specifically, overall herbivore damage to beech trees increased with increasing dominance of beech trees – suggesting the action of associational resistance processes – and decreased with harvest intensity. The density of leaf chewers and mines was positively related to leaf damage, and several arthropod groups responded to beech dominance and harvest intensity. The distribution of damage patterns was consistent with a vertical shift of herbivores to higher crown layers during the season and with higher beech dominance. By linking quantitative data on arthropod herbivore abundance and herbivory with tree species composition and harvesting activity in a wide variety of beech forests, our study helps to better understand the influence of forest management on interactions between a naturally dominant deciduous forest tree and arthropod herbivores.     

Moose and vole browsing patterns in experimentally assembled pure and mixed forest stands

Plants growing in diverse communities are believed to exhibit associational resistance to herbivores, but this hypothesis has seldom been tested experimentally for vertebrate herbivores in forest ecosystems. We examined browsing patterns of the two principal mammalian herbivores of Finnish boreal forests, moose and voles, in young stands where tree species diversity and composition were experimentally manipulated. The stands were composed either of monocultures or different 2–5 species mixtures of Norway spruce, Scots pine, Siberian larch, silver birch, and black alder. Voles and moose showed contrasting responses to stand diversity and species composition. In accordance with the predictions of the associational resistance hypothesis, vole damage was higher in tree monocultures than in mixed stands, although stand diversity effects were statistically significant only at one of the three study areas. Voles also damaged more trees in coniferous than in deciduous stands. In contrast, moose browsing tended to increase with the number of tree species in a stand and with the presence of the preferred tree species, birch, in a mixture. The observed differences in vole and moose responses to stand diversity and species composition are likely to be due to different feeding specialisation, foraging patterns, and movement ability of these herbivores. Voles switched to trees only when the supply of a more preferred food (grasses and forbs) was depleted and restricted their feeding choice only to the most palatable tree species regardless of the number of tree species present per stand. In contrast, tree branches and foliage represented an important part of moose diet throughout the year; moose may be able to tolerate secondary plant metabolites of different tree species better than voles and may thus benefit from diet broadening when more tree species are available. Furthermore, the home range size and foraging ability of these two very differently sized herbivores may partly explain the observed differences in utilisation of different tree species. Finally, both moose and voles showed high spatial and temporal variation in their feeding; in particular, vole damage was more influenced by tree species diversity in areas and years with high vole densities. Thus, diversification of forest stands may have very different effects on mammalian browsing depending on the herbivores present, their densities, and the tree species used in reforestation.     

Are Tree Species Diversity and Genotypic Diversity Effects on Insect Herbivores Mediated by Ants?

Plant diversity can influence predators and omnivores and such effects may in turn influence herbivores and plants. However, evidence for these ecological feedbacks is rare. We evaluated if the effects of tree species (SD) and genotypic diversity (GD) on the abundance of different guilds of insect herbivores associated with big-leaf mahogany (Swietenia macrophylla) were contingent upon the protective effects of ants tending extra-floral nectaries of this species. This study was conducted within a larger experiment consisting of mahogany monocultures and species polycultures of four species and –within each of these two plot types– mahogany was represented by either one or four maternal families. We selected 24 plots spanning these treatment combinations, 10 mahogany plants/plot, and within each plot experimentally reduced ant abundance on half of the selected plants, and surveyed ant and herbivore abundance. There were positive effects of SD on generalist leaf-chewers and sap-feeders, but for the latter group this effect depended on the ant reduction treatment: SD positively influenced sap-feeders under ambient ant abundance but had no effect when ant abundance was reduced; at the same time, ants had negative effects on sap feeders in monoculture but no effect in polyculture. In contrast, SD did not influence specialist stem-borers or leaf-miners and this effect was not contingent upon ant reduction. Finally, GD did not influence any of the herbivore guilds studied, and such effects did not depend on the ant treatment. Overall, we show that tree species diversity influenced interactions between a focal plant species (mahogany) and ants, and that such effects in turn mediated plant diversity effects on some (sap-feeders) but not all the herbivores guilds studied. Our results suggest that the observed patterns are dependent on the combined effects of herbivore identity, diet breadth, and the source of plant diversity.     

Species-specific herbivore–herbivore interactions and plant responses to sequential attack by multiple herbivores on a perennial woody plant

When plants are sequentially attacked by multiple herbivores, herbivore identity and host specialization can greatly influence the patterns of herbivore–herbivore and plant–herbivore interactions. However, how prior herbivory and the resulting induced plant responses potentially affect subsequent herbivores deserves further investigation. In this study, we conducted a common-garden experiment that manipulated sequential herbivory by the specialist caterpillar Gadirtha fusca Pogue (Lepidoptera: Nolidae) and the generalist caterpillar Spodoptera litura (Fabricius) (Lepidoptera: Noctuidae) on Chinese tallow, Triadica sebifera (L.) Small (Euphorbiaceae). We tested how prior exposure to herbivores with different levels of host specialization affected the performance of subsequently arriving con- and heterospecifics, as well as plant growth and defense responses under subsequent herbivory. We found that prior exposure to the specialist G. fusca facilitated the performance of subsequent conspecifics, resulting in a significant decrease in the growth (height and stem diameter at ground level) of tallow plants. However, prior exposure to the generalist S. litura did not affect the feeding of subsequent con- or heterospecifics or the growth of tallow plants. Sequential herbivory by specialist and generalist conspecifics resulted in lower levels of tannins and flavonoids, respectively, in leaves of tallow plants, whereas sequential herbivory by the two species did not affect the levels of tannins or flavonoids, compared to a single damage event. We conclude that herbivore species-specific plant responses appear to be more important than herbivore identity or specialization in determining herbivore–herbivore interactions and plant responses to sequential herbivore attack.     

No evidence for an ‘evolution of increased competitive ability’ for the invasive Lepidium draba

The ‘evolution of increased competitive ability’ (EICA) hypothesis states that reduced herbivory in the introduced range causes an evolutionary shift in resource allocation from herbivore defense to growth. Therefore, according to EICA, introduced genotypes are expected to grow more vigorously than conspecific native genotypes when cultivated under common standardized conditions. The EICA hypothesis also assumes that herbivores will perform better on introduced genotypes compared to native genotypes, because they are less well defended. However, selection for either defense or growth will depend on the type of defense (quantitative or qualitative) employed by the plant, and whether the plant is released from generalist or specialist herbivores. The predictions of the EICA hypothesis might be reversed if a plant experiences increased generalist herbivore pressure in the introduced range, and therefore invests more in qualitative defense. We examined this idea with the invasive perennial mustard, Lepidium draba. We grew a total of 16 populations of L. draba from both its native European and introduced western US ranges under common conditions in a greenhouse. We also tested for differences in plant resistance to the specialist herbivore, Psylliodes wrasei, by conducting a leaf disc feeding bioassay with native and introduced L. draba genotypes. Furthermore, we quantified the generalist herbivore load on L. draba in both ranges in order to assess the selection pressure for increased qualitative defense. Contrary to the original EICA prediction, all plant traits (biomass, number of shoots, length and diameter of longest leaf) tended to be greater for the native, rather than introduced L. draba genotypes. There was no significant difference in the proportion of leaf area consumed by the specialist herbivore between native and introduced genotypes. The generalist herbivore load on L. draba was significantly greater in the introduced range. Our data suggest that the EICA hypothesis does not explain the invasion success of L. draba in the US. Instead, we propose that the reduced vigor of introduced genotypes may be due to selection for increased defense against generalist herbivores in the introduced range.     

Specialist and generalist herbivores exert opposing selection on a chemical defense

* Plant defense traits often show high levels of genetic variation, despite clear impacts on plant fitness. This variation may be partly maintained by trade-offs in the defense against multiple herbivore species, for example between generalists and coevolved specialists. Despite a long-standing discussion in the literature on the subject, no study to date has specifically manipulated specialist and generalist herbivores independently of one another to determine whether the two guilds exert opposing selection pressures on specific defensive traits. * In two separate experiments, the dominant specialist and generalist herbivores of Brassica nigra were independently manipulated to test whether the composition of the herbivore community altered the direction of selection on a major defensive trait of the plant, sinigrin concentration. * It was found that generalist damage was negatively correlated but specialist loads were positively correlated with increasing sinigrin concentrations; and sinigrin concentration was favored when specialists were removed, disfavored (past an intermediate point) when generalists were removed and selectively neutral when plants faced both generalists and specialists.     

Increasing vole numbers cause more lethal damage to saplings in tree monocultures than in mixed stands

Overall, mammalian herbivores are more harmful in mixed plantations than in monocultures, but the effect of herbivore abundance has not been experimentally tested in this context. It has been proposed that there is a critical threshold density where herbivore pressure spreads from preferred plants to everything edible, leading to non-linear density effects on low-quality plants. We experimentally investigated whether survival of an unpalatable plant is similarly related to herbivore density in both monocultures and mixed stands. This we did by establishing monocultures of unpalatable black alder (Alnus glutinosa) and mixed stands of black alder and five more palatable tree species in enclosures, where Microtus voles were introduced and their abundances monitored.The effect of stand diversity tended to depend on vole abundance. Vole damage of tree saplings did not differ between monocultures and mixed stands, but at higher vole abundances attacks had a stronger effect on sapling survival in the monocultures. Sapling survival showed a significant drop in the monocultures at peak abundance of approximately 300 voles ha^?1. In monocultures herbivores do not have alternatives and therefore are forced to become deadlier consumers.     

Associational refuge in practice: can existing vegetation facilitate woodland restoration?

Herbivores can dramatically diminish revegetation success, but associational refuge theory predicts that neighbouring plants could hinder browsing of planted seedlings. The key to strategic restoration using associational refuge is to define which patch variables are effective against the appropriate herbivores, at multiple scales, and to understand which stages of the foraging process these variables disrupt. Our study aimed to test the capacity of existing vegetation to act as associational refuge for planted seedlings by affecting search, detection and consumption decisions, and more generally influence herbivore foraging patterns. We conducted a field trial with free‐ranging, mammalian herbivores and nursery‐raised, native tree seedlings. We quantified seedling browsing damage over time in relation to a suite of existing patch variables at two spatial scales (100 m² and 4 m²). After two months, 78% of seedlings were browsed, suffering mean foliage loss of 90.5%. Focal seedlings were almost exclusively consumed by swamp wallabies Wallabia bicolor, an abundant generalist browser. Once a swamp wallaby investigated a seedling, the probability of consumption was high (86%). At the large scale, browsing of seedlings was delayed in patches with lower canopy cover and fewer browsed plant species. Seedlings in fern‐dominated patches escaped browsing for longer than those in grass‐dominated patches. At the small scale, browsing was delayed with higher cover of understorey vegetation. Associational refuge was provided by vegetation with characteristics, and at spatial scales, consistent with disrupted search and detection of focal seedlings by herbivores. Thus strategic placement of seedlings in existing vegetation – based on understanding which herbivore species is responsible and how it responds to vegetation – can take advantage of associational refuge during restoration. However, given rapid seedling detection by herbivores, associational refuge may be inadequate in the long‐term under high browsing pressure unless high absolute numbers of seedlings are planted among refuge.     

Herbivore-herbivore interactions complicate links between soil fertility and pest resistance

Soil fertility is tightly linked with herbivore pressure because it affects the nutritional status of host plants as well as the production of anti-herbivore defenses. This in turn can influence whether herbivores in different feeding guilds render plants more or less susceptible to one another. Thus, growers’ fertility management choices may impact herbivores through a variety of indirect channels. We examined relationships between soil fertility and interactions between phloem-feeding and leaf-chewing herbivores on broccoli (Brassica oleracea) plants in the greenhouse, taking advantage of natural variation in nitrogen (N) and phosphorus (P) in soils from 20 working organic vegetable farms. Next, we experimentally fertilized soil in a field trial with N and/or P to examine the consequences of these nutrients for growth of and interactions between specialist and generalist herbivores. Soils on our cooperating farms varied widely in P and N concentrations, with 40% exceeding recommended pre-plant N concentrations and 90% exceeding P recommendations. In single-herbivore infestations, augmenting N in the soil increased caterpillar (Pieris rapae) growth, augmented N and P additively enhanced generalist green peach aphid (Myzus persicae) colonization, and augmented P (but not N) increased specialist cabbage aphid (Brevicoryne brassicae) growth. In dual-guild herbivore infestations, caterpillars facilitated specialist cabbage aphid growth in the absence of fertilizer, but this pattern disappeared under augmented N, and reversed under augmented P. We found that a complex web of indirect effects linked soil fertility to herbivore performance, depending on the identity of the nutrients being altered, the ecological roles of responding herbivore species (i.e., specialist versus generalist), and indirect interactions between chewing and sucking herbivores. More generally, we highlight that successful use of fertility management to improve pest resistance requires careful consideration of herbivore feeding niches and herbivore-herbivore interactions.