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Christer Björkman - One of the best experts on this subject based on the ideXlab platform.
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Plant sex effects on insect herbivores and biological control in a Short Rotation Coppice willow
Biological Control, 2017Co-Authors: Kim K. Moritz, Christer Björkman, Amy L. Parachnowitsch, Johan A. StenbergAbstract:In the wild, plant sex can affect plant-herbivore interactions and higher trophic levels, including natural enemies of the herbivores. However, the possibility of manipulating plant sex to improve biological control and reduce herbivory in domesticated dioecious crops remains unexplored. The dioecious bioenergy crop, Salix viminalis, is often planted in monoclonal, and thus monosexual, fields. We investigated whether using plant clones of either sex, or mixing plants of both sexes, reduced the performance and abundance of the herbivorous pest insect Phratora vulgatissima and its main natural enemy, Anthocoris nemorum, and whether predation was affected. The herbivore laid more eggs, and the predator survived longer, on female plants in the lab. However, these effects did not translate into differences in predation rates in laboratory experiments or differential insect abundances on plants of either sex or plantation sex composition in the field. Plant genotype did have a significant effect on insect abundances, but this was due to plant traits other than sex. The results indicate that manipulating plant sex will not lead to improved biological control or reduced insect herbivory in S. viminalis energy forestry, but suggest that a focus on plant genotypic differences offers promise for improving management practices.
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predator hunting mode and host plant quality shape attack abatement patterns of predation risk in an insect herbivore
Ecosphere, 2016Co-Authors: Jorg G Stephan, Johan A. Stenberg, Christer BjörkmanAbstract:Group formation reduces individual predation risk when the proportion of prey taken per predator encounter declines faster than the increase in group encounter rate (a ack-abatement). Despite a ack-abatement being an important component of group formation ecology, several key aspects have not been empirically studied, that is, interactions with the hunting mode of the predator and how these relationships are modi ed by local habitat quality. In 79 cage trials, we examined individual egg predation risk in di erent-sized egg clutches from the blue willow beetle Phratora vulgatissima for two predators with di erent hunting modes (consumption of full group [Orthotylus marginalis] vs. part group [Anthocoris nemorum]). Because these predators also take nutrients from plant sap, we could examine how the quality of alternative food sources (high- vs. low-quality host plant sap) in uenced a ack-abatement pa erns in the presence of di erent hunting strategies. For the O. marginalis predator, individual egg predation risk was largely independent of group size. For A. nemorum, egg predation risk clearly declined with increasing group size. However, approximately one-third of the grouping bene t was lost to an increase in group detectability. There were clear di erences in a ack-abatement pa erns between plants with high- vs. low-quality sap. When O. marginalis was the predator, there was no clear change in a ack-abatement in relation to host plant quality. However, for A. nemorum there was a clear reduction in overall predation risk and a stronger a ack-abatement pa ern with increasing group size when plant sap quality increased. This implies that the relative bene ts of prey grouping behavior for any species might show diurnal or seasonal changes as other aspects of resource/habitat quality change for the focal predator. Modulation of a ack-abatement by bo om-up e ects such as plant-based food resources is yet to be incorporated into general theory, despite the ubiquity of omnivorous predators and with omnivory being important for shaping food webs, ecosystem functions, and in biological control. Thus, ongoing re nement of a ack- abatement theory by focusing on bo om-up vs. top-down processes could have signi cant impacts on many important contemporary elds of study.
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Constitutive and herbivore-induced systemic volatiles differentially attract an omnivorous biocontrol agent to contrasting Salix clones.
AoB PLANTS, 2013Co-Authors: Anna Lehrman, Tina Boddum, Johan A. Stenberg, Colin M. Orians, Christer BjörkmanAbstract:While carnivores are known to be attracted to herbivore-induced plant volatiles, little is known about how such volatiles may affect the behaviour of omnivorous predators that may use both plants and herbivores as food. Here, we examine how systemically produced plant volatiles, in response to local herbivore damage, differentially attract a key omnivorous predator, Anthocoris nemorum (Heteroptera: Anthocoridae), to single clones of three species of Salix: S. viminalis, S. dasyclados and S. cinerea. The profiles of the plant volatiles produced were found to vary among Salix clones and between herbivore-damaged and intact plants. Anthocoris nemorum was attracted to the volatiles released from undamaged plants of all three species, but most strongly to a native S. cinerea clone. Plants damaged by the herbivorous leaf beetle Phratora vulgatissima (Coleoptera: Chrysomelidae) were generally more attractive than undamaged plants, with A. nemorum responding to systemic changes in the damaged plants where the experimental design specifically excluded volatiles released from the actual site of damage. When comparing damaged plants, the S. dasyclados clone was more attractive to A. nemorum than the S. viminalis clone—a somewhat surprising result since this Salix clone is considered relatively resistant to P. vulgatissima, and hence offers a limited amount of prey. Our experiments highlight that both constitutive and induced plant volatiles play a role in omnivore attraction, and this emphasizes the importance of considering odours of released volatiles when cropping and breeding Salix for increased resistance to herbivores.
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Host‐plant genotype mediates supply and demand of animal food in an omnivorous insect
Ecological Entomology, 2011Co-Authors: Johan A. Stenberg, Anna Lehrman, Christer BjörkmanAbstract:1. Omnivorous predators can protect plants from herbivores, but may also consume plant material themselves. Omnivores and their purely herbivorous prey have previously been thought to respond similarly to host-plant quality. However, different responses of omnivores and herbivores to their shared host plants may influence the fitness, trophic identity, and population dynamics of the omnivores. 2. The aim of the present study was to show that an omnivorous heteropteran (Anthocoris nemorum L.) and two strictly herbivorous prey species respond differently to different genotypes of their shared host plant, Salix. Some plant genotypes were sub-optimal for the omnivore, although suitable for the herbivores, and vice versa. 3. The contrasting patterns of plant suitability for the omnivore and the herbivores highlight an interaction between plant genotype and omnivores' access to animal food. Plant genotypes that were sub-optimal for the omnivore when herbivores were experimentally excluded became the best host plants when herbivores were present, as in the latter situation additional prey became available. By contrast, the quality of plant genotypes that were intrinsically suitable for omnivores, did not improve when herbivores were present as these plant genotypes were intrinsically sub-optimal for herbivores, thus providing omnivores with almost no additional animal food. 4. The differential responses of omnivores and their prey to the same host-plant genotypes should allow omnivores to colonise sub-optimal host plants in their capacity as predators, and to colonise more suitable host plants in their capacity as herbivores. It may thus be difficult for Salix to escape herbivory entirely, as it will rarely be unsuitable for both omnivores and pure herbivores at the same time.
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Plant defence: Feeding your bodyguards can be counter-productive
Basic and Applied Ecology, 2011Co-Authors: Johan A. Stenberg, Anna Lehrman, Christer BjörkmanAbstract:Insect predators often utilize plant cues to locate herbivores and plant food for sustenance when prey is scarce. The capacity of many plants to attract and support predators has sometimes been interpreted as an indirect plant defence system, which has evolved because it reduces the plants’ exposure to detrimental herbivores. In accordance with this hypothesis, plant breeders have begun to select for crop varieties that attract and support biocontrol agents. We have tested whether predator proneness to consume herbivores is affected by variation in the plant food quality provided by different Salix (willow) varieties. Plant suitability as food for the important biocontrol agent Anthocoris nemorum (Common Flower Bug) varied widely between the tested Salix varieties. The proneness of Anthocoris to consume eggs of the detrimental herbivore Phratora vulgatissima also varied between the varieties, consuming eggs at much lower rates on those that provided high quality plant food. The results suggest that plants that provide too good plant food may satiate their “bodyguards”, thereby reducing their motivation to hunt for herbivorous prey. Thus, the capacity of plants to support predators may sometimes result in a partially enemy-free space for herbivores – an outcome conflicting with the notion of “indirect defence”.
Johan A. Stenberg - One of the best experts on this subject based on the ideXlab platform.
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Plant sex effects on insect herbivores and biological control in a Short Rotation Coppice willow
Biological Control, 2017Co-Authors: Kim K. Moritz, Christer Björkman, Amy L. Parachnowitsch, Johan A. StenbergAbstract:In the wild, plant sex can affect plant-herbivore interactions and higher trophic levels, including natural enemies of the herbivores. However, the possibility of manipulating plant sex to improve biological control and reduce herbivory in domesticated dioecious crops remains unexplored. The dioecious bioenergy crop, Salix viminalis, is often planted in monoclonal, and thus monosexual, fields. We investigated whether using plant clones of either sex, or mixing plants of both sexes, reduced the performance and abundance of the herbivorous pest insect Phratora vulgatissima and its main natural enemy, Anthocoris nemorum, and whether predation was affected. The herbivore laid more eggs, and the predator survived longer, on female plants in the lab. However, these effects did not translate into differences in predation rates in laboratory experiments or differential insect abundances on plants of either sex or plantation sex composition in the field. Plant genotype did have a significant effect on insect abundances, but this was due to plant traits other than sex. The results indicate that manipulating plant sex will not lead to improved biological control or reduced insect herbivory in S. viminalis energy forestry, but suggest that a focus on plant genotypic differences offers promise for improving management practices.
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predator hunting mode and host plant quality shape attack abatement patterns of predation risk in an insect herbivore
Ecosphere, 2016Co-Authors: Jorg G Stephan, Johan A. Stenberg, Christer BjörkmanAbstract:Group formation reduces individual predation risk when the proportion of prey taken per predator encounter declines faster than the increase in group encounter rate (a ack-abatement). Despite a ack-abatement being an important component of group formation ecology, several key aspects have not been empirically studied, that is, interactions with the hunting mode of the predator and how these relationships are modi ed by local habitat quality. In 79 cage trials, we examined individual egg predation risk in di erent-sized egg clutches from the blue willow beetle Phratora vulgatissima for two predators with di erent hunting modes (consumption of full group [Orthotylus marginalis] vs. part group [Anthocoris nemorum]). Because these predators also take nutrients from plant sap, we could examine how the quality of alternative food sources (high- vs. low-quality host plant sap) in uenced a ack-abatement pa erns in the presence of di erent hunting strategies. For the O. marginalis predator, individual egg predation risk was largely independent of group size. For A. nemorum, egg predation risk clearly declined with increasing group size. However, approximately one-third of the grouping bene t was lost to an increase in group detectability. There were clear di erences in a ack-abatement pa erns between plants with high- vs. low-quality sap. When O. marginalis was the predator, there was no clear change in a ack-abatement in relation to host plant quality. However, for A. nemorum there was a clear reduction in overall predation risk and a stronger a ack-abatement pa ern with increasing group size when plant sap quality increased. This implies that the relative bene ts of prey grouping behavior for any species might show diurnal or seasonal changes as other aspects of resource/habitat quality change for the focal predator. Modulation of a ack-abatement by bo om-up e ects such as plant-based food resources is yet to be incorporated into general theory, despite the ubiquity of omnivorous predators and with omnivory being important for shaping food webs, ecosystem functions, and in biological control. Thus, ongoing re nement of a ack- abatement theory by focusing on bo om-up vs. top-down processes could have signi cant impacts on many important contemporary elds of study.
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Plant-sex-biased tritrophic interactions on dioecious willow.
Ecosphere, 2014Co-Authors: Faisal Kabir, Kim K. Moritz, Johan A. StenbergAbstract:Plant sex effects on herbivores are well studied, but little is known about these effects on predators and predator-herbivore dynamics. Here we take a holistic approach to study, simultaneously, plant sex effects on herbivore and predator preference and performance, as well as population densities and predation pressure in the field. For dioecious Salix cinerea (grey willow) we found that male plants represented higher host plant quality than females for an omnivorous predator (Anthocoris nemorum, common flower bug), while host plant quality for its herbivorous prey (Phratora vulgatissima, blue willow beetle) was not sex-biased. The herbivore strongly preferred the host plant sex (female) that was suboptimal for the predator, which in turn followed its prey to female plants, leading to plant-sex-biased predation. These results provide new insight into the far-reaching effects of plant sex on insect communities, and open up novel opportunities for improving biocontrol of the herbivore in Salix short rotation coppice.
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Constitutive and herbivore-induced systemic volatiles differentially attract an omnivorous biocontrol agent to contrasting Salix clones.
AoB PLANTS, 2013Co-Authors: Anna Lehrman, Tina Boddum, Johan A. Stenberg, Colin M. Orians, Christer BjörkmanAbstract:While carnivores are known to be attracted to herbivore-induced plant volatiles, little is known about how such volatiles may affect the behaviour of omnivorous predators that may use both plants and herbivores as food. Here, we examine how systemically produced plant volatiles, in response to local herbivore damage, differentially attract a key omnivorous predator, Anthocoris nemorum (Heteroptera: Anthocoridae), to single clones of three species of Salix: S. viminalis, S. dasyclados and S. cinerea. The profiles of the plant volatiles produced were found to vary among Salix clones and between herbivore-damaged and intact plants. Anthocoris nemorum was attracted to the volatiles released from undamaged plants of all three species, but most strongly to a native S. cinerea clone. Plants damaged by the herbivorous leaf beetle Phratora vulgatissima (Coleoptera: Chrysomelidae) were generally more attractive than undamaged plants, with A. nemorum responding to systemic changes in the damaged plants where the experimental design specifically excluded volatiles released from the actual site of damage. When comparing damaged plants, the S. dasyclados clone was more attractive to A. nemorum than the S. viminalis clone—a somewhat surprising result since this Salix clone is considered relatively resistant to P. vulgatissima, and hence offers a limited amount of prey. Our experiments highlight that both constitutive and induced plant volatiles play a role in omnivore attraction, and this emphasizes the importance of considering odours of released volatiles when cropping and breeding Salix for increased resistance to herbivores.
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Host‐plant genotype mediates supply and demand of animal food in an omnivorous insect
Ecological Entomology, 2011Co-Authors: Johan A. Stenberg, Anna Lehrman, Christer BjörkmanAbstract:1. Omnivorous predators can protect plants from herbivores, but may also consume plant material themselves. Omnivores and their purely herbivorous prey have previously been thought to respond similarly to host-plant quality. However, different responses of omnivores and herbivores to their shared host plants may influence the fitness, trophic identity, and population dynamics of the omnivores. 2. The aim of the present study was to show that an omnivorous heteropteran (Anthocoris nemorum L.) and two strictly herbivorous prey species respond differently to different genotypes of their shared host plant, Salix. Some plant genotypes were sub-optimal for the omnivore, although suitable for the herbivores, and vice versa. 3. The contrasting patterns of plant suitability for the omnivore and the herbivores highlight an interaction between plant genotype and omnivores' access to animal food. Plant genotypes that were sub-optimal for the omnivore when herbivores were experimentally excluded became the best host plants when herbivores were present, as in the latter situation additional prey became available. By contrast, the quality of plant genotypes that were intrinsically suitable for omnivores, did not improve when herbivores were present as these plant genotypes were intrinsically sub-optimal for herbivores, thus providing omnivores with almost no additional animal food. 4. The differential responses of omnivores and their prey to the same host-plant genotypes should allow omnivores to colonise sub-optimal host plants in their capacity as predators, and to colonise more suitable host plants in their capacity as herbivores. It may thus be difficult for Salix to escape herbivory entirely, as it will rarely be unsuitable for both omnivores and pure herbivores at the same time.
Peter Dalin - One of the best experts on this subject based on the ideXlab platform.
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K (2003) Generalist natural enemies of a willow leaf beetle (Phratora vulgatissima): abundance and feeding habits
2015Co-Authors: Peter Dalin, Karin EklundAbstract:The natural enemies attacking eggs (and young larvae) of the willow leaf beetle Phratora vulgatissima were identified in the field. Three heteropterans were common natural enemies. The mirid Orthotylus marginalis was the most abundant and had an intermediate consumption rate in the lab, whereas the mirid Closterotomus fulvomaculatus was the least abundant but had the high-est consumption rate. The anthocorid Anthocoris nemorum was intermediate in abundance but had the lowest consumption rate. However, the experimental situation (in petridish or on shoot) affected the ranking of the predators and illustrates behavioral differences. The anthocorid was very mobile and could be characterized as a “run and eat ” predator, whereas the mirids were less mobile and behaved to a “find and stay ” principle. Possible consequences of interspecific variation in behavior, from a biological control perspective, are discussed. KEY WORDS: Chrysomelidae; predation; Anthocoridae; Miridae; omnivorous; generalist predators
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Predator foraging strategy influences prey population dynamics: arthropods predating a gregarious leaf beetle
Animal Behaviour, 2006Co-Authors: Peter Dalin, Oskar Kindvall, Christer BjörkmanAbstract:We examined whether behavioural variation within an enemy complex attacking the willow leaf beetle, Phratora vulgatissima, influences the population dynamics of this gregarious prey. The most common enemies are three species of heteropteran arthropods: the two mirids Orthotylus marginalis and Closterotomus fulvomaculatus and the anthocorid Anthocoris nemorum. When attacking egg clusters on plants in the laboratory, the two mirids consumed a greater proportion of eggs within egg clusters than the anthocorid. The anthocorid visited and ate eggs from more egg clusters than both the mirids. The two foraging strategies have been characterized as ‘find and stay’ for the mirids and ‘run and eat’ for the anthocorid. By using a stochastic exponential growth model we showed that model prey experienced different temporal dynamics when exposed to predators that differ in the probabilities of finding prey aggregations and of consuming prey within aggregations. Model prey exposed to the find and stay type of predator was less likely to become established and to increase in abundance than model prey exposed to the run and eat type. In a field study, we found a correspondence between high abundance of find and stay mirids and low densities of leaf beetles. The results suggest that, even when average predation rate is constant, the foraging strategy of the predator can have population level consequences for the prey. The consumption of prey in dense patches seems to be important in the control of gregarious prey, especially in the early phase of prey population establishment.
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Generalist Natural Enemies of a Willow Leaf Beetle (Phratora vulgatissima): Abundance and Feeding Habits
Journal of Insect Behavior, 2003Co-Authors: Christer Björkman, Peter Dalin, Karin EklundAbstract:The natural enemies attacking eggs (and young larvae) of the willow leaf beetle Phratora vulgatissima were identified in the field. Three heteropterans were common natural enemies. The mirid Orthotylus marginalis was the most abundant and had an intermediate consumption rate in the lab, whereas the mirid Closterotomus fulvomaculatus was the least abundant but had the highest consumption rate. The anthocorid Anthocoris nemorum was intermediate in abundance but had the lowest consumption rate. However, the experimental situation (in petridish or on shoot) affected the ranking of the predators and illustrates behavioral differences. The anthocorid was very mobile and could be characterized as a “run and eat” predator, whereas the mirids were less mobile and behaved to a “find and stay” principle. Possible consequences of interspecific variation in behavior, from a biological control perspective, are discussed.
Anna Lehrman - One of the best experts on this subject based on the ideXlab platform.
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Constitutive and herbivore-induced systemic volatiles differentially attract an omnivorous biocontrol agent to contrasting Salix clones.
AoB PLANTS, 2013Co-Authors: Anna Lehrman, Tina Boddum, Johan A. Stenberg, Colin M. Orians, Christer BjörkmanAbstract:While carnivores are known to be attracted to herbivore-induced plant volatiles, little is known about how such volatiles may affect the behaviour of omnivorous predators that may use both plants and herbivores as food. Here, we examine how systemically produced plant volatiles, in response to local herbivore damage, differentially attract a key omnivorous predator, Anthocoris nemorum (Heteroptera: Anthocoridae), to single clones of three species of Salix: S. viminalis, S. dasyclados and S. cinerea. The profiles of the plant volatiles produced were found to vary among Salix clones and between herbivore-damaged and intact plants. Anthocoris nemorum was attracted to the volatiles released from undamaged plants of all three species, but most strongly to a native S. cinerea clone. Plants damaged by the herbivorous leaf beetle Phratora vulgatissima (Coleoptera: Chrysomelidae) were generally more attractive than undamaged plants, with A. nemorum responding to systemic changes in the damaged plants where the experimental design specifically excluded volatiles released from the actual site of damage. When comparing damaged plants, the S. dasyclados clone was more attractive to A. nemorum than the S. viminalis clone—a somewhat surprising result since this Salix clone is considered relatively resistant to P. vulgatissima, and hence offers a limited amount of prey. Our experiments highlight that both constitutive and induced plant volatiles play a role in omnivore attraction, and this emphasizes the importance of considering odours of released volatiles when cropping and breeding Salix for increased resistance to herbivores.
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Host‐plant genotype mediates supply and demand of animal food in an omnivorous insect
Ecological Entomology, 2011Co-Authors: Johan A. Stenberg, Anna Lehrman, Christer BjörkmanAbstract:1. Omnivorous predators can protect plants from herbivores, but may also consume plant material themselves. Omnivores and their purely herbivorous prey have previously been thought to respond similarly to host-plant quality. However, different responses of omnivores and herbivores to their shared host plants may influence the fitness, trophic identity, and population dynamics of the omnivores. 2. The aim of the present study was to show that an omnivorous heteropteran (Anthocoris nemorum L.) and two strictly herbivorous prey species respond differently to different genotypes of their shared host plant, Salix. Some plant genotypes were sub-optimal for the omnivore, although suitable for the herbivores, and vice versa. 3. The contrasting patterns of plant suitability for the omnivore and the herbivores highlight an interaction between plant genotype and omnivores' access to animal food. Plant genotypes that were sub-optimal for the omnivore when herbivores were experimentally excluded became the best host plants when herbivores were present, as in the latter situation additional prey became available. By contrast, the quality of plant genotypes that were intrinsically suitable for omnivores, did not improve when herbivores were present as these plant genotypes were intrinsically sub-optimal for herbivores, thus providing omnivores with almost no additional animal food. 4. The differential responses of omnivores and their prey to the same host-plant genotypes should allow omnivores to colonise sub-optimal host plants in their capacity as predators, and to colonise more suitable host plants in their capacity as herbivores. It may thus be difficult for Salix to escape herbivory entirely, as it will rarely be unsuitable for both omnivores and pure herbivores at the same time.
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Plant defence: Feeding your bodyguards can be counter-productive
Basic and Applied Ecology, 2011Co-Authors: Johan A. Stenberg, Anna Lehrman, Christer BjörkmanAbstract:Insect predators often utilize plant cues to locate herbivores and plant food for sustenance when prey is scarce. The capacity of many plants to attract and support predators has sometimes been interpreted as an indirect plant defence system, which has evolved because it reduces the plants’ exposure to detrimental herbivores. In accordance with this hypothesis, plant breeders have begun to select for crop varieties that attract and support biocontrol agents. We have tested whether predator proneness to consume herbivores is affected by variation in the plant food quality provided by different Salix (willow) varieties. Plant suitability as food for the important biocontrol agent Anthocoris nemorum (Common Flower Bug) varied widely between the tested Salix varieties. The proneness of Anthocoris to consume eggs of the detrimental herbivore Phratora vulgatissima also varied between the varieties, consuming eggs at much lower rates on those that provided high quality plant food. The results suggest that plants that provide too good plant food may satiate their “bodyguards”, thereby reducing their motivation to hunt for herbivorous prey. Thus, the capacity of plants to support predators may sometimes result in a partially enemy-free space for herbivores – an outcome conflicting with the notion of “indirect defence”.
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Uncoupling direct and indirect plant defences: Novel opportunities for improving crop security in willow plantations
Agriculture Ecosystems & Environment, 2010Co-Authors: Johan A. Stenberg, Anna Lehrman, Christer BjörkmanAbstract:Increased cropping security, i.e. minimized risk for detrimental stress events and hence improved yield stability, may be achieved by selecting resistant plant genotypes. However, strong direct defences against herbivores have been associated with negative effects on the natural enemies, resulting in weak indirect defences through these plant “bodyguards”. We compared the preference and performance of the most detrimental herbivore, the leaf beetle Phratora vulgatissima, and a biocontrol agent, the predatory bug Anthocoris nemorum, on four willow (Salix) genotypes used in short rotation coppicing. The biocontrol agent is omnivorous and survives on the plant in the absence of prey, without causing any apparent damage to the plant. Two of the genotypes intrinsically attracted and supported the bodyguard, and were at the same time partially capable of deterring and resisting the herbivore. In contrast, the third willow clone exhibited very limited direct and indirect resistance, while the fourth clone displayed intermediate levels of both. Thus, direct and indirect defence against herbivores need not be traded off in this system, but may be intertwined, resulting in “super plants” with higher probability of resisting detrimental herbivores, opening up novel possibilities for increased cropping security, without the use of insecticides.
Hans Johansson - One of the best experts on this subject based on the ideXlab platform.
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Spatial distribution of interacting insect predators: Possible roles of intraguild predation and the surrounding habitat
Basic and Applied Ecology, 2011Co-Authors: Christer Björkman, Hans Johansson, Tord SnällAbstract:Abstract Predator foraging behaviour affects the outcome of enemy–enemy interactions. Using a combination of fieldwork and laboratory experiments, we show that intraguild predation may be important in the field distribution of generalist predators that share a common prey: the eggs (and larvae) of the leaf beetle Phratora vulgatissima , a major insect pest in coppicing willow plantations. We focused on a species from the hoverfly genus Parasyrphus (Syrphidae), which may exhibit large temporal and spatial variation in density. Predator and prey densities were quantified in 40 field plots in willow plantations. The likelihood of finding hoverfly eggs declined with increasing densities of two predatory mirids, Orthotylus marginalis and Closterotomus fulvomaculatus , which exhibit less mobile behaviour similar to that of hoverfly larvae. The density of a more mobile predatory bug species, the anthocorid Anthocoris nemorum , was not associated with hoverfly occurrence. These results corroborate the hypothesis that less mobile predators should be stronger intraguild predators than mobilepredators. Further partial support for this hypothesis was obtained in the laboratory study where individual predators were presented with clutches of P. vulgatissima eggs containing one hoverfly egg: the less mobile C. fulvomaculatus and O. marginalis tended to consume the hoverfly egg more readily than the more mobile A. nemorum . However, most individuals of all three bug species consumed the egg of the potential competitor – the syrphid – within 24 h. The field study also showed that hoverfly occurrence was positively associated with the density of their prey and with the presence of nearby forests. We conclude that intraguild predation, abundance of prey and the surrounding habitat affect the distribution of hoverflies in this system and should be considered when developing biological control methods.
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Abundance of syrphid larvae in willow plantations
2009Co-Authors: Hans JohanssonAbstract:A large number of hover fly species are as larvae effective predators and often feed on insect pests in plantations. The leaf-beetle Phratora vulgatissima is a common pest in willow plantations and the aim of this study is to study what may affect the presence of hover flies in this system. The quality of the surrounding habitats, prey availability and interactions with other natural enemies are parameters that may explain variances in densities of hover flies and was here investigated at five localities whereas a total of 40 plots of 5 x 5 meters have been studied in detail. The heteropterans Anthocoris nemorum and Orthotylus marginalis are two natural enemies to P. vulgatissima and whose foraging behaviour differs and their interactions with the hover fly may therefore differ. The syrphid larva and the mirid display a similar foraging behaviour and interactions, e.g. competition and predation, between these two species are thus likely to occur. Experiments were performed in the laboratory to study if there were any differences in proportion of consumed syrphid eggs and in time to consumption of eggs between anthocorids and mirids. The results showed that all plots in field where syrphid eggs were found were surrounded by a high proportion of forest while no eggs were found in plots next to arable land. Leaf beetle egg clutches with syrphid eggs were in average larger in size compared with clutches without. The variation in density of leaf beetle eggs was small between localities and plots, which could explain the lack of association between high densities and abundance of syrphid eggs. The abundance of other enemies, e.g. heteropterans, seemed to have a negative effect on the hover fly since no hover fly eggs were found in plots with high densities of other enemies. Both species of heteropterans consumed a large proportion of syrphid eggs. Contrary to my expectations none of the species had a larger negative impact on the survival of the syrphid neither when considering time to consumption of the egg nor the proportion of consumed syrphid eggs. This study shows that a higher diversity within the enemy-complex may have negative effects on the biological control of pests and that predators within the same guild are susceptible to predation in certain developmental stages. Hover flies are probably of little importance in the biocontrol of leaf beetles in situations with high densities of other natural enemies but have the potential to be particularly important as predators in recently harvested plantations i.e. plantations with low densities of heteropterans.