Adaptive aggregation by spider mites under predation risk

Conspecific cues mediate habitat selection and reproductive performance in a haplodiploid spider mite

Selection of a suitable habitat by animals before settlement is critical for their survival and reproduction. In silk-spinning arthropods like spider mites, denser webs offer protection from predation and serve as a dispersal mode. Settling in habitats with the presence of conspecifics and silk webs can benefit the habitat-searching females. Silk and conspecifics usually coexist, but their distinct effects on female colonization have received little attention. In this study, we used a haplodiploid spider mite, Tetranychus ludeni Zacher (Acari: Tetranychidae), to examine the impact of conspecific cues, including cues from ovipositing conspecifics and silk, on habitat selection and subsequent reproductive performance of females. Results show that females significantly preferred habitats with cues from neighboring conspecifics and silk and neighboring conspecifics induced additive effect to that of silk on habitat selection. Conspecific cues did not boost female reproduction but facilitated females laying larger eggs that were more likely to be fertilized and to develop into daughters. When given a choice between silk-covered and clean habitats, females preferred silk-covered habitats, laid a similar number of eggs with similar size, but produced more daughters, suggesting that T. ludeni females can adjust the size threshold for fertilization in response to the current social environment. Knowledge of this study improves our understanding of spider mite habitat selection and post-settlement reproductive performance behaviors.

Exudate droplets incorporated on eggs by Raoiella indica Hirst female during oviposition may avoid the predation of Amblyseius largoensis (Muma)

Raoiella indica Hirst has rapidly and widely spread throughout the New World since 2004, primarily infesting coconut palms and interacting with the predator Amblyseius largoensis (Muma). Although A. largoensis feeds on R. indica at all stages of development, it cannot naturally reduce its population to levels that do not impact the host plant. Raoiella indica possesses dorsal setae that secrete exudates during all post-embryonic developmental stages, and females have a behavior that deliberately deposits droplets on the freshly laid egg, possibly as a defense strategy against predation in vulnerable stages. In this context, we analyzed whether the presence or absence of droplets in R. indica eggs affects predation using A. largoensis as a biological model. Thus, we evaluated whether some biological and behavioral characteristics of A. largoensis could be affected by the consumption of R. indica egg masses washed or unwashed with water. Also, we performed a chemical analysis of the droplets exuded by R. indica and provided a description of the oviposition behavior of R. indica. The predator showed a higher consumption rate and preference for washed eggs. The results suggest that the exudate droplets have defensive functions, which are incorporated by the female onto the egg during oviposition and subsequently during a patrolling behavior, as they lose their effect after being washed with water. Although the droplets do not prevent the predator from feeding, they reduce the number of R. indica eggs consumed without affecting the growth of A. largoensis.

Cascading social-ecological benefits of biodiversity for agriculture

Cultivated species diversity can provide numerous benefits to agricultural systems. Many ecological theories have been proposed to understand the relationships between plant species diversity and trophic interactions. However, extending such theories to socioeconomic systems has been rare for agriculture. Here, we establish ten hypotheses (e.g., the natural enemy hypothesis, resource concentration hypothesis, insurance hypothesis, and aggregation hypothesis) about the relationships between cultivated species diversity (i.e., crop diversification, co-cultures of crops and domestic animals, and co-cultures of crops and edible fungi) and trophic cascades of crops, invertebrate herbivores and natural enemies in cropping systems. We then explore the socioeconomic advantages (e.g., yield, economic and environmental performance) of these trophic cascades. Finally, we propose a multi-perspective framework to promote the cascading social-ecological benefits of species diversity for agricultural sustainability. Integrating the benefits of trophic cascades into agricultural socioeconomic systems requires policies and legislation that support multi-species co-culture practices and the willingness of consumers to pay for these practices through higher prices for agricultural products.

Locally adaptive aggregation of organisms under death risk in rock-paper-scissors models

We run stochastic simulations of the spatial version of the rock-paper-scissors game, considering that individuals use sensory abilities to scan the environment to detect the presence of enemies. If the local dangerousness level is above a tolerable threshold, individuals aggregate instead of moving randomly on the lattice. We study the impact of the locally adaptive aggregation on the organisms' spatial organisation by measuring the characteristic length scale of the spatial domains occupied by organisms of a single species. Our results reveal that aggregation is beneficial if triggered when the local density of opponents does not exceed 30%; otherwise, the behavioural strategy may harm individuals by increasing the average death risk. We show that if organisms can perceive further distances, they can accurately scan and interpret the signals from the neighbourhood, maximising the effects of the locally adaptive aggregation on the death risk. Finally, we show that the locally adaptive aggregation behaviour promotes biodiversity independently of the organism's mobility. The coexistence probability rises if organisms join conspecifics, even in the presence of a small number of enemies. We verify that our conclusions hold for more complex systems by simulating the generalised rock-paper-scissors models with five and seven species. Our discoveries may be helpful to ecologists in understanding systems where organisms' self-defence behaviour adapts to local environmental cues.

Predation stress experienced as immature mites extends their lifespan

The early-life experience is important in modulating the late-life performance of individuals. It has been predicted that there were trade-offs between early-life fitness and late-life success. Most of the studies on senescence have focused on the trade-offs between the reproduction and lifespan, and the influences of diet, mating, and other factors. Because the negative, non-consumptive effects of predators could also modulate the behaviour and underlying mechanisms of the prey, this study aimed to examine the different effects of predator-induced stress experienced in the early life compared with later life of the prey. The prey (Tyrophagus putrescentiae) was exposed to predation stress from the predator (Neoseiulus cucumeris) during different periods of its life (immature, oviposition period, and post-oviposition period). The results showed that the predation stress experienced during immature stages delayed development by 7.3% and prolonged lifespan by 9.7%, while predation stress experienced in the adult stage (both oviposition and post-oviposition periods) decreased lifespans of T. putrescentiae (by 24.8% and 28.7%, respectively). Predation stress experienced during immature stages also reduced female fecundity by 7.3%, whereas that experienced during the oviposition period reduced fecundity of the prey by 50.7%. This study demonstrated for the first time lifespan extension by exposure to predation stress when young and highlighted the importance of early-life experience to aging and lifespan.

Predator- and killed prey-induced fears bear significant cost to an invasive spider mite: Implications in pest management

BACKGROUND

The success of biological control using predators is normally assumed to be achieved through direct predation. Yet it is largely unknown how the predator- and killed prey-induced stress to prey may contribute to biological control effectiveness. Here, we investigate variations in life-history traits and offspring fitness of the spider mite Tetranychus ludeni in response to cues from the predatory mite Phytoseiulus persimilis and killed T. ludeni, providing knowledge for evaluation of the nonconsumptive contribution to the biological control of T. ludeni and for future development of novel spider mite control measures using these cues.

RESULTS

Cues from predators and killed prey shortened longevity by 23-25% and oviposition period by 35-40%, and reduced fecundity by 31-37% in T. ludeni females. These cues significantly reduced the intrinsic rate of increase (rm ) and net population growth rate (R0 ), and extended time to double the population size (Dt ). Predator cues significantly delayed lifetime production of daughters. Mothers exposed to predator cues laid significantly smaller eggs and their offspring developed significantly more slowly but these eggs had significantly higher hatch rate.

CONCLUSION

Predator- and killed prey-induced fears significantly lower the fitness of T. ludeni, suggesting that these nonconsumptive effects can contribute to the effectiveness of biological control to a great extent. Our study provides critical information for evaluation of biological control effectiveness using predators and paves the way for identification of chemical odors from the predator and killed prey, and development of new materials and methods for the control of spider mite pests. © 2022 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Level-dependent effects of predation stress on prey development, lifespan and reproduction in mites

In predator-prey interactions, non-consumptive effects of predators have been less studied than consumptive effects. However, non-consumptive effects may have significant influences on prey and can change different aspects of their life history such as development, reproduction and lifespan. The odour and other cues associated with a predator, without direct contact, could induce stress in prey, leading to phenotypic changes in life history traits. In this study, we investigate how mild and strong predator-induced stress could affect prey life history. The prey (Tyrophagus putrescentiae) was exposed, from hatching to death, to three different levels of predation stress from its predator (Neoseiulus cucumeris) (1, 3 or 5 predator adults in an adjacent cage separated by a mesh screen). Compared with the control, both males and females under predator-induced stress had longer developmental time and shorter lifespan when the level of predation stress increased, showing significant level-dependence. In addition, females had reduced fecundity under predation stress. Sex-specific response to predation stress was observed under a low level of predation stress: females had greater reduction in lifespan than males. Furthermore, the reduction in female lifespan was due more from the decrease in the post-oviposition period than the decrease in the oviposition period. Future studies applying even milder levels of predation press, such as exposure of prey to predator cues only during part of the prey lifespan, may provide additional insights.

Toxicity and Control Efficacy of an Organosilicone to the Two-Spotted Spider Mite Tetranychus urticae and Its Crop Hosts

Organosilicone molecules represent important components of surfactants added to pesticides to improve pest control efficiency, but these molecules also have pesticidal properties in their own right. Here, we examined toxicity and control efficacy of Silwet 408, a trisiloxane ethoxylate-based surfactant, to the two-spotted spider mite (TSSM), Tetranychus urticae and its crop hosts. Silwet 408 was toxic to nymphs and adults of TSSM but did not affect eggs. Field trials showed that the control efficacy of 1000 mg/L Silwet 408 aqueous solution reached 96% one day after spraying but declined to 54% 14 days after spraying, comparable to 100 mg/L cyetpyrafen, a novel acaricide. A second spraying of 1000 mg/L Silwet 408 maintained control efficacy at 97% when measured 14 days after spraying. However, Silwet 408 was phytotoxic to eggplant, kidney bean, cucumber, and strawberry plants, although phytotoxicity to strawberry plants was relatively low and declined further seven days after application. Our study showed that while the organosilicone surfactant Silwet 408 could be used to control the TSSM, its phytotoxicity to crops should be considered.

Mobility-limiting antipredator response in the rock-paper-scissors model

Antipredator behavior is present in many biological systems where individuals collectively react to an imminent attack. The antipredator response may influence spatial pattern formation and ecosystem stability but requires an organism's cost to contribute to the collective effort. We investigate a nonhierarchical tritrophic system, whose predator-prey interactions are described by the rock-paper-scissors game rules. In our spatial stochastic simulations, the radius of antipredator response defines the maximum prey group size that disturbs the predator's action, determining the individual cost to participate in antipredator strategies. We consider that each organism contributes equally to the collective effort, having its mobility limited by the proportion of energy devoted to the antipredator reaction. Our outcomes show that the antipredator response leads to spiral patterns, with the segregation of organisms of the same species occupying departed spatial domains. We found that a less localized antipredator response increases the average size of the single-species patches, improving the protection of individuals against predation. Finally, our findings show that although the increase of the predation risk for a more localized antipredator response, the high mobility constraining benefits species coexistence. Our results may help ecologists understand the mechanisms leading to the stability of biological systems where locality is crucial to behavioral interactions among species.

Cooperative Behaviors in Group-Living Spider Mites

Cooperative behaviors are evolutionary stable if the direct and/or indirect fitness benefits exceed the costs of helping. Here we discuss cooperation and behaviors akin to cooperation in subsocial group-living species of two genera of herbivorous spider mites (Tetranychidae), i.e., the largely polyphagous Tetranychus spp. and the nest-building Stigmaeopsis spp., which are specialized on grasses, such as bamboo. These spider mites are distributed in patches on various spatial scales, that is, within and among leaves of individual host plants and among individual hosts of single or multiple plant species. Group-living of spider mites is brought about by plant-colonizing foundresses ovipositing at local feeding sites and natal site fidelity, and by multiple individuals aggregating in the same site in response to direct and/or indirect cues, many of which are associated with webbing. In the case of the former, emerging patches are often composed of genetically closely related individuals, while in the case of the latter, local patches may consist of kin of various degrees and/or non-kin and even heterospecific spider mites. We describe and discuss ultimate and proximate aspects of cooperation by spider mites in host plant colonization and exploitation, dispersal, anti-predator behavior, and nesting-associated behaviors and conclude with theoretical and practical considerations of future research on cooperation in these highly rewarding model animals.

Antipredator behavior in the rock-paper-scissors model

When faced with an imminent risk of predation, many animals react to escape consumption. Antipredator strategies are performed by individuals acting as a group to intimidate predators and minimize the damage when attacked. We study the antipredator prey response in spatial tritrophic systems with cyclic species dominance using the rock-paper-scissors game. The impact of the antipredator behavior is local, with the predation probability reducing exponentially with the number of prey in the predator's neighborhood. In contrast to the standard Lotka-Volterra implementation of the rock-paper-scissors model, where no spiral waves appear, our outcomes show that the antipredator behavior leads to spiral patterns from random initial conditions. The results show that the predation risk decreases exponentially with the level of antipredator strength. Finally, we investigate the coexistence probability and verify that antipredator behavior may jeopardize biodiversity for high mobility. Our findings may help biologists to understand ecosystems formed by species whose individuals behave strategically to resist predation.

Integrating top-down and bottom-up effects of local density across scales and a complex life cycle

Effects of group size (local conspecific density) on individual performance can be substantial, yet it is unclear how these translate to larger-scale and longer-term outcomes. Effects of group size can be mediated by both top-down and bottom-up interactions, can change in type or direction across the life cycle, and can depend on the spatial scale at which group size is assessed. Only by determining how these different processes combine can we make predictions about how selection operates on group size or link hierarchical patterns of density dependence with population dynamics. We manipulated the density of a leaf beetle, Leptinotarsa juncta, at three nested spatial scales (patch, plant within a patch, and leaf within plant) to investigate how conspecific density affects predator-mediated survival and resource-mediated growth during different life stages and across multiple spatial scales. We then used data from field predation experiments to assess how L. juncta densities at hierarchical scales affect different aspects of predation. Finally, we incorporated predator- and resource-mediated effects of density in a model to explore how changes in group size due to density-dependent predation might affect mass at pupation for survivors. The effects of L. juncta density on predation risk differed among scales. Per capita predation risk of both eggs and late instars was lowest at high patch-scale densities, but increased with plant-scale density. The final mass of late instars declined with increasing plant-scale larval density, potentially because of truncated development of high-density larvae. Predation incidence (i.e., group attack rate) increased with larval density at all spatial scales. A high coefficient of variation (i.e., greater aggregation) of L. juncta density was associated with lower predation incidence at some scales. Our model suggested that predator- and resource-mediated effects of density interact: lower survival at high larval density is mitigated by high final mass of larvae in the resulting smaller groups. Our results emphasize the importance of spatial scale and demonstrate that effects of top-down and bottom-up interactions are not necessarily independent. To understand how group size influences fitness, predator- and resource-mediated effects of density should be measured in their demographic and spatial context, and not in isolation.

Using mathematical models to describe aerial dispersal and silk ball formation of peanut red spider mite, Tetranychus ogmophallos (Acari: Tetranychidae)

Peanut red spider mite, Tetranychus ogmophallos, exhibits a peculiar dispersal behavior using silk balls, which involves clustering of mites and spinning of webs at the top of plants. Such a dispersal mechanism has not been studied for this species yet. Therefore, this study aimed at using mathematical models to describe aerial dispersal and silk ball formation of peanut red spider mite on peanut plants. The influence of wind speed, generated by a wind tunnel, on the dispersal of mites was studied in two experiments, one with 500 mites per plant and one with 1000 mites per plant, and six wind speeds (5, 10, 15, 20, 25, and 30 km h^-1) for each mite density. The proportion of displaced mites and the distance they were blown were measured. Another series of experiments considered the formation of silk balls to assess how fast balls were formed as a function of time and the number of mites present on a peanut plant. Data from the wind tunnel experiments were analyzed by logistic regression and multiple regression to assess the proportion of displaced mites and the distance moved, respectively, as functions of wind speed and the initial density of mites on the donor plant. The distribution of dispersal distances from the donor plant was fitted by a mathematical model proposed by Ricker (J Fish Res Board Can 11:559-623, 1954). The number of mites moving upwards on a plant to be involved in silk ball formation was modeled as a function of time based on the initial number of spider mites and their estimated birth, death and movement rates per capita. Logistic regression was used to analyze the presence of balls as a function of time elapsed since a plant was infested with spider mites. Finally, non-linear regression was applied to link ball size to the total number of mites occupying the ball. The data analyses revealed that wind speed had a significant positive effect on take-off probability and distance moved by individual mites, whereas mite density had little influence. Ricker's model adequately described the distribution of dispersal distances. The models describing silk ball formation also described data very well. Ball size was found to increase almost linearly with the number of mites found in the ball. We expect that the knowledge provided by the present study will help to develop efficient management strategies against T. ogmophallos in peanut crops as dispersal seems to be a key factor in the species' capability to become a serious pest.

Predator-induced stress responses in insects: A review

Predators can induce extreme stress and profound physiological responses in prey. Insects are the most dominant animal group on Earth and serve as prey for many different predators. Although insects have an extraordinary diversity of anti-predator behavioral and physiological responses, predator-induced stress has not been studied extensively in insects, especially at the molecular level. Here, we review the existing literature on physiological predator-induced stress responses in insects and compare what is known about insect stress to vertebrate stress systems. We conclude that many unrelated insects share a baseline pathway of predator-induced stress responses that we refer to as the octopamine-adipokinetic hormone (OAH) axis. We also present best practices for studying predator-induced stress responses in prey insects. We encourage investigators to compare neurophysiological responses to predator-related stress at the organismal, neurohormonal, tissue, and cellular levels within and across taxonomic groups. Studying stress-response variation between ecological contexts and across taxonomic levels will enable the field to build a holistic understanding of, and distinction between, taxon- and stimulus-specific responses relative to universal stress responses.

Spatial structure arising from chase-escape interactions with crowding

Movement of individuals, mediated by localised interactions, plays a key role in numerous processes including cell biology and ecology. In this work, we investigate an individual-based model accounting for various intraspecies and interspecies interactions in a community consisting of two distinct species. In this framework we consider one species to be chasers and the other species to be escapees, and we focus on chase-escape dynamics where the chasers are biased to move towards the escapees, and the escapees are biased to move away from the chasers. This framework allows us to explore how individual-level directional interactions scale up to influence spatial structure at the macroscale. To focus exclusively on the role of motility and directional bias in determining spatial structure, we consider conservative communities where the number of individuals in each species remains constant. To provide additional information about the individual-based model, we also present a mathematically tractable deterministic approximation based on describing the evolution of the spatial moments. We explore how different features of interactions including interaction strength, spatial extent of interaction, and relative density of species influence the formation of the macroscale spatial patterns.

Preference and performance of the two-spotted spider mite Tetranychus urticae (Acari: Tetranychidae) on strawberry cultivars

The two-spotted spider mite (TSSM), Tetranychus urticae Koch (Acari: Tetranychidae), is one of the most serious pests of strawberry worldwide. Understanding the preference of TSSM for particular cultivars of strawberry and performance on them helps identify host-plant resistance to this pest mite. In this study, we tested preference, developmental duration, fecundity and population levels of TSSM on 14 strawberry cultivars. TSSM showed strong preference for the Chinese cultivars of Yanxiang, Baixuegongzhu, and Jingtaoxiang. Development of TSSM on the cultivars varied from 32.32 to 36.82 days; it was longest on the cultivars Hongxiutianxiang and Baixuegongzhu, and shortest on Yanxiang, Jingzangxiang, and Darselect as well as on a wild variety (Wuye). TSSM had high fecundity on the cultivars Yanxiang, Taoxun, Hongxiutianxiang, Jingzangxiang, Albion and Baixuegongzhu as well as on Wuye, whereas egg production was lowest on Sweet Charlie, Portola, Akihime, and Benihoppe. After 28 days of plant infestation with 10 pairs of adults, the cultivars Yanxiang, Taoxun, Jingzangxiang, Jingtaoxiang, and Baixuegongzhu had the highest number of mites (> 1000 per plant), whereas mite numbers on Albion and Camarosa were low. The population size of TSSM was correlated with fecundity, but no correlation was found between other preference/performance measures. Our study suggests that a rapid increase of population size of TSSM on cultivars of strawberry is related to high fecundity, and also that there are substantial differences in preference and performance across cultivars.