Asymmetrical Intraguild Predation Research Articles

Dynamic intraguild interactions between two sympatric and congeneric coccinellid species associated with aphids could explain their coexistence in citrus agroecosystems

Scymnus interruptus and Scymnus subvillosus (Coleoptera: Coccinellidae) are the most frequent aphid predators in Western Mediterranean citrus agroecosystems. Intraguild predation (IGP) between them would be expected, but how IGP might affect their coexistence and biological control function remains unknown. In this work, we assess the frequency of predator encounters in the field and investigate intraguild predation between these two predators in laboratory settings. Different field prey scenarios derived from the field observations were simulated with the two dominant aphid species in this agroecosystem, Aphis gossypii and Aphis spiraecola.Scymnus subvillosus and S. interruptus engaged in IGP but the magnitude and symmetry varied with the prey species and abundance. At high aphid densities, IGP was symmetric and weak, whereas, at low aphid densities, IGP was strong and very asymmetric, with S. interruptus taking the role of the intraguild predator. The developmental time of S. subvillosus was reduced when competing with S. interruptus at high extraguild prey densities. The adult size of both predators under weak IGP also increased even when the developmental time had been reduced.We predict that high aphid infestations in the spring would benefit S. subvillosus, while the strong and asymmetric IGP at low aphid infestations would hinder its coexistence with S. interruptus. Our results give a theoretical explanation to what is observed in the field: S. subvillosus is typically dominant during the high aphid infestations of the spring, whereas S. interruptus increases in relative dominance during the summer and fall when aphid abundance is much less.

Modeling and bifurcation analysis of an intraguild predation system

This paper proposes a mathematical model of an asymmetric intraguild (IG) predation system with an exclusive alternative resource. In particular, this paper analyzes the effects that the exclusive alternative resource has on the consumption/predation behaviors of both the IG predator and IG prey species in the system. The results presented on this paper show that, if the IG predator is less competitive in consumption and less efficient in conversion of the shared resource than that of the IG prey, then there exists a lower bound on the value of the predation rate parameter that should be maintained by IG predator species to ensure its survival and co-existence in the system.

Interaction between Ceraeochrysa valida and Chrysoperla externa (Neuroptera: Chrysopidae), Natural Enemies of Diaphorina citri (Hemiptera: Liviidae)

This work evaluated, under controlled conditions, the predatory capacity and the intraguild predation of Ceraeochrysa valida and Chrysoperla externa, with Diaphorina citri as prey. Statistical analysis indicates an asymmetric intraguild predation in favor of Ch. externa when prey density is low. There were no statistical differences in the predation of D. citri between both chrysopids when preying upon D. citri individually or in combination. It is concluded that the intraguild predation in favor of Ch. externa over Ce. valida depends on the density of the prey, and that the population of D. citri can be decreased by effect of a single chrysopid species.

Asymmetrical intra-guild predation and niche differentiation in two pit-building antlions

Abstract Co-occurring species often compete with each other directly and indirectly. Intra-guild predation (IGP) is an extreme manifestation of direct competition, which involves the attack, killing and eating of potential competitors. We studied the competitive interactions between two pit-building antlion species that co-occur in the Israeli desert: Myrmeleon hyalinus residing in the more productive sandy soil, and Cueta lineosa solely inhabiting poorer loess soils. To understand the mechanisms driving C. lineosa away from the more productive habitat, we explored the factors triggering IGP of one antlion species on the other. We tested whether IGP is affected by soil type, depth and temperature. IGP was asymmetrical with M. hyalinus preying on C. lineosa, and it intensified as the size difference favoring the former increased. Interactive rather than additive effects governed IGP, which was lowest in sandy soil combined with low temperature, and highest in shallow loess soil. C. lineosa possesses a smaller head and thorax relative to its abdomen compared to M. hyalinus, providing a possible explanation for the advantage of M. hyalinus in direct competition. We then focused on the weaker competitor, C. lineosa, examining how it copes with competition induced by M. hyalinus. Both the growth and survival rates of C. lineosa declined in the presence of M. hyalinus. The asymmetrical IGP C. lineosa experiences from M. hyalinus combined with its competitive inferiority may explain why it is mostly found in poor habitats, while its intra-guild competitor is abundant in the more productive habitats.

The buoyancy-based biotope axis of the evolutionary radiation of Antarctic cryonotothenioid fishes

In the absence of any prior comprehensive analysis, I evaluate divergence along the biotope axis in the habitat stage of the evolutionary radiation of Antarctic cryonotothenioids. I utilize the available percentage buoyancy (%B) measurements as habitat proxies for recognition of the pelagic, semipelagic, demersal, and benthic biotopes that include, respectively, 5%, 10%, 73%, and 12% of the 59 species and 1749 specimens in the study. The majority of species retain the ancestral demersal biotope of Eleginops maclovinus, and this probably enhances ecological plasticity. Divergence into the pelagic biotope is the most distinctive organismal feature of the radiation and, although only 5% of species are pelagic, this biotope is not depauperate in global comparisons. Pelagic or potentially pelagic species are Dissostichus mawsoni, D. eleginoides, Pleuragramma antarctica, Aethotaxis mitopteryx, and Gvozdarus svetovidovi. Small ontogenetic changes in %B with growth are typical; however, this is extensive in D. mawsoni, a species with the potential to transition through benthic to pelagic biotopes over ontogeny. Occupation of the pelagic biotope by large D. mawsoni may be impermanent as it is lipid-dependent, a contingency reliant on the availability of P. antarctica as prey. In unusual conditions, the specialized sacs of P. antarctica can also yield their lipid for metabolism with possible loss of buoyancy. Pelagic species are inordinately important in the food web. In the southwestern Ross Sea a guild of large mammalian and avian predators, which includes D. mawsoni, is reliant on lipid-rich, energy-dense cryonotothenioid prey. This includes asymmetrical intraguild predation on D. mawsoni, with P. antarctica as a basal resource for the guild.

Competition and Intraguild Predation Between Beetles, Pterostichus stygicus (Coleoptera: Carabidae) and Centipedes, Scolopocryptops sexspinosus (Scolopemdromorpha: Scolopocryptopidae)

Abstract Studying interactions between distantly related species is necessary to understand the complexity of food webs. Generalist predator interactions, such as intraguild predation (IGP) and competition, can alleviate predation pressure and weaken top–down control that predators have on lower trophic levels. Centipedes (Chilopoda) and carabid beetles (Coleoptera) are common deciduous forest floor generalist predators that may interact by competing for resources beneath rocks and logs on the forest floor, especially during dry periods when prey become confined to such microhabitats. We used laboratory and field studies to determine whether the carabid beetle, Pterostichus stygicus (Say), and the centipede, Scolopocryptops sexspinosus (Say) co-occur under artificial cover. Additionally, a laboratory mesocosm experiment was used to examine competitive interactions in intra- and interspecific trials. There was significant negative co-occurrence of beetles and centipedes beneath cover objects in the field and laboratory. Pairings of S. sexspinosus and P. stygicus within mesocosms resulted in high mortality of P. stygicus, and reciprocal but asymmetric IGP. Centipedes maintained weight within solitary, intra- and interspecific mesocosm treatments, however, beetles lost mass in all treatments. Scolopocryptops sexspinosus responded more favorably to intra- and interspecific competition than did P. stygicus. Analysis of the leaf litter mesofauna indicated that these predators consumed similar prey in laboratory mesocosms. Our results suggest that species with very different trophic morphology have the potential to compete for shared microhabitat and prey.

Temporal shifts in intraguild predation pressure between beluga whales and Greenland halibut in a changing Arctic.

Asymmetrical intraguild predation (AIGP), which combines both predation and competition between predator species, is pervasive in nature with relative strengths varying by prey availability. But with species redistributions associated with climate change, the response by endemic predators within an AIGP context to changing biotic-abiotic conditions over time (i.e. seasonal and decadal) has yet to be quantified. Furthermore, little is known on AIGP dynamics in ecosystems undergoing rapid directional change such as the Arctic. Here, we investigate the flexibility of AIGP among two predators in the same trophic guild: beluga (Delphinapterus leucas) and Greenland halibut (Reinhardtius hippoglossoides), by season and over 30 years in Cumberland Sound-a system where forage fish capelin (Mallotus villosus) have recently become more available. Using stable isotopes, we illustrate different predator responses to temporal shifts in forage fish availability. On a seasonal cycle, beluga consumed less Greenland halibut and increased consumption of forage fish during summer, contrasting a constant consumption rate of forage fish by Greenland halibut year-round leading to decreased AIGP pressure between predators. Over a decadal scale (1982-2012), annual consumption of forage fish by beluga increased with a concomitant decline in the consumption of Greenland halibut, thereby indicating decreased AIGP pressure between predators in concordance with increased forage fish availability. The long-term changes of AIGP pressure between endemic predators illustrated here highlights climate-driven environmental alterations to interspecific intraguild interactions in the Arctic.

Optimal harvesting of a prey–predator model with variable carrying capacity

This work deals with a prey–predator model in an environment where the carrying capacities are assumed to be variable with time and one species feeds upon the other. Independent harvesting efforts are applied in either species and asymmetrical intraguild predation occurs. A common resource is consumed by two competing species and at the same time predator also consumes the prey. At first we discuss the model under constant carrying capacity and make the conclusion that no limit cycle exists in this case. Then we discuss the model without intraspecific competition. Our main concern is to cover the above mentioned two cases together, i.e. the model with variable carrying capacity and intraspecific competition. We determine the steady states and examine the dynamical behavior. We also analyze the local and global stability of the interior equilibrium by Routh–Hurwitz criterion and a suitable Lyapunov function respectively. A Hopf bifurcation occurs with respect to a parameter which is the ratio of predator’s and prey’s intrinsic growth rate. The possibility of bionomic equilibrium has been considered. The optimal harvest policy is formulated and solved with Pontryagin’s maximum principle. Some numerical simulations are given to explain most of the analytical results.

Pesticide-mediated displacement of a phytoseiid predator, Neoseiulus womersleyi, by another phytoseiid predator, N. californicus (Acari: Phytoseiidae).

Neoseiulus womersleyi and N. californicus are two predators that are frequently used to control spider mites in fruit-tree orchards. Neoseiulus womersleyi used to be the dominant predator species in Japan, but since the 1990s in central and southwestern Japan, N. californicus populations have been increasing and have displaced populations of N. womersleyi. We previously observed the same phenomenon under laboratory conditions when these species were released at a 1:1 ratio, and attributed the displacement to asymmetrical intraguild predation. However, the ratio in fruit-tree orchards could be different from 1:1. Therefore, we hypothesized that differential susceptibilities to pesticides might accelerate species displacement of N. womersleyi by N. californicus, even if the ratio between these two species was extremely skewed in favor of N. womersleyi and no species displacement occurred otherwise. We examined the effects of 21 pesticides on egg-to-adult and adult survivorship in N. womersleyi and N. californicus. Among these pesticides, two neonicotinoids (acetamiprid and imidacloprid) had much severer effects on N. womersleyi than on N. californicus and thus could possibly account for the species displacement. When the two species were released onto leaf arenas at an N. californicus: N. womersleyi ratio of 1:9 in the absence of insecticide, no displacement was observed. However, just after acetamiprid or imidacloprid application, the proportion of N. californicus increased, causing N. californicus to displace N. womersleyi. Our results indicate that displacement in predator complexes of fruit-tree orchards could be due to different degrees of pesticide susceptibility.

Comparing the influence of native and invasive intraguild predators on a rare native oyster

Invasive species can cause complex, unpredictable changes in community dynamics because they do not share an evolutionary history with native species, meaning interactions could be much stronger or weaker than expected. A field experiment tested hypotheses generated from previous laboratory experiments about interactions in an invaded tri-trophic intertidal food chain that is also characterized by asymmetric intra-guild predation. Cages controlled access of a top predator (native cancrid crabs) and an intermediate (or intraguild) prey (invasive oyster drills, Ocenebra inornata) to a resource (native oysters, Ostrea lurida) in order to explore the separate and combined effects of these predators on a native ecosystem engineer of conservation concern. Though crabs were predicted to have a strong negative effect on oysters via direct predation, the presence of oyster drills had the strongest impact on oyster survival. Drills consumed up to 80% of oysters in experimental cages per month and accounted for an average of 70% of total mortality when they were present. Contrary to the hypothesis, crabs almost never attacked oysters directly, and consumed drills primarily during only one out of four months. Crabs also did not appear to reduce individual drill feeding rates (i.e. an intimidation effect) or initiate a strong indirect positive effect on oyster survival. This experiment demonstrates that the role of the invasive predator in IGP as well as the strength of the interaction between the native and invasive species combine to influence the dynamics of the system. In addition, these observations underscore the importance of considering non-native predators as obstacles to the recovery of threatened species, as well the value of experimentally identifying, in situ, which of the possible interactions in an invaded food web are ecologically important.

Dynamics of asymmetric intraguild predation with time lags in reproduction and maturation

A three dimensional (3D) stage-structured predator–prey model is proposed and analyzed to study the effect of intraguild predation with harvesting of the adult species. Time lags in reproduction and maturation of the organism are introduced in the system and conditions for local asymptotic stability of steady states of delay differential forms of the ODE model are derived. The length of the delay preserving the stability is also estimated. Moreover, it is shown that the system undergoes a Hopf bifurcation when the time lags cross certain critical values. The stability and direction of the Hopf bifurcations are determined by applying the normal form method and the center manifold theory. Computer simulations have been carried out to illustrate various analytical results.

Intraguild predation and partial consumption of blue sharks Prionace glauca by Cape fur seals Arctocephalus pusillus pusillus

The top-down effects of predators on ecosystem structure and dynamics have been studied increasingly. However, the nature and consequence of trophic interactions between upper-trophic-level predators have received considerably less attention. This is especially the case in marine systems due to the inherent challenges of studying highly mobile marine species. Here we describe the first documentation of asymmetrical intraguild predation by a pinniped predator on a mid-sized predatory shark. The report is based on direct observations in South African waters, in which free-swimming blue sharks Prionace glauca were captured and partially consumed by Cape fur seals Arctocephalus pusillus pusillus. These observations are important not just for understanding the interactions between these two species but more broadly for their implications in understanding the trophic ecology of pinnipeds, many populations of which have increased while numerous shark populations have declined.

Influence of density on intraguild predation of aquatic Hemiptera (Heteroptera): implications in biological control of mosquito

The water bugs <em>Diplonychus rusticus</em> (Fabricius) (Heteroptera: Belostomatidae) and <em>Anisops bouvieri</em> (Kirkaldy) (Heteroptera: Notonectidae) co-occur in wetlands sharing mosquito larvae as prey. As a consequence, an asymmetrical intraguild predation (IGP) involving D. rusticus as IG predator and <em>A. bouvieri</em> as IG prey can be possible, the outcome of which may vary with the relative density of interacting species. Based on this proposition density dependent effects on the IG prey and shared prey mortality were assessed in the laboratory using varying numbers of IG predator and shared prey (IV instar <em>Culex quinquefasciatus</em> larva). In contrast to single predator system, mosquito larvae were proportionately less vulnerable to predation in IGP, at low density of shared prey. An increase in density of mosquito decreased the mortality of IG prey (<em>A. bouvieri</em>), but the mean mortality of the IG prey increased with the density of IG predator, in IGP system. Increase in density of mosquito and <em>D. rusticus</em> enhanced risk to predation of mosquito while reducing the mortality of <em>A. bouvieri</em>. Interaction between <em>D.</em> <em>rusticus</em> and <em>A. bouvieri</em> as a part of IGP system provides a possible reason of coexistence of mosquito immature along with predators in wetlands. Biological regulation of mosquitoes may be affected, if appropriate predator numbers are not available in the habitats.

Asymmetrical intraguild predation between natural enemies of the citrus mealybug

AbstractThe ability of a predator to discriminate against parasitized prey determines the extent of asymmetrical intraguild predation, which is often crucial for the outcome of biological control. Anagyrus nr. pseudococci (Girault) (Hymenoptera: Encyrtidae), a parasitoid of the citrus mealybug, Planococcus citri (Risso) (Hemiptera: Pseudococcidae), suffers from intraguild predation by coccinellids occurring in the same habitat. The level of intraguild predation on A. nr. pseudococci by Nephus includens (Kirsch) (Coleoptera: Coccinellidae) at different immature stages has been investigated with and without simultaneous offer of extraguild prey. Larvae of A. nr. pseudococci appeared to face increased intraguild predation at early developmental stages, whereas mummification provided adequate protection against the predatory coccinellid. Different predation levels on unparasitized vs. parasitized hosts at various developmental stages in choice assays indicated that N. includens preferences might be determined not solely by palatability of the prey but also by its ability to protect itself.

Factors determining species displacement of related predatory mite species (Acari: Phytoseiidae)

Neoseiulus womersleyi (Acari: Phytoseiidae) used to be the dominant species in fruit-tree orchards throughout Japan, but starting in the 1990s, N. womersleyi began to be displaced by Neoseiulus californicus in central and southwestern Japan. The present study was conducted to examine factors explaining the displacement of N. womersleyi by N. californicus. First, we confirmed under laboratory conditions that N. californicus could exclude N. womersleyi if they initially coexisted in a 1:1 ratio. During a 2-h continuous observation period, none of the heterospecific pairs had copulated and after 5days together with heterospecific males, none of the females had laid eggs. When these females were placed with conspecific males, normal numbers of offspring were produced. Moreover, conspecific matings were not substantially disturbed in the presence of heterospecific males or females. Total fecundity was significantly lower in N. womersleyi than in N. californicus, but their r m values did not differ from each other. On the other hand, the frequency of intraguild predation by N. californicus on N. womersleyi was significantly higher than vice versa. From these results, we concluded that not reproductive interference nor differential female fecundity but asymmetrical intraguild predation seemed to explain the competitive exclusion of N. womersleyi by N. californicus.

Thermal adaptation affects interactions between a range‐expanding and a native odonate species

Summary1. Increasing temperature and invading species may interact in their effects on communities. In this study, we investigated how rising temperatures alter larval interactions between a naturally range‐expanding dragonfly, Crocothemis erythraea, and a native northern European species, Leucorrhinia dubia. Initial studies revealed that C. erythraea grow up to 3.5 times faster than L. dubia at temperatures above 16 °C. As a result, we hypothesised that divergent temperature responses would lead to rapid size differences between coexisting larvae and, consequently, to asymmetric intraguild predation at higher ambient temperatures.2. Mortality and growth rates were measured in interaction treatments (with both species present) and non‐interaction controls (one species present) at four different temperature regimes: at an ambient temperature representative of central Germany, where both species overlap in distribution, and at temperatures increased by 2, 4 and 6 °C.3. The mortality of C. erythraea did not differ between treatment and control. In contrast, mortality of L. dubia remained similar over all temperatures in the controls, but increased with temperature in the presence of the other species and was significantly higher there than in the controls. We concluded that L. dubia suffered asymmetric intraguild predation, particularly at increased temperature. Reduced growth rate of L. dubia in the interaction treatment at higher temperatures also suggested asymmetric competition for prey in the first phase of the experiment.4. The results imply that the range expansion of C. erythraea may cause reduction in population size of syntopic L. dubia when temperature rises by more than 2 °C. The consequences for future range patterns, as well as other factors that may influence the interaction in nature, are discussed.

Plant quality effects on intraguild predation between Orius laevigatus and Aphidoletes aphidimyza

AbstractTo understand the influence of plant quality on intraguild predation and consequently on the suppression of a shared prey population as well as on plant yield, the interactions between Aphis gossypii Glover (Hemiptera: Aphididae) (shared prey), Aphidoletes aphidimyza (Rondani) (Diptera: Cecidomyiidae) (intermediate predator), and Orius laevigatus Fieber (Hemiptera: Anthocoridae) (top predator) were investigated in 25‐day experiments on cucumber, Cucumis sativus L. (Cucurbitaceae) at various N fertilization levels (90, 150, and 190 p.p.m.) in microcosm set‐ups under greenhouse conditions. The final aphid population size was significantly affected by an interactive effect of N fertilization and predator application. Regardless of the N fertilization levels, O. laevigatus alone was more effective in aphid suppression than A. aphidimyza alone. In addition, the risk for aphids of being predated upon by both predators together was significantly reduced in the low and medium‐N fertilization levels, whereas it was additive in the high‐N fertilization treatment. The A. aphidimyza population was suppressed by O. laevigatus in both the 90 and 150 p.p.m. N treatments. However, there was no intraguild predation of O. laevigatus on A. aphidimyza at the 190 p.p.m. N level. Total plant yield depended on predator treatments and N fertilization levels, with the highest yield produced at the 150 p.p.m. N fertilization level in treatments with either O. laevigatus alone or with both predators together. Our results demonstrate that the weak asymmetric intraguild predation among A. aphidimyza and O. laevigatus does not influence the ability of both predators together to diminish bottom‐up effects on aphid populations and the yield losses associated with aphid infestations.

SUPPRESSION OF SOYBEAN APHID BY GENERALIST PREDATORS RESULTS IN A TROPHIC CASCADE IN SOYBEANS

Top-down regulation of herbivores in terrestrial ecosystems is pervasive and can lead to trophic cascades that release plants from herbivory. Due to their relatively simplified food webs, agroecosystems may be particularly prone to trophic cascades, a rationale that underlies biological control. However, theoretical and empirical studies show that, within multiple enemy assemblages, intraguild predation (IGP) may lead to a disruption of top-down control by predators. We conducted a factorial field study to test the separate and combined effects of predators and parasitoids in a system with asymmetric IGP. Specifically we combined ambient levels of generalist predators (mainly Coccinellidae) of the soybean aphid, Aphis glycines Matsumura, with controlled releases of the native parasitoid Lysiphlebus testaceipes (Cresson) and measured their impact on aphid population growth and soybean biomass and yield. We found that generalist predators provided strong, season-long aphid suppression, which resulted in a trophic cascade that doubled soybean biomass and yield. However, contrary to our expectations, L. testaceipes provided minor aphid suppression and only when predators were excluded, which resulted in nonadditive effects when both groups were combined. We found direct and indirect evidence of IGP, but because percentage parasitism did not differ between predator exclusion and ambient predator treatments, we concluded that IGP did not disrupt parasitism during this study. Our results support theoretical predictions that intraguild predators which also provide strong herbivore suppression do not disrupt top-down control of herbivores.

Intraguild predation of parasitoids by Solenopsis invicta: a non‐disruptive interaction

AbstractThe outcome of intraguild predation among natural enemies can have significant ramifications for herbivore suppression and biological control. Manipulating habitat complexity may alter the strength of intraguild predation, since changes in habitat complexity are often associated with concomitant changes in natural enemy abundance. Using a combination of greenhouse and field experiments, we determined if asymmetric intraguild predation by a pervasive generalist predator, the red imported fire ant, Solenopsis invicta Buren (Hymenoptera: Formicidae), disrupts important parasitoids in a collard, Brassica oleracea L. (Brassicaceae), agroecosystem. The effect of habitat complexity on this interaction was assessed by conducting field experiments in a simple, collard monocrop and a more complex, collard‐white clover intercrop. Neither the density of adult parasitoids nor the percentage parasitism of caterpillars was affected when S. invicta abundance was manipulated. Solenopsis invicta reduced the survival of the diamondback moth larvae, Plutella xylostella (L.) (Lepidoptera: Plutellidae) by 26% and 42% in greenhouse and field experiments, respectively, but there was no preference of S. invicta for parasitized or unparasitized caterpillars. An increase in habitat complexity significantly affected the abundance of both S. invicta and parasitoids, but had no overall effect on their interaction. The results of this study suggest that although S. invicta is an intraguild predator of parasitoids because it preys upon parasitized caterpillars, the action of S. invicta may not compromise overall biological control. This study's findings are important, because they suggest that the presence and conservation of multiple natural enemies may result in sustained pest suppression in agroecosystems, even complex systems containing many species of natural enemies including strong intraguild predators such as S. invicta.

Interactions between a hunting spider and a web‐builder: consequences of intraguild predation and cannibalism for prey suppression

Abstract. 1. Antagonistic interactions among invertebrate predators such as intraguild predation and cannibalism have the potential to dampen top‐down impacts on shared prey at lower trophic levels. Two abundant spider predators, the large wolf spiderPardosa littoralisand the small sheet‐web builderGrammonota trivitattaco‐occur on the salt marshes of eastern North America where they both attack planthoppers (Prokelisiaspp.), the dominant herbivores on the marsh. Experiments both in the laboratory and field were used to assess the incidence of intraguild predation and cannibalism in these spiders and elucidate how such antagonistic interactions influence planthopper suppression.2. Functional response experiments showed that with an increase in planthopper prey density,Grammonotacaptured more prey but not a higher proportion of that offered.Pardosaexhibited the same response whenGrammonotawere offered as intraguild prey. Both functional responses were type I over the range of prey densities offered.3. Grammonotais moderately cannibalistic, and the presence of planthopper prey reduced the incidence of cannibalism.4. Factorial experiments in the laboratory showed thatPardosabut notGrammonotareduced planthopper prey populations when prey density was low. By contrast, at high prey densities, bothPardosaandGrammonotahad significant adverse effects on planthopper populations. Moreover, there was an interactive effect such thatGrammonotareduced planthopper populations relatively more whenPardosawas absent than when it was present.5. There was direct evidence for the intraguild predation ofGrammonotabyPardosasuch that fewerGrammonotasurvived in the presence ofPardosathan when it was absent. This result occurred whether planthopper prey were abundant or not.6. Field releases ofGrammonotain open plots resulted in significant but small decreases in the density of planthopper prey, both nymphs and adults.7. Enhancing densities ofPardosain open plots resulted inGrammonotasuppression. The intraguild predation ofGrammonotaat this enhancedPardosadensity, however, did not precludePardosafrom significantly reducing planthopper populations.8. Although there was evidence thatGrammonotareduced planthopper populations and that the intraguild predation ofGrammonotabyPardosaoccurred, the strength of these interactions was relatively weak given the low consumption rate of planthoppers byGrammonota(< 3 day–1) andGrammonotabyPardosa(≈ 2 day−1). Thus, weak asymmetric intraguild predation among spiders on the marsh likely dampens but does not eliminate the ability ofPardosato exert significant top‐down control on planthopper populations.