Effect of size,extraguild prey and habitat complexity on intraguild interactions: a case study with the invasive ladybird <Emphasis Type="Italic">Harmonia axyridis</Emphasis> and the hoverfly <Emphasis Type="Italic">Episyrphus balteatus</Emphasis>

Intraguild predation (IGP) is a widespread interaction between predatory arthropods, and is influenced by several factors. The harlequin ladybird, Harmonia axyridis (Coleoptera: Coccinellidae), has frequently been reported as an intraguild predator of other Coccinellidae, but little is known about its interactions with other aphidophagous predators, including syrphids. This study investigated the incidence of IGP between H. axyridis and Episyrphus balteatus (Diptera: Syrphidae), the most abundant hoverfly species in Europe and a commercially available aphid biocontrol agent. The influence of size, presence of extraguild prey and habitat complexity were investigated through laboratory experiments in Petri dishes and on potted broad bean plants. In both types of arenas, IGP between H. axyridis and E. balteatus was found to be asymmetric, with the coccinellid in the majority of cases being the intraguild predator. There was a significant effect of size on the frequency of IGP. The efficiency of H. axyridis as an intraguild predator increased with the developmental stage. Early instars of E. balteatus were the most vulnerable to IGP. Pupae of either species were not attacked. In the presence of extraguild prey, the frequency of IGP was substantially reduced. However, IGP still occurred, mainly in combinations of older larvae of H. axyridis with first or second instars of E. balteatus. The size of the arena affected the incidence of IGP in combinations with second instars of E. balteatus, but not in combinations with third instars. Field research is needed to elucidate the ecological relevance of IGP among these predators.     

Intraguild predation between the aphidophagous ladybird beetles Harmonia axyridis and Coccinella undecimpunctata (Coleoptera: Coccinellidae): The role of intra and extraguild prey densities

Laboratory cages were used to evaluate the influence of extraguild (EGprey) and intraguild prey (IGprey) densities on the direction, symmetry and magnitude of the intraguild predation (IGP) of the aphidophagous Harmonia axyridis Pallas on Coccinella undecimpunctata L. and vice versa. In order to understand the role of competition between IGprey, the experiments included treatments with one IGpredator, with one or four IGprey and EGprey (Aphis fabae Scopoli) ranging from zero to sufficient aphids to satiate the predators for 12, 24 or 48 h. Increases in EGprey and IGprey densities did not alter the direction, but decreased the magnitude and symmetry of IGP. Predation on one individual of IGprey decreased from more than 80%, in the absence of EGprey, to from 6% to 53%, at higher EGprey densities. Decrease in IGP was less when H. axyridis was the IGpredator. Even at high EGprey densities, eggs and 2nd larval stages of C. undecimpunctata were vulnerable to IGP and the level of predation was 40% and 53%. The presence of more than one IGprey increased the magnitude of IGP mainly at EGprey densities sufficient to satiate the predators for 12 and 24 h, suggesting that competition between the IGpredator and IGprey may be one of the processes promoting IGP. These results and those of other authors suggest that H. axyridis has the potential to be an IGpredator, mainly of the most vulnerable stages of IGprey. Thus, H. axyridis may negatively affect the survival of C. undecimpunctata, when these two species exploit the same resources.     

Density-dependent intraguild predation of an aphid parasitoid

A growing body of research has examined the effect of shared resource density on intraguild predation (IGP) over relatively short time frames. Most of this work has led to the conclusion that when the shared resource density is high, the strength of IGP should be lower, due to prey dilution. However, experiments addressing this topic have been done using micro- or mesocosms that excluded the possibility of intraguild predator aggregation. We examined the effect of shared resource density on IGP of an aphid parasitoid in an open field setting where the effects of prey dilution and predator aggregation could occur simultaneously. We brought potted soybean plants with 2, 20, or 200 soybean aphids (Aphis glycines) and 20 pupae (‘mummies’) of the soybean aphid parasitoid Binodoxys communis into soybean fields in Minnesota, USA. We monitored predator aggregation onto the potted plants, predation of parasitoid mummies, and successful adult emergence of B. communis. We found that predator aggregation was higher at the higher aphid densities on our experimental plants and that this coincided with lower adult emergence of B. communis, indicating that even if a prey dilution effect occurred in our study, it was overcome by short-term predator aggregation. Our results suggest that the effect of shared resource density on IGP may be more nuanced in a field setting than in microcosms due to predator aggregation.     

Interaction between Podisus maculiventris and Harmonia axyridis two predators used in augmentative biological control in greenhouse crops

Intraguild predation (IGP) betweenthe pentatomid Podisus maculiventris(Say) and the coccinellid Harmoniaaxyridis (Pallas) in the absence or presenceof the extraguild prey Spodopteralittoralis (Boisduval) and Myzuspersicae (Sulzer) was studied in thelaboratory. Interactions were asymmetric infavor of the pentatomid. Podisusmaculiventris readily fed upon eggs and larvaeof H. axyridis, but adult beetles wererarely attacked. Success of attacks by P.maculiventris was stage dependent, fourthinstars and adults being more successful inkilling ladybeetle larvae than second instars.Attacks by H. axyridis on the pentatomidwere rare and none of them were successful. Theeffect of introducing extraguild prey on thelevel of IGP was tested both in petri dishesand on caged sweet pepper plants. Whensufficient numbers of S. littoralislarvae were present to satiate the pentatomid,predation on H. axyridis larvae decreasedsignificantly, indicating that the coccinellidis a less preferred or less vulnerable prey.When the aphid M. persicae was presentedas extraguild prey, levels of IGP were notaltered. Nymphs of P. maculiventrissuccessfully completed development whenexclusively fed on larvae of H. axyridis,but developmental time was longer than onlepidopteran prey. No pentatomid nymphs reachedadulthood on aphids alone. IGP by P.maculiventris on H. axyridis may be ofsome importance in greenhouse crops, where bothpredators are being used increasingly inaugmentative biological control programs.Nonetheless, it is expected that in practicelarger larvae and adults of H. axyridiswill escape most attacks by the pentatomid.     

Asymmetrical intraguild predation between natural enemies of the citrus mealybug

The 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.     

Resource quality,resource availability,and intraguild predation among omnivorous mirids

Intraguild predation (IGP) among predatory species can influence many plant-arthropod associations. However, the relevance of IGP is poorly understood for truly omnivorous species such as those that can complete development on both animal and plant diets. Here we test the hypothesis that IGP among two omnivorous mirids is more common when extraguild food is either absent or not suitable. Laboratory experiments were performed in experimental cages in order to determine the effect of intraguild prey densities and diet availability on direction and intensity of IGP between Dicyphus tamaninii and Macrolophus caliginosus (Heteroptera: Miridae). Intraguild predation was symmetrical between the two mirid species in the absence of alternative food. Increasing densities of intraguild prey enhanced drastically the incidence of IGP. Intraguild predation was reduced when mirids were in the presence of green or red tomato fruits, but the presence of any other extraguild resources had no impact on IGP level. However, when given before the experiments, all resources with the exception of tomato leaves significantly reduced IGP. A second experiment was performed on live plants to compare the results of the previous trials with that obtained in a more natural setting. No IGP was observed when both mirid species were present on a plant. However, development of the intraguild prey (the more vulnerable stage) was hindered by the presence of the intraguild predator. The potential of such results is discussed from community ecology and biological control perspective.     

Development and application of molecular gut‐content analysis to detect aphid and coccinellid predation by Harmonia axyridis (Coleoptera: Coccinellidae) in Italy

Despite their positive effect in reducing pest populations, exotic generalist predators sometimes become invasive and contribute to the displacement of indigenous species in the same trophic level. Although laboratory experiments have linked intraguild predation (IGP) to these interactions, field evidence and quantification of IGP are still lacking for most systems. The recent establishment of the exotic Harmonia axyridis (Pallas) (Coleoptera: Coccinellidae) in Italy raises concern about the detrimental effect that the ladybird could have on native coccinellids. Here we assessed, under laboratory conditions, the acceptability and suitability of eggs of 2 native ladybirds, Adalia bipunctata L. and Oenopia conglobata (L.), as prey items for H. axyridis larvae. Then we developed primers for molecular gut‐content analysis to detect predation by H. axyridis on the 2 ladybirds and on the aphid Eucallipterus tiliae L. Species‐specific 16S primers were developed for the 3 species and laboratory feeding trials were conducted to quantify the rate of prey DNA breakdown in the gut of H. axyridis. Moreover, to field evaluate primers, H. axyridis 4th instars (n = 132) were systematically collected from linden trees in northern Italy and screened for the presence of prey DNA. Seventy‐three percent and 7% of field collected H. axyridis were positive for aphid and coccinellid DNA, respectively. Predation upon aphid and A. bipunctata was lower than predicted if density dependent consumption was expected, while predation upon O. conglobata was significantly higher. Here, we provided the first evidence of IGP among feral populations of H. axyridis and indigenous ladybird beetles, occurring in Italy.     

Impact of intraguild predation by adult Harmonia axyridis (Coleoptera: Coccinellidae) on Aphis glycines (Hemiptera: Aphididae) biological control in cage studies

The soybean aphid, Aphis glycines Matsumura, has become a principal arthropod pest of soybean in the U.S. since its first detection in 2000. This species threatens soybean production through direct feeding damage and virus transmission. A diverse guild of insect predators feeds on soybean aphid in Michigan including the exotic coccinellid Harmonia axyridis, the native gall midge Aphidoletes aphidimyza and the native lacewing Chrysoperla carnea. In addition to feeding on A. glycines some members of this guild may also engage in intraguild predation. These interactions may produce positive, negative, or neutral impacts on A. glycines biological control. We explored the impact of intraguild predation on soybean aphid population dynamics by comparing aphid populations in microcosms with either A. aphidimyza larvae or C. carnea larvae alone, with both a H. axyridis adult and either A. aphidimyza or C. carnea larvae, and without predators. When H. axyridis was present with larval A. aphidimyza or C. carnea, the lady beetle acted as an intraguild predator. However, intraguild feeding did not result in a release of aphid populations compared with microcosms containing only the intraguild and aphid prey. A similar result was found in field cages. Cages allowing large predators had reduced numbers of A. aphidimyza and C. carnea larvae but also significantly fewer aphids compared with predator exclusion cages. Thus, in both lab and field studies the direct impact of H. axyridis on A. glycines overcame its negative impact as an intraguild predator. Together, these studies indicate that while the exotic H. axyridis does act as an intraguild predator and may contribute to local declines in A. aphidimyza and C. carnea, it is also currently important in overall biological control of A. glycines.     

Alkaloids provide evidence of intraguild predation on native coccinellids by <Emphasis Type="Italic">Harmonia axyridis</Emphasis> in the field

Over a period of less than 5 years, Belgium was thoroughly invaded by the multicolored Asian coccinellid, Harmonia axyridis. At the same time, a decline of some native coccinellid species was observed in tree habitats. One hypothesis about the cause of this decline was that it might have been due to intraguild predation (IGP) by H. axyridis. In natural conditions, IGP between coccinellids can be tracked by using defensive compounds. Exogenous alkaloids in H. axyridis were therefore examined by gas chromatography-mass spectrometry (GC–MS), using individuals sampled from lime trees that were also occupied by other species of coccinellids. Harmonia axyridis was the dominant species at all life stages, in terms of both numbers of sites occupied and local abundance. The GC–MS analysis of H. axyridis larvae revealed traces of exogenous alkaloids from 19 of the 20 sites and, in nine of those 19 sites, more than 30% of the larvae analyzed contained exogenous alkaloids. Three alkaloids were detected: adaline from Adalia spp., calvine from Calvia spp. and propyleine from Propylea quatuordecimpunctata. Predation by H. axyridis on two different coccinellid species was also detected in the same larva, reinforcing the status of H. axyridis as a top predator. A generalized linear model indicated that IGP frequency was positively influenced by two variables: the abundance of extraguild and intraguild prey; and the interaction between these two variables. Our results therefore support the hypothesis that IGP by H. axyridis on native coccinellids in tree habitats has led to the decline of several of these species.     

Combining lacewings and parasitoids for biological control of foxglove aphids in sweet pepper

The role of natural enemy diversity in biological pest control has been debated in many studies, and understanding how interactions amongst predators and parasitoids affect herbivore populations is crucial for pest management. In this study, we assessed the individual and combined use of two species of natural enemies, the parasitoid Aphidius ervi Haliday, and the predatory brown lacewing Micromus variegatus (Fabricius), on their shared prey, the foxglove aphid, Aulacorthum solani (Kaltenbach), on sweet pepper. We hypothesized that the presence of intraguild predation (IGP) and predator facilitation (through induced aphid dropping behaviour) might have both negative and positive effects on aphid control, respectively. Our greenhouse trial showed that overall, the greatest suppression of aphids occurred in the treatment with both the parasitoid and the lacewing. While the combination of lacewings and parasitoids significantly increased aphid control compared to the use of parasitoids alone, the effect was not significantly different to the treatment with only predators, although there was a clear trend of enhanced suppression. Thus, the combined effects of both species of natural enemies were between additive and non‐additive, suggesting that the combination is neither positive nor negative for aphid control. High levels of IGP, as proven in the laboratory, were probably compensated for by the strong aphid suppression provided by the lacewings, whether or not supplemented with some level of predator facilitation. For aphid management over a longer time scale, it might still be useful to combine lacewings and parasitoids to ensure stable and resilient aphid control.     

Intra‐ and interspecific interactions between aphidophagous ladybirds: the role of prey in predator coexistence

Cannibalism (CANN) and intraguild predation (IGP) may provide energy and nutrients to individuals and eliminate potential competitors. These negative competitive interactions could also affect the coexistence of predatory species. The co‐occurrence of aphidophagous ladybird species in crops creates opportunities for CANN and IGP, especially when aphids become scarce. The Lotka–Volterra model predicts the coexistence of two species if intraspecific competition is stronger than interspecific interference interactions. Cycloneda sanguinea L. and Eriopis connexa (Germar) (both Coleoptera: Coccinellidae) coexist in sweet pepper crops in La Plata (Argentina) consuming mainly Myzus persicae (Sulzer) (Hemiptera: Aphididae). The present study used laboratory experiments to estimate levels of CANN and IGP by adults and larvae on eggs, and by adults on larvae, in both the presence and absence of prey (i.e., M. persicae), to explain the effect of prey on coexistence of these two predators. Levels of CANN by C. sanguinea and E. connexa were high in the absence of aphids, and decreased when prey was present. Intraguild predation was bidirectional and asymmetric. Adults and larvae of E. connexa were more voracious IG predators of C. sanguinea than vice versa, the former being the stronger IG predator and interference competitor. Eriopis connexa always won when larvae of the same instar were compared, whereas the larger larva always won when larvae were of different instars, regardless of species. In the presence of prey, CANN by both species decreased, but IGP by E. connexa on C. sanguinea remained high, suggesting that E. connexa could displace C. sanguinea via interspecific interference competition. Other factors potentially affecting the coexistence of C. sanguinea and E. connexa in sweet pepper crops are discussed.     

Reciprocal intraguild predation between two mirid predators,Cyrtorhinus lividipennis and Tytthus chinensis (Hemiptera: Miridae)

Interactions between predator species shape ecological communities and are crucial in the relationship between biodiversity and ecosystem functioning. They contribute to improving the natural biological control. Cyrtorhinus lividipennis Reuter and Tytthus chinensis Stål (Hemiptera: Miridae) are important natural enemies of brown planthopper, Nilaparvata lugens Stål (Hemiptera: Delphacidae), the most destructive rice pest in Asia. Comparative functional responses of both sexes of the two mirids at different development stages were estimated and the direction, symmetry, as well as intensity of intraguild predation (IGP) were studied in the laboratory. Prey preferences of the two mirid species and the influence of alternative prey on functional responses were also evaluated. The mirids exhibited reciprocal IGP in addition to consuming eggs of BPH. All predator–prey combinations exhibited a type II functional response. Females and fourth instar nymphs of C. lividipennis showed greater voracity for eggs of BPH than of T. chinensis. The two mirids were more voracious to intraguild prey than to extraguild prey. Alternative prey released the functional responses of C. lividipennis to both intra- and extraguild prey. In contrast, alternative prey had no effect on the functional response of T. chinensis. RIGP was symmetrical in the absence of extraguild prey. And in the presence of extraguild prey that was common in the field, resulted in different responses by the mirids.     

Coccinellid interactions mediated by vegetation heterogeneity

Environmental heterogeneity can have profound effects on agroecosystem function and it is important for improving ecosystem services such as biological control. Promoting system diversity via non‐crop plants is one method for increasing habitat heterogeneity within farmscapes. Non‐crop plants provide access to refuges and alternative food resources provide multiple benefits to enhance populations of arthropod predators. In this study, we examined the effects of small‐scale spatial structure on life‐stage specific interactions between the native coccinellid, Hippodamia convergensGuérin‐Méneville, and the exotic Harmonia axyridis (Pallas) (both Coleoptera: Coccinellidae), which overlap in spatial distribution in many crop systems. Squash [Cucurbita pepo L. (Cucurbitaceae)] and non‐crop mugwort [Artemisia vulgaris L. (Asteraceae)] plants with and without aphids were used as a model of spatial heterogeneity in micro‐ and mesocosm experiments. In response to factorial treatment combinations, we evaluated oviposition behavior, egg predation, larval survival, and larval predator‐prey and predator‐predator interactions. Adult H. convergens displayed higher foraging activity on aphids when exposed to complex habitats containing a non‐crop plant. In the presence of the exotic coccinellid, H. convergens preferred to deposit eggs on the non‐crop plant. Furthermore, a combination of spatial heterogeneity and prey availability reduced larval intraguild predation and cannibalism, and improved reproductive output of H. convergens by reducing intra‐ and interspecific egg predation. Our results provide evidence that life‐stage‐specific intraguild interactions are mediated by access to non‐crop plants. Thus, the introduction or maintenance of non‐crop plants has the potential to enhance coexistence of multiple natural enemies and improve top‐down control of pests.     

Intraguild interactions and aphid predators: biological efficiency of Harmonia axyridis and Episyrphus balteatus

Introductions of the harlequin ladybird Harmonia axyridis into Belgium for aphid biological control have been followed by declines in native aphid natural enemies. We first examined, in laboratory and field conditions, the impact of larval mobility of either H. axyridis or Episyrphus balteatus, the most abundant native hoverfly in central Europe, on aphid suppression. The hoverfly larvae consumed more aphids (Megoura viciae) in a short term, whereas the ladybird ones were more efficient in a long term. We second investigated the intraguild interactions between H. axyridis and E. balteatus larvae and adults. In larva pairings between the two species in laboratory microcosms containing aphid (M. viciae) infested broad bean (Vicia faba) plants, H. axyridis had an intraguild predation (IGP) advantage over the hoverfly. When conspecific larvae were paired together on aphid‐infested plants, no cannibalism between them was detected. The presence of either H. axyridis or E. balteatus larvae on aphid‐infested plants negatively influenced the ovipositional behaviour of H. axyridis and E. balteatus females; lower numbers of laid eggs were recorded compared to control treatment. Moreover, eggs laid by E. balteatus females were also dropped as victims of predation mainly by H. axyridis larvae. Our results suggest that while the exotic ladybird was more efficient in aphid biological control, larvae and eggs of the native hoverfly species face increased IGP by H. axyridis, which would contribute, as a consequence, to the decline in E. balteatus population following invasion.     

Intraguild Predation among the Hoverfly Episyrphus balteatus de Geer (Diptera: Syrphidae) and Other Aphidophagous Predators

Aphidophagous predators compete for the same prey species. During their foraging activity they frequently encounter heterospecific aphid predators. These situations can lead to intraguild predation and may disrupt biological control efforts against aphids where more than one predator species is present. We investigated the behavior of larvae of the hoverfly Episyrphus balteatus de Geer and its interaction with three other aphid predators: the ladybird Coccinella septempunctata L., the lacewing Chrysoperla carnea Stephens, and the gall midge Aphidoletes aphidimyza (Rondani). Interspecific interactions between predators were examined in arenas of different sizes and in the presence of extraguild prey. The outcome of interactions between E. balteatus larvae and the other predators depended predominantly on the relative body size of the competitors. Relatively large individuals acted as intraguild predators, while relatively smaller individuals became intraguild prey. Eggs and first- as well as second-instar larvae of E. balteatus were highly susceptible to predation by all other predators, whereas pupae of E. balteatus were preyed upon only by the larvae of C. carnea. Interactions between A. aphidimyza and E. balteatus were asymmetric and always favored the latter. Eggs and first- as well as second-instar larvae of E. balteatus sustained intraguild predation irrespective of the size of the arena or the presence of extraguild prey. However, the frequency of predation on third-instar larvae of E. balteatus was significantly reduced. This study indicated that the same species can be both intraguild predator and intraguild prey. It is suggested that combinations of predators must be carefully chosen for success in biological control of aphids.     

Intraguild predation and successful invasion by introduced ladybird beetles

Introductions of two ladybird beetle (Coleoptera: Coccinellidae) species, Coccinella septempunctata and Harmonia axyridis, into North America for aphid biocontrol have been followed by declines in native species. We examined intraguild predation (IGP) between larvae of these two exotic species and larvae of the two most abundant native coccinellids in eastern Washington State, C. transversoguttata and Hippodamia convergens. In pairings between the two native species in laboratory microcosms containing pea (Pisum sativum) plants, neither native had a clear advantage over the other in IGP. When the natives were paired with either Harmonia axyridis or C. septempunctata, the natives were more frequently the victims than perpetrators of IGP. In contrast, in pairings between the exotic species, neither had an IGP advantage, although overall rates of IGP between these two species were very high. Adding alternative prey (aphids) to microcosms did not alter the frequency and patterns of relative IGP among the coccinellid species. In observations of encounters between larvae, the introduced H. axyridis frequently survived multiple encounters with the native C. transversoguttata, whereas the native rarely survived a single encounter with H. axyridis. Our results suggest that larvae of the native species face increased IGP following invasion by C. septempunctata and H. axyridis, which may be contributing to the speed with which these exotic ladybird beetles displace the natives following invasion.     

Influence of extraguild prey density on intraguild predation by heteropteran predators: A review of the evidence and a case study

Heteropteran predators constitute an important component of predatory guilds in terrestrial and aquatic ecosystems. Most heteropteran species have generalist diets, and intraguild predation has been documented in most heteropteran families. Zoophytophagous species also frequently engage in intraguild interactions. An increase in extraguild prey density is often predicted to reduce intraguild predation between guild members by providing abundant alternate prey. However, an increase of extraguild prey density may also be associated with an increase in the density of intraguild predators, which could instead strengthen intraguild predation. Evaluating the combined effect of these potentially opposing influences on intraguild predation is difficult. Most studies have been carried out in the laboratory, using artificially simplified communities of predators and prey and employing spatial and temporal scales that may not reflect field conditions. We review experimental studies examining how extraguild prey density influences the intensity of intraguild predation and then report an observational case study examining the influence of extraguild prey density on the intensity of intraguild predation at larger spatial and temporal scales in unmanipulated cotton fields. Fields with more abundant extraguild prey (aphids, mites) were not associated with elevated densities of intraguild predators, and were strongly associated with increased survival of intraguild prey (lacewing larvae). In this system, the ability of extraguild prey to relax the intensity of intraguild predation, as previously documented in small-scale field experiments, also extends to the larger spatial and temporal scales of commercial agriculture.     

A sublethal effect on native Anthocoris nemoralis through competitive interactions with invasive Harmonia axyridis

1. The invasive ladybird Harmonia axyridis Pallas (Coleoptera: Coccinellidae) is implicated in declines of native ladybirds with intraguild predation identified as an underlying mechanism, however, less studied are the effects on non‐coccinellid predators. Intraguild predation between Anthocoris nemoralis Fabricius (Heteroptera: Anthocoridae) and H. axyridis and the relative effects of inter‐ and intraspecific competition on predators on potted Tilia cordata Mill. was investigated, at two aphid densities. 2. Intraguild predation was unidirectional in favour of H. axyridis in Petri dishes although a low level of ladybird egg predation by A. nemoralis occurred. However, on trees intraguild predation was rare. 3. A positive effect of aphid density on H. axyridis weight gain, and increased survival when reared with A. nemoralis on potted trees indicated stronger effects of intraspecific competition on H. axyridis. Consistent suppression of aphids by ladybirds revealed their superior competition for aphids. 4. Anthocoris nemoralis weight gain was reduced when reared with H. axyridis on potted trees, indicating a potential for a sublethal effect through interspecific competition. Survivorship of A. nemoralis did not differ between con‐ and heterospecific treatments, but high aphid density enhanced A. nemoralis survival. Overall, high mortality among nymphs across treatments suggested effects related to both inter‐ and intraspecific competition for resources. 5. Results provide evidence of a sublethal effect by invasive H. axyridis through resource competition, suggesting that where these predators co‐occur, competitive interactions for shared prey are more likely to affect A. nemoralis populations than intraguild predation by H. axyridis.     

Invasion of the coccinellid community associated with a tropical annual agroecosystem

1. Detailed understanding of the mechanisms enabling or limiting the impacts of invasive generalist predators is needed. Harmonia axyridis is an invasive generalist coccinellid predator that may have destabilised coccinellid communities worldwide and is an excellent candidate for investigating invasion dynamics.
2. We evaluated four hypotheses: (1) Nine years after being detected in Central Brazil, Ha. axyridis has dominated the coccinellid community. This is facilitated by (2) its broader aphid prey range than the other aphidophagous coccinellids, (3) its superiority as an asymmetrical intraguild predator, and (4) the invasibility of the present coccinellid community.
3. We sampled the invertebrate communities associated with six organic vegetable farms in Brazil during 2017 and 2018, conducted a feeding trial to measure prey niche breadth and overlap, and estimated intra- and interspecific interaction strengths corresponding to first-order Lotka-Volterra parameters to evaluate the strength of intraguild predation and create a community matrix.
4. We found (1) Ha. axyridis comprised ≤8.3% of the coccinellid community. (2) Hi. convergens had the broadest prey niche and dominated the prey niche of Ha. axyridis. (3) We could not determine from the population data if Ha. axyridis was the dominant intraguild predator because its density was too low and constant. (4) The community matrix indicated that the present-day coccinellid community was stable.
5. We conclude Ha. axyridis has not become invasive in Central Brazil, in part because it does not have the broadest prey niche breadth and the present coccinellid community is stable to the present perturbation of Ha. axyridis.

     

Effect of pollen supplement on intraguild predatory interactions between two omnivores: The importance of spatial dynamics

Arthropods often engage in complex trophic interactions such as intraguild predation (IGP), true omnivory (i.e., feeding on plants and prey), and apparent competition. Theoretical treatments of the effects of such interactions on herbivore populations have been concerned almost entirely with equilibrium conditions. Yet these interactions are common in non-equilibrium settings such as agroecosystems, where they are likely to have a strong influence on pest populations. We therefore tested short-term effects of IGP and food supplementation on interactions between two predators (the phytoseiid mite Neoseiulus cucumeris and the anthocorid bug Orius laevigatus) and their shared prey, Frankliniella occidentalis, on strawberry plants. All three consumers feed on strawberry pollen, both mites and bugs prey on thrips, and the bug also feeds on the mites (IGP). Strong IGP on mites (IG prey) by the bugs (IG predator) was recorded in structurally-simple arenas. In a more complex setting (whole-plants), however, the intensity of IGP differed among plant structures. Likewise, pollen supplementation reduced both IGP and predation on thrips in a structurally simple setting. In the whole-plant experiment, IGP was more intense on pollen-bearing than pollen-free flowers. The study illustrated how spatial dynamics, generated when consumers track food sources differently in the habitat and possibly when herbivorous and IG prey alter their distribution to escape predation, led to site-specific configuration of interacting populations. The intensity of resulting trophic interactions was weakened by food supplementation and by increased complexity of the habitat.