Invasion success in communities with reciprocal intraguild predation depends on the stage structure of the resident population

Review: predatory soil mites as biocontrol agents of above- and below-ground plant pests

Biological pest control is becoming increasingly important for sustainable agriculture. Although many species of natural enemies are already being used commercially, efficient biological control of various pests is still lacking, and there is a need for more biocontrol agents. In this review, we focus on predatory soil mites, their role as natural enemies, and their biocontrol potential, mainly in vegetable and ornamental crops, with an emphasis on greenhouse systems. These predators are still underrepresented in biological control, but have several advantages compared to predators living on above-ground plant parts. For example, predatory soil mites are often easy and affordable to mass rear, as most of them are generalist predators, which also means that they may be used against various pests and can survive periods of pest scarcity by feeding on alternative prey or food. Many of them can also endure unfavourable conditions, making it easier for them to establish in various crops. Based on the current literature, we show that they have potential to control a variety of pests, both in greenhouses and in the field. However, more research is needed to fully understand and appreciate their potential as biocontrol agents. We review and discuss several methods to increase their efficiency, such as supplying them with alternative food and changing soil/litter structure to enable persistence of their populations. We conclude that predatory soil mites deserve more attention in future studies to increase their application in agricultural crops.

The multiple meanings of omnivory influence empirical, modular theory and whole food web stability relationships

The persistence of whole communities hinges on the presence of select interactions which act to stabilize communities making the identification of these keystone interactions critical. One potential candidate is omnivory, yet theoretical research on omnivory thus far has been dominated by a modular theory approach whereby an omnivore and consumer compete for a shared resource. Empirical research, however, has highlighted the presence of a broader suite of omnivory modules. Here, we integrate empirical data analysis and mathematical models to explore the influence of both omnivory module (including classic, multi-resource, higher level, mutual predation and cannibalism) and omnivore-resource interaction type on food web stability. We use six classic empirical food webs to examine the prevalence of the different types of omnivory, a multi-species consumer-resource model to determine the stability of these different kinds of omnivory within a module context, and finally extend these models to a 50 species, whole food web model to examine the influence of omnivory on whole food web persistence. Our results challenge the concept that omnivory is broadly stabilizing. In particular, we demonstrate that the impact of omnivory depends on the type of omnivory being examined with multi-resource omnivory having the largest correlation with whole food web persistence. Moreover, our results highlight that we need to exercise caution when scaling modular theory to whole food web theory. Cannibalism, for example, was the most persistent and stable omnivory module in the modular theory analysis, but only demonstrated a weak correlation with whole food web persistence. Lastly, our results demonstrate that the frequency of omnivory modules are more important for whole food web persistence than the frequency of omnivore-resource interactions. Together, these results demonstrate that the role of omnivory often depends both on the type of omnivory being examined and the food web within which it is nested. In whole food web models, omnivory acts less as a keystone interaction, rather, specific types of omnivory, particularly multi-resource omnivory, act as keystone modules. Future work integrating module and whole food web theory is critical for resolving the role of key interactions in food webs.

Enhanced recruitment of larger predators in the presence of large prey

Most carnivores undergo diet shift from smaller to larger prey items during ontogeny. The trophic relationship between a growing carnivore and larger prey is representative of a size-structured predator-prey interaction. The strength of this interaction is, in part, determined by the recruitment of individuals from smaller predatory size classes into larger predatory size classes. Therefore, it is interesting to investigate how larger prey alter the recruitment of smaller predator size classes into larger predator size classes, since this can affect their own future predation risk. Past empirical studies have exclusively documented that large prey reduce predator recruitment by decreasing growth and/or survival of the smaller predators. In this study, we provide empirical evidence of the contrasting pattern: large prey enhance the recruitment of smaller predators into larger predators even though they increase cannibalism mortality of the smaller predators. We have done this here by studying the trophic interaction between predatory salamander larvae Hynobius retardatus and the frog tadpoles Rana pirica that represent their large prey. In a field experiment in which salamander hatchlings were exposed to the presence or absence of large frog tadpoles, we found that more giant salamanders emerged in the presence of frog tadpoles than in their absence. Reassignment of frog tadpoles (to both treatments) in the subsequent experimental period showed that the enhanced emergence of giant salamanders in the presence of frog tadpoles leads to the intensification of salamander predation on the frog tadpoles. In an additional laboratory experiment, to better understand the underlying mechanisms, we manipulated both the presence of frog tadpoles and the occurrence of cannibalism between salamander hatchlings. This experiment revealed that frog tadpoles intensify the cannibalism of salamander larvae during their hatchling stage, thus allowing more salamander larvae to become large-sized predators. Our results suggest that frog tadpoles can inadvertently intensify their own future predation risk by intensifying cannibalistic interactions among predatory salamander hatchlings, thereby enhancing the degree of predator recruitment to a larger size class. Hence, large prey can enhance the recruitment of individuals from small predator size classes into larger predator size classes.

Ontogenetic stage-specific reciprocal intraguild predation

The size or stage of interacting individuals is known to affect the outcome of ecological interactions and can have important consequences for population dynamics. This is also true for intraguild predation (the killing and eating of potential competitors), where the size or ontogenetic stage of an individual determines whether it is the intraguild predator or the intraguild prey. Studying size- or stage-specific interactions is therefore important, but can be challenging in species with complex life histories. Here, we investigated predatory interactions of all feeding stages of the two predatory mite species Neoseiulus californicus and Phytoseiulus macropilis, both of which have complex life cycles, typical for predatory arthropods. Populations of these two species compete for two-spotted spider mites, their prey. We evaluated both the capacity to kill stages of the other predator species and the capacity to benefit from feeding on these stages, both prerequisites for the occurrence of intraguild predation. Ontogeny played a critical role in the occurrence of intraguild predation. Whereas the juveniles of P. macropilis developed from larva until adulthood when feeding on N. californicus eggs, interestingly, adult female P. macropilis did not feed on the smaller stages of the other species. We furthermore show that intraguild predation was reciprocal: both juveniles and adult females of N. californicus preyed on the smallest stages of P. macropilis. These results suggest that a proper analysis of the interactions between pairs of species involved in intraguild predation should start with an inventory of the interactions among all ontogenetic stages of these species.

Reciprocal intraguild predation and predator coexistence

Intraguild predation is a mix of competition and predation and occurs when one species feeds on another species that uses similar resources. Theory predicts that intraguild predation hampers coexistence of species involved, but it is common in nature. It has been suggested that increasing habitat complexity and the presence of alternative food may promote coexistence. Reciprocal intraguild predation limits possibilities for coexistence even further. Habitat complexity and the presence of alternative food are believed to promote coexistence. We investigated this using two species of predatory mites, Iphiseiodes zuluagai and Euseius concordis, by assessing co-occurrence in the field and on arenas differing in spatial structure in the laboratory. The predators co-occured on the same plants in the field. In the laboratory, adults of the two mites fed on juveniles of the other species, both in the presence and the absence of a shared food source, showing that the two species are involved in reciprocal intraguild predation. Adults of I. zuluagai also attacked adults of E. concordis. This suggests limited possibilities for coexistence of the two species. Indeed, E. concordis invariably went extinct extremely rapidly on arenas without spatial structure with populations consisting of all stages of the two predators and with a shared resource. Coexistence was prolonged on host plant leaves with extra food sources, but E. concordis still went extinct. On small, intact plants, coexistence of the two species was much longer, and ended with the other species, I. zuluagai, often going extinct. These results suggest that spatial structure and the presence of alternative food increase the coexistence period of intraguild predators.

Deadly competition and life-saving predation: the potential for alternative stable states in a stage-structured predator-prey system

Predators often undergo complete ontogenetic diet shifts, engaging in resource competition with species that become their prey during later developmental stages. Theory posits that this mix of stage-specific competition and predation, termed life-history intraguild predation (LHIGP), can lead to alternative stable states. In one state, prey exclude predators through competition (i.e. juvenile competitive bottleneck), while in the alternative, adult predators control prey density to limit competition and foster coexistence. Nevertheless, the interactions leading to these states have not been demonstrated in an empirical LHIGP system. To address this gap, we manipulated densities of cannibalistic adult cyclopoid copepods (Mesocyclops edax) and their cladoceran prey (Daphnia pulex) in a response-surface design and measured the maturation and survival of juvenile copepods (nauplii). We found that Daphnia reduced and even precluded both nauplii maturation and survival through depletion of a shared food resource. As predicted, adult copepods enhanced nauplii maturation and survival through Daphnia consumption, yet this positive effect was dependent on the relative abundance of Daphnia as well as the absolute density of adult copepods. Adult copepods reduced nauplii survival through cannibalism at low Daphnia densities and at the highest copepod density. This work demonstrates that predation can relax a strong juvenile competitive bottleneck in freshwater zooplankton, though cannibalism can reduce predator recruitment. Thus, our results highlight a key role for cannibalism in LHIGP dynamics and provide evidence for the interactions that drive alternative stable states in such systems.

How to evaluate the potential occurrence of intraguild predation

Intraguild predation is the combination of exploitative competition and predation among potential competitors that use similar resources. It has the potential to shape population dynamics and community structure. Although there is much empirical evidence for the occurrence of intraguild predation in natural ecosystems, the study of its effects is mainly limited to short-term microcosm experiments. There is, therefore, certain skepticism about its actual significance in nature. A relevant concern is that there is no consensus regarding criteria to evaluate the possible occurrence of intraguild predation in short-term experiments, and methodological differences may therefore underlie apparent inconsistencies among studies. Our purpose here was to evaluate existing criteria to offer guidance for the design of experiments to determine whether two species may potentially engage in intraguild predation. The criteria are based on the condition that intraguild predators need to experience immediate energetic gains when feeding on the intraguild prey. Thus, a relevant experimental design must quantify predation but also fitness benefits of feeding on the other species, i.e. increases in reproduction, somatic growth, or survival.

Intraguild predation between phytoseiid mite species might not be so common

It is widely acknowledged that intraguild predation (IGP) occurs frequently between species of phytoseiid mites. However, in the presence of a shared resource many species of phytoseiid mites considerably reduce, or even cease, predation on each other. That being the case, IGP would then be minimal, or non-existent, and its theoretical effects on communities negligible. The aim of this work was two-fold. On the one hand, we aimed at determining the occurrence of IGP between two species of phytoseiid mites that inhabit avocado agro-ecosystems (Euseius stipulatus and E. scutalis) while considering the influence of abiotic conditions. On the other hand, we aimed at evaluating the occurrence of IGP between species of phytoseiid mites through a literature search of studies to determine whether methodologies and results in these papers supported the extended idea of IGP being widespread in the Phytoseiidae family. Our results suggested that in the presence of the shared resource predation on the IG-prey was negligible and both species seem to forage preferentially on pollen. Therefore, the interaction that most likely drives the dynamics of these two species in the field is exploitative resource competition. The literature search revealed that caution should be taken when assuming that IGP between phytoseiid mites is widespread, because only few works used experimental set ups with the adequate array of treatments allowing to assess whether IG-predators fed or not on both the IG-prey and the shared resource.

Distribution and oviposition site selection by predatory mites in the presence of intraguild predators

When intraguild prey and intraguild predators feed and reproduce in the same habitat and relatively immobile juveniles are the vulnerable stage, predation risk depends on oviposition site selection by the adult females. We studied how the availability of oviposition sites affected the distribution of two predatory mite species, Neoseiulus cucumeris (Oudemans) and Iphiseius degenerans (Berlese), over two patches that both contained food. The two plant-inhabiting species feed on pollen and thrips, prey on each other's juveniles, and prefer to oviposit on hairy parts of the leaf. When an artificial oviposition site was provided on one of two connected patches, both predator species strongly preferred this patch. Whereas the distributions of adults and eggs of N. cucumeris over the two patches were not affected by the presence of heterospecifics, the proportions of adults and eggs of I. degenerans on the patch with an oviposition site were reduced by the presence of N. cucumeris. A similar change in distribution was induced by cues of N. cucumeris on the oviposition site, without these mites being present. Hence, intraguild prey can weaken the strength of intraguild predation through patch selection, which in turn may promote coexistence of the two predator species.

Role of supplemental foods and habitat structural complexity in persistence and coexistence of generalist predatory mites

Plant traits can influence the interactions between herbivore arthropods and their natural enemies. In these interactions generalist predators are often present, preying on herbivores and also on other arthropods in the same trophic guild. Variation in the strength of intraguild predation (IGP) may be related to habitat structural complexity and to additional resources outside the narrow predator-prey relationship. In this paper we study the food web interactions on grape, which involves two generalist predatory mites. We evaluated the effects of grape powdery mildew (GPM) as supplemental food, and habitat structural complexity provided by domatia. The inoculation of GPM resulted in higher predatory mite densities and reduced the negative impact of unfavorable leaf structure for one species. Access to domatia was the main factor in promoting population abundance and persistence of predatory mites. Access to domatia and GPM availability favored the coexistence of predatory mites at a low density of the intraguild prey. Our findings suggest that structural and nutritional diversity/complexity promote predatory mite abundance and can help to maintain the beneficial mites - plants association. The effect of these factors on coexistence between predators is influenced by the supplemental food quality and relative differences in body size of interacting species.

Predation on heterospecific larvae by adult females of Kampimodromus aberrans, Amblyseius andersoni, Typhlodromus pyri and Phytoseius finitimus (Acari: Phytoseiidae)

The predatory mites Kampimodromus aberrans (Oudemans), Amblyseius andersoni (Chant), Typhlodromus pyri Scheuten and Phytoseius finitimus Ribaga are important biological control agents in orchards and vineyards in Europe and elsewhere. They can coexist in the same habitat and engage in intraguild predation (IGP). In the laboratory we evaluated the longevity, fecundity and prey consumption of females of these predatory mites fed with heterospecific larvae considered as intraguild prey (IG-prey). The survival and age-specific oviposition curves of predatory mites fed with pollen were compared with those obtained on different IG-prey. We assessed the prey conversion rate into eggs expressed by the different IG-predator as an indicator of their capacity to persist when prey is diminishing. Results suggest that A. andersoni should be considered the superior intraguild predator but the least efficient in food conversion. Phytoseius finitimus appeared to suffer from intraguild predation, and its efficiency in food conversion was not superior to that of K. aberrans and T. pyri. The profiles of K. aberrans and T. pyri were less definite. The comparison between pollen and IG-prey diets confirmed the positive effect of pollen on the fecundity of all four predatory mite species. Fecundity was higher on pollen than on IG-prey. We can suggest that A. andersoni have the potential to exclude the other predatory mites only at high food resource availability, whereas low levels of food availability can favor the other species in IGP.

Phenotypic plasticity in anti-intraguild predator strategies: mite larvae adjust their behaviours according to vulnerability and predation risk

Interspecific threat-sensitivity allows prey to maximize the net benefit of antipredator strategies by adjusting the type and intensity of their response to the level of predation risk. This is well documented for classical prey-predator interactions but less so for intraguild predation (IGP). We examined threat-sensitivity in antipredator behaviour of larvae in a predatory mite guild sharing spider mites as prey. The guild consisted of the highly vulnerable intraguild (IG) prey and weak IG predator Phytoseiulus persimilis, the moderately vulnerable IG prey and moderate IG predator Neoseiulus californicus and the little vulnerable IG prey and strong IG predator Amblyseius andersoni. We videotaped the behaviour of the IG prey larvae of the three species in presence of either a low- or a high-risk IG predator female or predator absence and analysed time, distance, path shape and interaction parameters of predators and prey. The least vulnerable IG prey A. andersoni was insensitive to differing IGP risks but the moderately vulnerable IG prey N. californicus and the highly vulnerable IG prey P. persimilis responded in a threat-sensitive manner. Predator presence triggered threat-sensitive behavioural changes in one out of ten measured traits in N. californicus larvae but in four traits in P. persimilis larvae. Low-risk IG predator presence induced a typical escape response in P. persimilis larvae, whereas they reduced their activity in the high-risk IG predator presence. We argue that interspecific threat-sensitivity may promote co-existence of IG predators and IG prey and should be common in predator guilds with long co-evolutionary history.

Predator-prey role reversals, juvenile experience and adult antipredator behaviour

Although biologists routinely label animals as predators and prey, the ecological role of individuals is often far from clear. There are many examples of role reversals in predators and prey, where adult prey attack vulnerable young predators. This implies that juvenile prey that escape from predation and become adult can kill juvenile predators. We show that such an exposure of juvenile prey to adult predators results in behavioural changes later in life: after becoming adult, these prey killed juvenile predators at a faster rate than prey that had not been exposed. The attacks were specifically aimed at predators of the species to which they had been exposed. This suggests that prey recognize the species of predator to which they were exposed during their juvenile stage. Our results show that juvenile experience affects adult behaviour after a role reversal.

Habitat complexity does not promote coexistence in a size-structured intraguild predation system

Size-dependent interactions and habitat complexity have been identified as important factors affecting the persistence of intraguild predation (IGP) systems. Habitat complexity has been suggested to promote intraguild (IG) prey and intraguild predator coexistence through weakening trophic interactions particularly the predation link. Here, we experimentally investigate the effects of habitat complexity on coexistence and invasion success of differently sized IG-predators in a size-structured IGP system consisting of the IG-predator Poecilia reticulata and a resident Heterandria formosa IG-prey population. The experiments included medium-long and long-term invasion experiments, predator-prey experiments and competition experiments to elucidate the mechanisms underlying the effect of prey refuges. Habitat complexity did not promote the coexistence of IG-predator and IG-prey, although the predation link was substantially weakened. However, the presence of habitat structure affected the invasion success of large IG-predators negatively and the invasion success of small IG-predators positively. The effect of refuges on size-dependent invasion success could be related to a major decrease in the IG-predator's capture rate and a shift in the size distribution of IG-predator juveniles. In summary, habitat complexity had two main effects: (i) the predation link was diminished, resulting in a more competition driven system and (ii) the overall competitive abilities of the two species were equalized, but coexistence was not promoted. Our results suggest that in a size-structured IGP system, individual level mechanisms may gain in importance over species level mechanisms in the presence of habitat complexity.