The predatory mite Phytoseiulus persimilis adjusts patch-leaving to own and progeny prey needs

Age and Density of Mated Females Affect Dispersal Strategies in Spider Mite Tetranychus ludeni Zacher

The dispersal strategies of a species can affect its invasion success. Investigations into the dispersal strategies of invasive species in relation to different factors help improve our understanding of invasion mechanisms and provide knowledge for population management and invasion evaluation. Tetranychus ludeni Zacher (Acari: Tetranychidae) is an invasive species which is native to Europe but is now cosmopolitan. Here, we examined the effects of age and density on dispersal in mated females. Our results show that older females that are capable of producing more eggs within 24 h were more likely to disperse and moved longer distances than younger ones with fewer eggs. Older females spread most of their eggs out of their natal habitats and over longer distances, which reduced competition and increased offspring fitness. Females exhibited significantly increased dispersal probability and distances with an increase in population density to avoid crowding. The synchronization of dispersal and reproduction, along with the positive density-dependent dispersal strategy, may facilitate the habitat colonization and invasion speed of T. ludeni.

Heat waves affect prey and predators differently via developmental plasticity: who may benefit most from global warming?

BACKGROUND

Climate warming is considered to affect the characteristics of heat waves by increasing their duration, frequency and intensity, which can have dramatic consequences for ectothermic arthropods. However, arthropods may respond to heat waves via plastic modifications, which could differently affect a predator and its prey. We examined this assumption using prominent counterparts in biological control, the predatory mite Phytoseiulus persimilis and its prey, the spider mite Tetranychus urticae. Individuals of both species were separately exposed to mild and extreme heat waves during their juvenile development.

RESULTS

Both species developed faster during extreme heat waves, but the proportional increase of the developmental rates was higher in the prey. Independent of sex, P. persimilis reached smaller size at maturity under extreme heat waves, whereas the body size modifications were sex-dependent in T. urticae: males became smaller, but females were able to maintain their size.

CONCLUSIONS

An accelerated development may result in the reduction of the exposure time of susceptible juvenile stages to heat waves and prey stages to predators. Plastic size adjustments caused a shift in the female predator-prey body size ratio in favor of the prey, which may lead to higher heat resistance and reduced predation risk for prey females under extreme heat waves. In conclusion, our findings indicate that species-specific shifts in age and size at maturity may result in lower suppression efficacy of the predator P. persimilis against its prey T. urticae with severe consequences for biological control of spider mites, if global warming continues.

Maternal effect determines drought resistance of eggs in the predatory mite Phytoseiulus persimilis

The ability of an organism to adapt to short-term environmental changes within its lifetime is of fundamental importance. This adaptation may occur through phenotypic plasticity. Insects and mites, in particular, are sensitive to changes in temperature and humidity, especially during the juvenile stages. We studied the role of phenotypic plasticity in the adaptation of eggs to different relative humidity conditions, in the predatory mite Phytoseiulus persimilis, used worldwide as a biological control agent of the spider mite Tetranychus urticae. The biocontrol efficacy of P. persimilis decreases under dry conditions, partly because P. persimilis eggs are sensitive to drought. We exposed P. persimilis adult females from two different strains to constant and variable humidity regimes and evaluated the hatching rate of their eggs in dry conditions, as well as the survival and oviposition rates of these females. Whereas the eggs laid by P. persimilis females exposed to constant high humidity did not survive in dry conditions, females exposed to constant low humidity started laying drought-resistant eggs after 24 h of exposure. Survival and oviposition rates of the females were affected by humidity: females laid fewer eggs under constant low humidity and had a shorter lifespan under constant high and constant low humidity. The humidity regimes tested had similar effects across the two P. persimilis strains. Our results demonstrate that transgenerational phenotypic plasticity, called maternal effect, allows P. persimilis females to prepare their offspring for dry conditions.

Assessing the augmentation of Amblydromalus limonicus with the supplementation of pollen, thread, and substrates to combat greenhouse whitefly populations

Due to issues with establishment and persistence of natural enemies in biological control, the provision of alternative food sources and oviposition sites are important factors to enhance pest control. In this study, three different supplementation treatments were examined for their ability to increase the populations of the predatory mite Amblydromalus limonicus, and its implications for greenhouse whitefly control on peppers and eggplants. These were: (1) pollen (Typha orientalis), (2) pollen and thread, (3) pollen, thread, and a substrate mixture of buckwheat, gorse, and rice husks, which were compared to a control treatment that had no supplementation. Significant treatment effects were found on pepper for A. limonicus (mite eggs p = 0.008, mobile mites p = <0.0001). The predatory mite successfully established and persisted at high population levels in the pollen-thread, and pollen-thread-substrate treatments. All supplementation treatments were able to control whitefly populations on peppers, while the control treatment failed to. The results obtained were formulated into possible application techniques for greenhouse growers to utilise.

Maternal intraguild predation risk affects offspring anti-predator behavior and learning in mites

Predation risk is a strong selective force shaping prey morphology, life history and behavior. Anti-predator behaviors may be innate, learned or both but little is known about the transgenerational behavioral effects of maternally experienced predation risk. We examined intraguild predation (IGP) risk-induced maternal effects on offspring anti-predator behavior, including learning, in the predatory mite Phytoseiulus persimilis. We exposed predatory mite mothers during egg production to presence or absence of the IG predator Amblyseius andersoni and assessed whether maternal stress affects the anti-predator behavior, including larval learning ability, of their offspring as protonymphs. Protonymphs emerging from stressed or unstressed mothers, and having experienced IGP risk as larvae or not, were subjected to choice situations with and without IG predator traces. Predator-experienced protonymphs from stressed mothers were the least active and acted the boldest in site choice towards predator cues. We argue that the attenuated response of the protonymphs to predator traces alone represents optimized risk management because no immediate risk existed. Such behavioral adjustment could reduce the inherent fitness costs of anti-predator behaviors. Overall, our study suggests that P. persimilis mothers experiencing IGP risk may prime their offspring to behave more optimally in IGP environments.

Food stress causes sex-specific maternal effects in mites

Life history theory predicts that females should produce few large eggs under food stress and many small eggs when food is abundant. We tested this prediction in three female-biased size-dimorphic predatory mites feeding on herbivorous spider mite prey: Phytoseiulus persimilis, a specialized spider mite predator; Neoseiulus californicus, a generalist preferring spider mites; Amblyseius andersoni, a broad diet generalist. Irrespective of predator species and offspring sex, most females laid only one small egg under severe food stress. Irrespective of predator species, the number of female but not male eggs decreased with increasing maternal food stress. This sex-specific effect was probably due to the higher production costs of large female than small male eggs. The complexity of the response to the varying availability of spider mite prey correlated with the predators' degree of adaptation to this prey. Most A. andersoni females did not oviposit under severe food stress, whereas N. californicus and P. persimilis did oviposit. Under moderate food stress, only P. persimilis increased its investment per offspring, at the expense of egg number, and produced few large female eggs. When prey was abundant, P. persimilis decreased the female egg sizes at the expense of increased egg numbers, resulting in a sex-specific egg size/number trade-off. Maternal effects manifested only in N. californicus and P. persimilis. Small egg size correlated with the body size of daughters but not sons. Overall, our study provides a key example of sex-specific maternal effects, i.e. food stress during egg production more strongly affects the sex of the large than the small offspring.

Intra- and trans-generational costs of reduced female body size caused by food limitation early in life in mites

Background

Food limitation early in life may be compensated for by developmental plasticity resulting in accelerated development enhancing survival at the expense of small adult body size. However and especially for females in non-matching maternal and offspring environments, being smaller than the standard may incur considerable intra- and trans-generational costs.

Methodology/Principal Findings

Here, we evaluated the costs of small female body size induced by food limitation early in life in the sexually size-dimorphic predatory mite Phytoseiulus persimilis. Females are larger than males. These predators are adapted to exploit ephemeral spider mite prey patches. The intra- and trans-generational effects of small maternal body size manifested in lower maternal survival probabilities, decreased attractiveness for males, and a reduced number and size of eggs compared to standard-sized females. The trans-generational effects of small maternal body size were sex-specific with small mothers producing small daughters but standard-sized sons.

Conclusions/Significance

Small female body size apparently intensified the well-known costs of sexual activity because mortality of small but not standard-sized females mainly occurred shortly after mating. The disadvantages of small females in mating and egg production may be generally explained by size-associated morphological and physiological constraints. Additionally, size-assortative mate preferences of standard-sized mates may have rendered small females disproportionally unattractive mating partners. We argue that the sex-specific trans-generational effects were due to sexual size dimorphism – females are the larger sex and thus more strongly affected by maternal stress than the smaller males – and to sexually selected lower plasticity of male body size.

Smells familiar: group-joining decisions of predatory mites are mediated by olfactory cues of social familiarity

Group-living animals frequently have to trade off the costs and benefits of leaving an established group and joining another group. Owing to their high fitness relevance, group-joining decisions are commonly nonrandom and may be based on traits of both individual members and the group such as life stage, body size, social status and group density or size, respectively. Many group-living animals are able to recognize and to associate preferentially with familiar individuals, i.e. those encountered before. Hence, after dispersing from established groups, animals commonly have to decide whether to join a new familiar or unfamiliar group. Using binary choice situations we assessed the effects of social familiarity on group-joining behaviour of the plant-inhabiting predatory mite Phytoseiulus persimilis. Group living in P. persimilis is brought about by the patchy distribution of its spider mite prey and mutual conspecific attraction. In the first experiment, gravid predator females given a choice between spider mite patches occupied by unfamiliar and familiar groups of females strongly preferred to join familiar groups and to deposit their eggs in these patches. Preference for socially familiar groups was robust across biases of spider mite prey densities between choice options. The second experiment revealed that the predatory mite females can smell social familiarity from a distance. Females subjected to odour choice situations in artificial cages were more strongly attracted to the odour of familiar than unfamiliar groups. We argue that P. persimilis females preferentially join socially familiar groups because a familiar social environment relaxes competition and optimizes foraging and reproduction.

•

Group-joining decisions should be based on cues indicating fitness prospects.

•

Group-living predatory mites preferentially join socially familiar groups.

•

Attraction to familiar groups from the distance is mediated by olfactory cues.

•

Previous work showed that familiar individuals reproduce more than unfamiliar ones.

•

Thus we argue that preferentially joining socially familiar groups is adaptive.

Social familiarity relaxes the constraints of limited attention and enhances reproduction of group-living predatory mites

In many group-living animals, within-group associations are determined by familiarity, i.e. familiar individuals, independent of genetic relatedness, preferentially associate with each other. The ultimate causes of this behaviour are poorly understood and rigorous documentation of its adaptive significance is scarce. Limited attention theory states that focusing on a given task has interrelated cognitive, behavioural and physiological costs with respect to the attention paid to other tasks. In multiple signal environments attention has thus to be shared among signals. Assuming that familiar neighbours require less attention than unfamiliar ones, associating with familiar individuals should increase the efficiency in other tasks and ultimately increase fitness. We tested this prediction in adult females of the group-living, plant-inhabiting predatory mite Phytoseiulus persimilis. We evaluated the influence of social familiarity on within-group association behaviour, activity, predation and reproduction. In mixed groups (familiar and unfamiliar), familiar predator females preferentially associated with each other. In pure groups (either familiar or unfamiliar), familiar predator females produced more eggs than unfamiliar females at similar predation rates. Higher egg production was correlated with lower activity levels, indicating decreased restlessness. In light of limited attention theory, we argue that the ability to discriminate between familiar and unfamiliar individuals and preferential association with familiar individuals confers a selective advantage because familiar social environments are cognitively and physiologically less taxing than unfamiliar social environments.

Integration of multiple intraguild predator cues for oviposition decisions by a predatory mite

In mutual intraguild predation (IGP), the role of individual guild members is strongly context dependent and, during ontogeny, can shift from an intraguild (IG) prey to a food competitor or to an IG predator. Consequently, recognition of an offspring's predator is more complex for IG than classic prey females. Thus, IG prey females should be able to modulate their oviposition decisions by integrating multiple IG predator cues and by experience. Using a guild of plant-inhabiting predatory mites sharing the spider mite Tetranychus urticae as prey and passing through ontogenetic role shifts in mutual IGP, we assessed the effects of single and combined direct cues of the IG predator Amblyseius andersoni (eggs and traces left by a female on the substrate) on prey patch selection and oviposition behaviour of naïve and IG predator-experienced IG prey females of Phytoseiulus persimilis. The IG prey females preferentially resided in patches without predator cues when the alternative patch contained traces of predator females or the cue combination. Preferential egg placement in patches without predator cues was only apparent in the choice situation with the cue combination. Experience increased the responsiveness of females exposed to the IG predator cue combination, indicated by immediate selection of the prey patch without predator cues and almost perfect oviposition avoidance in patches with the cue combination. We argue that the evolution of the ability of IG prey females to evaluate offspring's IGP risk accurately is driven by the irreversibility of oviposition and the functionally complex relationships between predator guild members.

Social familiarity governs prey patch-exploitation, -leaving and inter-patch distribution of the group-living predatory mite Phytoseiulus persimilis

BACKGROUND

In group-living animals, social interactions and their effects on other life activities such as foraging are commonly determined by discrimination among group members. Accordingly, many group-living species evolved sophisticated social recognition abilities such as the ability to recognize familiar individuals, i.e. individuals encountered before. Social familiarity may affect within-group interactions and between-group movements. In environments with patchily distributed prey, group-living predators must repeatedly decide whether to stay with the group in a given prey patch or to leave and search for new prey patches and groups.

METHODOLOGY/PRINCIPAL FINDINGS

Based on the assumption that in group-living animals social familiarity allows to optimize the performance in other tasks, as for example predicted by limited attention theory, we assessed the influence of social familiarity on prey patch exploitation, patch-leaving, and inter-patch distribution of the group-living, plant-inhabiting predatory mite Phytoseiulus persimilis. P. persimilis is highly specialized on herbivorous spider mite prey such as the two-spotted spider mite Tetranychus urticae, which is patchily distributed on its host plants. We conducted two experiments with (1) groups of juvenile P. persimilis under limited food on interconnected detached leaflets, and (2) groups of adult P. persimilis females under limited food on whole plants. Familiar individuals of both juvenile and adult predator groups were more exploratory and dispersed earlier from a given spider mite patch, occupied more leaves and depleted prey more quickly than individuals of unfamiliar groups. Moreover, familiar juvenile predators had higher survival chances than unfamiliar juveniles.

CONCLUSIONS/SIGNIFICANCE

We argue that patch-exploitation and -leaving, and inter-patch dispersion were more favorably coordinated in groups of familiar than unfamiliar predators, alleviating intraspecific competition and improving prey utilization and suppression.

Social familiarity modulates group living and foraging behaviour of juvenile predatory mites

Environmental stressors during early life may have persistent consequences for phenotypic development and fitness. In group-living species, an important stressor during juvenile development is the presence and familiarity status of conspecific individuals. To alleviate intraspecific conflicts during juvenile development, many animals evolved the ability to discriminate familiar and unfamiliar individuals based on prior association and use this ability to preferentially associate with familiar individuals. Assuming that familiar neighbours require less attention than unfamiliar ones, as predicted by limited attention theory, assorting with familiar individuals should increase the efficiency in other tasks. We assessed the influence of social familiarity on within-group association behaviour, development and foraging of juvenile life stages of the group-living, plant-inhabiting predatory mite Phytoseiulus persimilis. The observed groups consisted either of mixed-age familiar and unfamiliar juvenile mites or of age-synchronized familiar or unfamiliar juvenile mites or of pairs of familiar or unfamiliar larvae. Overall, familiar mites preferentially grouped together and foraged more efficiently, i.e. needed less prey at similar developmental speed and body size at maturity, than unfamiliar mites. Preferential association of familiar mites was also apparent in the inter-exuviae distances. Social familiarity was established by imprinting in the larval stage, was not cancelled or overridden by later conspecific contacts and persisted into adulthood. Life stage had an effect on grouping with larvae being closer together than nymphal stages. Ultimately, optimized foraging during the developmental phase may relax within-group competition, enhance current and future food supply needed for optimal development and optimize patch exploitation and leaving under limited food.

Threat-sensitive anti-intraguild predation behaviour: maternal strategies to reduce offspring predation risk in mites

Predation is a major selective force for the evolution of behavioural characteristics of prey. Predation among consumers competing for food is termed intraguild predation (IGP). From the perspective of individual prey, IGP differs from classical predation in the likelihood of occurrence because IG prey is usually more rarely encountered and less profitable because it is more difficult to handle than classical prey. It is not known whether IGP is a sufficiently strong force to evolve interspecific threat sensitivity in antipredation behaviours, as is known from classical predation, and if so whether such behaviours are innate or learned. We examined interspecific threat sensitivity in antipredation in a guild of predatory mite species differing in adaptation to the shared spider mite prey (i.e. Phytoseiulus persimilis, Neoseiulus californicus and Amblyseius andersoni). We first ranked the players in this guild according to the IGP risk posed to each other: A. andersoni was the strongest IG predator; P. persimilis was the weakest. Then, we assessed the influence of relative IGP risk and experience on maternal strategies to reduce offspring IGP risk: A. andersoni was insensitive to IGP risk. Threat sensitivity in oviposition site selection was induced by experience in P. persimilis but occurred independently of experience in N. californicus. Irrespective of experience, P. persimilis laid fewer eggs in choice situations with the high- rather than low-risk IG predator. Our study suggests that, similar to classical predation, IGP may select for sophisticated innate and learned interspecific threat-sensitive antipredation responses. We argue that such responses may promote the coexistence of IG predators and prey.

Arbuscular mycorrhizal symbiosis increases host plant acceptance and population growth rates of the two-spotted spider mite Tetranychus urticae

Most terrestrial plants live in symbiosis with arbuscular mycorrhizal (AM) fungi. Studies on the direct interaction between plants and mycorrhizal fungi are numerous whereas studies on the indirect interaction between such fungi and herbivores feeding on aboveground plant parts are scarce. We studied the impact of AM symbiosis on host plant choice and life history of an acarine surface piercing-sucking herbivore, the polyphagous two-spotted spider mite Tetranychus urticae. Experiments were performed on detached leaflets taken from common bean plants (Phaseolus vulgaris) colonized or not colonized by the AM fungus Glomus mosseae. T. urticae females were subjected to choice tests between leaves from mycorrhizal and non-mycorrhizal plants. Juvenile survival and development, adult female survival, oviposition rate and offspring sex ratio were measured in order to estimate the population growth parameters of T. urticae on either substrate. Moreover, we analyzed the macro- and micronutrient concentration of the aboveground plant parts. Adult T. urticae females preferentially resided and oviposited on mycorrhizal versus non-mycorrhizal leaflets. AM symbiosis significantly decreased embryonic development time and increased the overall oviposition rate as well as the proportion of female offspring produced during peak oviposition. Altogether, the improved life history parameters resulted in significant changes in net reproductive rate, intrinsic rate of increase, doubling time and finite rate of increase. Aboveground parts of colonized plants showed higher concentrations of P and K whereas Mn and Zn were both found at lower levels. This is the first study documenting the effect of AM symbiosis on the population growth rates of a herbivore, tracking the changes in life history characteristics throughout the life cycle. We discuss the AM-plant-herbivore interaction in relation to plant quality, herbivore feeding type and site and the evolutionary implications in a multi-trophic context.

The spatial and temporal distribution of predatory and phytophagous mites in field-grown strawberry in the UK

Extensive sampling of strawberry plants in everbearing and June-bearing strawberry plantations and on potted plants showed that different species of mites were spatially separated. Of the two phytophagous species recorded, Tetranychus urticae was most abundant on old leaves and Phytonemus pallidus on folded leaves and flower/fruit clusters. Predatory phytoseiid mites were found on all plant parts but different species were spatially separated; Neoseiulus cucumeris and N. aurescens were found mostly on folded leaves and clusters, and N. californicus and Phytoseiulus persimilis on old and medium aged leaves. No Typhlodromus pyri were found in the field plantations. These patterns of distribution did not change over sampling dates in summer and early autumn. An understanding of this within-plant zonation of mite species is important when studying predator-prey interactions and when designing sampling strategies for strawberry. A programme to sample the entire mite system on strawberry should be stratified to include all the above mentioned parts of the plant. Different sampling protocols, as appropriate, are required for sampling different pest species and their associated predators.