Patterns of Microbiome Composition Vary Across Spatial Scales in a Specialist Insect.

Microbial communities associated with animals vary based on both intrinsic and extrinsic factors. Of many possible determinants affecting microbiome composition, host phylogeny, host diet, and local environment are the most important. How these factors interact across spatial scales is not well understood. Here, we seek to identify the main influences on microbiome composition in a specialist insect, the western corn rootworm (WCR; Diabrotica virgifera virgifera), by analyzing the bacterial communities of adults collected from their obligate host plant, corn (Zea mays), across several geographic locations and comparing the patterns in communities to its congeneric species, the northern corn rootworm (NCR; Diabrotica barberi). We found that bacterial communities of WCR and NCR shared a portion of their bacterial communities even when collected from disparate locations. However, within each species, the location of collection significantly influenced the composition of their microbiome. Correlations of geographic distance between sites with WCR bacterial community composition revealed different patterns at different spatial scales. Community similarity decreased with increased geographic distance at smaller spatial scales (~25 km between the nearest sites). At broad spatial scales (>200 km), community composition was not correlated with distances between sites, but instead reflected the historical invasion path of WCR across the United States. These results suggest bacterial communities are structured directly by dispersal dynamics at small, regional spatial scales, while landscape-level genetic or environmental differences may drive community composition across broad spatial scales in this specialist insect.

Interrelationship of soil moisture and temperature to sylvatic plague cycle among prairie dogs in the Western United States.

Plague, caused by Yersinia pestis, is a flea-borne disease that is endemic in areas throughout the world due to its successful maintenance in a sylvatic cycle, mainly in areas with temperate climates. Burrowing rodents are thought to play a key role in the enzootic maintenance as well as epizootic outbreaks of plague. In the United States, prairie dogs (Cynomys), rodents (Muridae), and ground squirrels (Spermophilus) are susceptible to infection and are parasitized by fleas that transmit plague. In particular, prairie dogs can experience outbreaks that rapidly spread, which can lead to extirpation of colonies. A number of ecological parameters, including climate, are associated with these epizootics. In this study, we asked whether soil parameters, primarily moisture and temperature, are associated with outbreaks of plague in black-tailed prairie dogs and Gunnison's prairie dogs in the Western United States, and at what depth these associations were apparent. We collected publicly available county-level information on the occurrence of population declines or colony extirpation, while historical soil data was collected from SCAN and USCRN stations in counties and states where prairie dogs have been located. The analysis suggests that soil moisture at lower depths correlates with colony die-offs, in addition to temperature near the surface, with key differences within the landscape ecology that impact the occurrence of plague. Overall, the model suggests that the burrow environment may play a significant role in the epizootic spread of disease amongst black-tailed and Gunnison's prairie dogs.

Presence-Absence Sampling Plans for Stink Bugs (Hemiptera: Pentatomidae) in the Midwest Region of the United States.

Stink bugs represent an increasing risk to soybean production in the Midwest region of the United States. The current sampling protocol for stink bugs in this region is tailored for population density estimation and thus is more relevant to research purposes. A practical decision-making framework with more efficient sampling effort for management of herbivorous stink bugs is needed. Therefore, a binomial sequential sampling plan was developed for herbivorous stink bugs in the Midwest region. A total of 146 soybean fields were sampled across 11 states using sweep nets in 2016, 2017, and 2018. The binomial sequential sampling plans were developed using combinations of five tally thresholds at two proportion infested action thresholds to identify those that provided the best sampling outcomes. Final assessment of the operating characteristic curves for each plan indicated that a tally threshold of 3 stink bugs per 25 sweeps, and proportion infested action thresholds of 0.75 and 0.95 corresponding to the action thresholds of 5 and 10 stink bugs per 25 sweeps, provided the optimal balance between highest probability of correct decisions (≥ 99%) and lowest probability of incorrect decisions (≤ 1%). In addition, the average sample size for both plans (18 and 12 sets of 25 sweeps, respectively) was lower than that for the other proposed plans. The binomial sequential sampling plan can reduce the number of sample units required to achieve a management decision, which is important because it can potentially reduce risk/cost of management for stink bugs in soybean in this region.

Host resistance to Bacillus thuringiensis is linked to altered bacterial community within a specialist insect herbivore.

Evolution of resistance to transgenic crops producing toxins from Bacillus thuringiensis (Bt) threatens the sustainability of the technology. Examination of resistance mechanisms has largely focused on characterization of mutations in proteins serving as Bt toxin binding sites. However, insect microbial communities have the potential to provide host resistance to pesticides in a myriad of ways. Previous findings suggest the killing mechanism of Bt relies on enteric bacteria becoming pathogenic in the disrupted gut environment of the insect following Bt intoxication. Thus, here we hypothesized that resistance to Bt would alter the microbiome composition of the insect. Previous studies have manipulated the microbiome of susceptible insects and monitored their response to Bt. In our study, we characterized the associated bacterial communities of Bt-resistant and -susceptible western corn rootworms, a widespread pest of maize in the United States. We found resistant insects harbor a bacterial community that is less rich and distinct from susceptible insects. After feeding on Bt-expressing maize, susceptible insects exhibited dysbiosis of the associated bacterial community, whereas the community within resistant insects remained relatively unchanged. These results suggest resistance to Bt produces alterations in the microbiome of the western corn rootworm that may contribute to resistance. We further demonstrated that by itself, feeding on Bt toxin-expressing seedlings caused a shift in the microbiota. This work provides a broader picture of the effect stressors have on microbiome composition, and the potential heritable changes induced as a result of intense selection.

Effects of Plant Stress on Aphid-Parasitoid Interactions: Drought Improves Aphid Suppression.

Drought events have the potential to mediate tri-trophic interactions. Changes in plant quality influence herbivore performance, which affects prey availability and quality for natural enemies. Parasitoid wasps are particularly vulnerable to these changes since their development is inextricably linked to that of their prey. However, we know little about the indirect effects of plant water limitation on parasitoid performance, especially the consequences of mild water limitation. This study investigated the ability of a parasitoid wasp (Aphidius colemani Viereck (Hymenoptera: Braconidae)) to suppress aphid (Rhopalosiphum padi L. (Hemiptera: Aphididae)) populations on well-watered, mildly stressed, or highly stressed wheat (Triticum aestivum L. (Poales: Poaceae)). We then investigated the role that aphid body size and behavioral interactions might play in wasp acceptance of aphid hosts. We found improved aphid suppression under mild and high-water limitation, but the underlying mechanisms were different. Mild-stress appeared to maximize parasitoid performance, as indicated by the highest mummy production. Aphids were larger when feeding on mildly stressed plants than on any other treatment, which suggests improved aphid quality for parasitoids. Improved aphid suppression under high-stress may have been driven by enhanced nonconsumptive effects. Despite improved aphid suppression, mummy formation was lowest on highly stressed plants. High-stress conditions have been shown to negatively affect aphid performance, so improved aphid suppression may be driven by poor aphid performance exacerbating the cost of interactions with parasitoids, such as stinging. No differences were observed in parasitoid foraging behaviors such as antennation or stinging across any treatments. This study highlights the importance of plant water stress intensity in affecting outcomes of parasitoid-host interactions.

Primacy of plants in driving the response of arthropod communities to drought.

Drought threatens arthropod communities worldwide. Water limitation affects the quantity and quality of plants available to herbivores as food, and can also affect higher trophic-level consumers through variability in prey quality and reduced availability of suitable habitats. Our study assessed the response of an arthropod community to water limited wheat (Triticum aestivum L.) in a field setting. We used rainout shelters to exclude precipitation, irrigated raised bed plots to create three levels of water availability, and monitored arthropod community development over 8 weeks. First, we compared arthropod communities in habitats with different levels of water limitation and found that community composition was reliant on the magnitude of the water stress. This difference was largely due to the loss of piercing-sucking herbivores and predators in high-stress environments. Next, we focused on aphids and their natural enemies to investigate the underlying mechanisms driving community responses using structural equation modeling (SEM). Aphid abundance was negatively affected by water limitation, and this response was primarily associated with stress-induced plant physiological changes and not plant biomass or natural enemy abundance. Natural enemy abundance was also reduced in water-limited habitats, but natural enemies responded to plant biomass and not prey availability. These effects were exacerbated as water stress increased. The absence of natural enemy effects on aphids indicates that top-down predation effects were dampened by strong bottom-up effects of plant water limitation. This study revealed the importance of considering water stress intensity when predicting outcomes of droughts for arthropod communities.

Non-consumptive effects stabilize herbivore control over multiple generations.

Understanding the factors that influence predator-prey dynamics requires an investigation of oscillations in predator and prey population sizes over time. However, empirical studies are often performed over one or fewer predator generations. This is particularly true for studies addressing the non-consumptive effects of predators on prey. In a previous study that lasted less than one predator generation, we demonstrated that two species of parasitoid wasps additively suppressed aphid populations through a combination of consumptive and non-consumptive effects. However, the non-consumptive effects of one wasp reduced the reproductive success of the other, suggesting that a longer-term experiment may have revealed antagonism between the wasps. The goal of our current study is to evaluate multi-generation consumptive and non-consumptive interactions between pea aphids (Acyrthosiphon pisum) and the wasps Aphidius ervi and Aphidius colemani. Aphidius ervi is a common natural enemy of pea aphids. Aphidius colemani is a non-consumptive enemy that does not consume pea aphids, but negatively affects pea aphid performance through behavioral disturbance. Large field cages were installed to monitor aphid abundance in response to the presence and absence of both species of wasp over four weeks (two parasitoid generations). We found that the non-consumptive enemy A. colemani initially controlled the pea aphid population, but control in the absence of parasitism was not sustainable over the long term. Aphidius ervi suppressed pea aphids through a combination of consumptive and non-consumptive effects. This suppression was more effective than that of A. colemani, but aphid abundance fluctuated over time. Suppression by A. ervi and A. colemani together was complementary, leading to the most effective and stable control of pea aphids. Therefore, promoting a diverse natural enemy community that contributes to pest control through consumptive and non-consumptive interactions may enhance the stability of herbivore population suppression over time.

Parasitism of Adult Pentatomidae by Tachinidae in Soybean in the North Central Region of the United States.

Stink bugs (Hemiptera: Pentatomidae) are agricultural pests of increasing significance in the North Central Region of the United States, posing a threat to major crops such as soybean. Biological control can reduce the need for insecticides to manage these pests, but the parasitism of stink bugs by Tachinidae (Diptera) is poorly characterized in this region. The objective of this study was to evaluate the rate of parasitism of stink bugs by tachinids over 2 yr from nine states across the North Central Region. Parasitism was assessed by quantifying tachinid eggs on the integument of stink bug adults. Parasitism rates (i.e., percent of adult stink bugs with tachinid eggs) were compared across stink bug species, states, stink bug sex, and years. The mean percent parasitism of stink bugs by tachinids was about 6% across the region and did not differ among stink bug species. Mean percent parasitism was significantly higher in Missouri than in northern and western states. In addition, male stink bugs had significantly higher mean percent parasitism than females. Stink bug species commonly found in soybean in the region showed some parasitism and are therefore potentially vulnerable to oviposition by these parasitoids. This is the first study to characterize the level of parasitism of stink bugs by tachinids across the North Central Region.

Author Correction: Multidimensional approach to formulating a specialized diet for northern corn rootworm larvae.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Tri-trophic interactions: bridging species, communities and ecosystems.

A vast body of research demonstrates that many ecological and evolutionary processes can only be understood from a tri-trophic viewpoint, that is, one that moves beyond the pairwise interactions of neighbouring trophic levels to consider the emergent features of interactions among multiple trophic levels. Despite its unifying potential, tri-trophic research has been fragmented, following two distinct paths. One has focused on the population biology and evolutionary ecology of simple food chains of interacting species. The other has focused on bottom-up and top-down controls over the distribution of biomass across trophic levels and other ecosystem-level variables. Here, we propose pathways to bridge these two long-standing perspectives. We argue that an expanded theory of tri-trophic interactions (TTIs) can unify our understanding of biological processes across scales and levels of organisation, ranging from species evolution and pairwise interactions to community structure and ecosystem function. To do so requires addressing how community structure and ecosystem function arise as emergent properties of component TTIs, and, in turn, how species traits and TTIs are shaped by the ecosystem processes and the abiotic environment in which they are embedded. We conclude that novel insights will come from applying tri-trophic theory systematically across all levels of biological organisation.

Survey of bacteria associated with western corn rootworm life stages reveals no difference between insects reared in different soils.

Western corn rootworm (Diabrotica virgifera virgifera LeConte) is a serious pest of maize (Zea mays L.) in North America and parts of Europe. With most of its life cycle spent in the soil feeding on maize root tissues, this insect is likely to encounter and interact with a wide range of soil and rhizosphere microbes. Our knowledge of the role of microbes in pest management and plant health remains woefully incomplete, yet that knowledge could play an important role in effective pest management strategies. For this study, insects were reared on maize in soils from different locations. Insects from two different laboratory colonies (a diapausing and a non-diapausing colony) were sampled at each life stage to determine the possible core bacteriome. Additionally, soil was sampled at each life stage and resulting bacteria were identified to determine the possible contribution of soil to the rootworm bacteriome, if any. We analyzed the V4 hypervariable region of bacterial 16S rRNA genes with Illumina MiSeq to survey the different species of bacteria associated with the insects and the soils. The bacterial community associated with insects was significantly different from that in the soil. Some differences appear to exist between insects from non-diapausing and diapausing colonies while no significant differences in community composition existed between the insects reared on different soils. Despite differences in the bacteria present in immature stages and in male and female adults, there is a possible core bacteriome of approximately 16 operational taxonomic units (i.e., present across all life stages). This research may provide insights into Bt resistance development, improved nutrition in artificial rearing systems, and new management strategies.

Species interactions affect the spread of vector-borne plant pathogens independent of transmission mode.

Within food webs, vectors of plant pathogens interact with individuals of other species across multiple trophic levels, including predators, competitors, and mutualists. These interactions may in turn affect vector-borne pathogens by altering vector fitness and behavior. Predators, for example, consume vectors and reduce their abundance, but often spur movement of vectors as they seek to avoid predation. However, a general framework to predict how species interactions affect vectors of plant pathogens, and the resulting spread of vector-borne pathogens, is lacking. Here we developed a mathematical model to assess whether interactions such as predation, competition, and mutualism affected the spread of vector-borne plant pathogens with nonpersistent or persistent transmission modes. We considered transmission mode because interactions affecting vector-host encounter rates were expected to most strongly affect nonpersistent pathogens that are transmitted with short feeding bouts; interactions that affect vector feeding duration were expected to most strongly affect persistent pathogens that require long feeding bouts for transmission. Our results show that interactions that affected vector behavior (feeding duration, vector-host encounter rates) substantially altered rates of spread for vector-borne plant pathogens, whereas those affecting vector fitness (births, deaths) had relatively small effects. These effects of species interactions were largely independent of transmission mode, except when interactions affected vector-host encounter rates, where effects were strongest for nonpersistent pathogens. Our results suggest that a better understanding of how vectors interact with other species within food webs could enhance our understanding of disease ecology.

The 'Botanical Triad': The Presence of Insectary Plants Enhances Natural Enemy Abundance on Trap Crop Plants in an Organic Cabbage Agro-Ecosystem.

Habitat manipulation through the incorporation of non-crop plants such as trap crops (to lure pests away from the cash crop) and insectary plants (to provide resources for natural enemies) into agro-ecosystems is an ecological approach to pest management. In a field-scale study, we quantified the effects of integrating the use of trap crops with insectary plants as a novel method to control pest herbivores in an organic cabbage agro-ecosystem. We hypothesized that pests would be concentrated in the trap crop habitat and suppressed by insectary-subsidized natural enemies in situ. We documented arthropod abundance (both adults and immature stages) associated with (1) two insectary plant species (sweet alyssum, Lobularia maritima, and buckwheat, Fagopyrum esculentum) either alone or in combination; (2) a trap crop mixture of mighty mustard (Brassica juncea), red Russian kale (Brassica oleracea var. acephala), and glossy collards (Brassica oleracea var. italica), and (3) cabbage cash crop (Brassica oleracea var. capitata). Trap crops were more attractive to pests than the cash crop. On a per-plant basis, densities of the herbivores Evergestis rimosalis, Trichoplusia ni, and Plutella xylostella were 154, 37, and 161× greater on the kale trap crop than on the cabbage cash crop, and 54, 18, and 89× greater on the collards trap crop than on the cash crop. Insectary plants contributed to the consumption of pests that aggregated on the trap crop. Parasitism of E. rimosalis by the braconid wasp Cotesia orobenae was significantly increased, and the abundance of eggs and larvae of the predatory coccinellid beetle Coleomegilla maculata was greater on the trap crop in the presence of insectary plants compared to trap crops that lacked insectary plants. The 'Botanical Triad' of cash crop, trap crop, and insectary plants represents a new type of agro-ecosystem manipulation that integrates ecosystem service providers (e.g., predators and parasitoids) within the cropping system.

Community Composition, Abundance, and Phenology of Stink Bugs (Hemiptera: Pentatomidae) in Soybean in the North Central Region of the United States.

Stink bugs (Hemiptera: Pentatomidae) are an increasing threat to soybean (Fabales: Fabaceae) production in the North Central Region of the United States, which accounts for 80% of the country's total soybean production. Characterization of the stink bug community is essential for development of management programs for these pests. However, the composition of the stink bug community in the region is not well defined. This study aimed to address this gap with a 2-yr, 9-state survey. Specifically, we characterized the relative abundance, richness, and diversity of taxa in this community, and assessed phenological differences in abundance of herbivorous and predatory stink bugs. Overall, the stink bug community was dominated by Euschistus spp. (Hemiptera: Pentatomidae) and Chinavia hilaris (Say) (Hemiptera: Pentatomidae). Euschistus variolarius (Palisot de Beauvois) (Hemiptera: Pentatomidae), C. hilaris and Halyomorpha halys (Stål) (Hemiptera: Pentatomidae) were more abundant in the northwestern, southeastern and eastern parts, respectively, of the North Central Region of the United States. Economically significant infestations of herbivorous species occurred in fields in southern parts of the region. Species richness differed across states, while diversity was the same across the region. Herbivorous and predatory species were more abundant during later soybean growth stages. Our results represent the first regional characterization of the stink bug community in soybean fields and will be fundamental for the development of state- and region-specific management programs for these pests in the North Central Region of the United States.

Spatial Patterns and Sequential Sampling Plans for Estimating Densities of Stink Bugs (Hemiptera: Pentatomidae) in Soybean in the North Central Region of the United States.

Stink bugs are an emerging threat to soybean (Fabales: Fabaceae) in the North Central Region of the United States. Consequently, region-specific scouting recommendations for stink bugs are needed. The aim of this study was to characterize the spatial pattern and to develop sampling plans to estimate stink bug population density in soybean fields. In 2016 and 2017, 125 fields distributed across nine states were sampled using sweep nets. Regression analyses were used to determine the effects of stink bug species [Chinavia hilaris (Say) (Hemiptera: Pentatomidae) and Euschistus spp. (Hemiptera: Pentatomidae)], life stages (nymphs and adults), and field locations (edge and interior) on spatial pattern as represented by variance-mean relationships. Results showed that stink bugs were aggregated. Sequential sampling plans were developed for each combination of species, life stage, and location and for all the data combined. Results for required sample size showed that an average of 40-42 sample units (sets of 25 sweeps) would be necessary to achieve a precision of 0.25 for stink bug densities commonly encountered across the region. However, based on the observed geographic gradient of stink bug densities, more practical sample sizes (5-10 sample units) may be sufficient in states in the southeastern part of the region, whereas impractical sample sizes (>100 sample units) may be required in the northwestern part of the region. Our findings provide research-based sampling recommendations for estimating densities of these emerging pests in soybean.

Multidimensional approach to formulating a specialized diet for northern corn rootworm larvae.

The northern corn rootworm (NCR), Diabrotica barberi Smith & Lawrence, is a major pest of maize (Zea mays L.). This pest has developed resistance to insecticides and adapted to crop rotation and may already be in the early stages of adaptation to toxins produced by Bacillus thuringiensis (Bt). Toxicity bioassays using artificial diet have proven to be valuable for monitoring resistance in many species, but no artificial diet has been developed specifically for NCR larvae. Toward this end, we first evaluated known Diabrotica diets to identify a starting media. We then developed a specialized diet for NCR using an iterative approach. Screening designs including 8 diet components were performed to identify the principal nutritional components contributing to multiple developmental parameters (survival, weight, and molting). We then applied mixture designs coupled with response surface modeling to optimize a blend of those components. Finally, we validated an improved NCR diet formulation that supports approximately 97% survival and molting, and a 150% increase in larval weight after 10 days of feeding compared with the best previously published artificial diet. This formulation appears suitable for use in diet bioassays as a tool for evaluating the resistance of NCR populations to insecticides.

Multi-species suppression of herbivores through consumptive and non-consumptive effects.

Most studies investigating the importance of non-consumptive interactions for herbivore suppression focus on pairwise interactions between one predator and one prey, ignoring any community context. Further, the potential for non-consumptive interactions to arise between herbivores and non-enemy organisms is commonly overlooked. We investigated the relative contributions of consumptive and non-consumptive effects to aphid suppression by a wasp assemblage containing both enemies and non-enemies. We examined the suppression of two aphid species with different defensive strategies, pea aphids (Acyrthosiphon pisum), which drop from their host plant to the ground, and green peach aphids (Myzus persicae), which remain on the plant and merely walk away. The expectation was that riskier defensive behaviors, like abandoning the plant, would result in larger non-consumptive effects. We found that the outcome of multi-species interactions differed depending on the mechanism of suppression, with interference among wasps in their consumptive effects and additivity in their non-consumptive effects. We also found that, despite differences in defensive strategies, the non-consumptive effects of wasps on aphid abundance were significant for both aphid species. Furthermore, when part of a multi-species assemblage, non-enemies enhanced aphid suppression via complementary non-consumptive effects with lethal enemies, but this increase in suppression was offset by disruption in the consumptive suppression of aphids by lethal enemies. We conclude that non-consumptive effects arise from interactions with both enemy and non-enemy species and that both can contribute to herbivore suppression when part of a broader community. We predict that encouraging the presence of non-enemy organisms may provide insurance against fluctuations in the size of consumptive enemy populations and buffer against herbivore outbreaks.

The Influence of Exotic Lady Beetle (Coleoptera: Coccinellidae) Establishment on the Species Composition of the Native Lady Beetle Community in Missouri.

The diversity and abundance of native lady beetles (Coccinellidae) in North America has declined in recent decades. This decline is often correlated with the introduction and establishment of exotic lady beetle species, including Coccinella septempunctata L. and Harmonia axyridis Pallas, suggesting that exotic species precipitated the decline of native lady beetles. We examined species records of native coccinellids in Missouri over 118 yr and asked whether the species composition of the community experienced a shift following the establishment of the exotic species. We found that the contemporary native coccinellid community is different from the community that was present nearly a century ago. However, there was no evidence for a recent abrupt shift in composition triggered by the establishment of exotic species. Instead, our data suggest that the native lady beetle community has been undergoing consistent and gradual change over time, with some species decreasing in abundance and others increasing. While not excluding exotic species as a factor contributing to the decline of native lady beetle species, our findings suggest that other continuous factors, like land use change, may have played a more influential role in determining the composition of the native coccinellid communities within our region.

Predators indirectly reduce the prevalence of an insect-vectored plant pathogen independent of predator diversity.

A widely cited benefit of predator diversity is greater suppression of insect herbivores, with corresponding increases in plant biomass. In the context of a vector-borne pathogen system, predator species richness may also influence plant disease risk via the direct effects of predators on the abundance and behavior of herbivores that also act as pathogen vectors. Using an assemblage of generalist insect predators, we examined the relationship between predator species richness and the prevalence of the aphid-vectored cereal yellow dwarf virus in wheat. We found that increasing predator richness enhanced suppression of the vector population and that pathogen prevalence was reduced when predators were present, but the reduction in prevalence was independent of predator species richness. To determine the mechanism(s) by which predator species richness contributes to vector suppression, but not pathogen prevalence, we evaluated vector movement and host plant occupancy in response to predator treatments. We found that pathogen prevalence was unrelated to vector suppression because host plant occupancy by vectors did not vary as a function of vector abundance. However, the presence of predators reduced pathogen prevalence because predators stimulated greater plant-to-plant movement by vectors, which likely diminished vector feeding time and reduced the transmission efficiency of this persistent pathogen. We conclude that community structure (i.e., the presence of predators), but not predator diversity, is a potential factor influencing local plant infection by this insect-vectored pathogen.

Contribution of predator identity to the suppression of herbivores by a diverse predator assemblage.

Studies manipulating predator diversity and measuring the impact on herbivore abundance have found that enhancing predator species richness often increases the strength of prey suppression. This relationship may be due to mechanisms such as complementarity or facilitation, which are considered "true" benefits of diversity because greater prey suppression is an emergent property of the multispecies predator community. Or it may be due to an identity effect, an "apparent" benefit of diversity that results from the greater likelihood of including one particularly voracious predator species as the total number of predator species increases. In separate greenhouse and field experiments, we simultaneously manipulated the species richness and species composition of predators attacking bird cherry-oat aphids (Rhopalosiphum padi) (L.) on wheat (Triticum aestivum L.). We found that on average aphid suppression by species-rich predator assemblages was greater than suppression by single-species monocultures. However, the performance of individual predator species varied and the species-rich assemblages did not outperform all single-species compositions, suggesting an identity effect. In particular, single-species compositions of the lady beetle Coleomegilla maculata (DeGeer) exhibited high performance across experiments, and on average predator assemblages that contained a lady beetle predator had lower overall aphid abundance than compositions where lady beetles were absent. Taken together, these results provide evidence for the dominant role of lady beetles, especially C. maculata, in natural pest suppression and suggest that predator species composition and identity are important factors to consider in efforts to conserve this valuable ecosystem service.