The predatory mite Phytoseiulus persimilis manipulates imprinting among offspring through egg placement

A thelytokous predatory mite is more cannibalistic towards distant kin

Kin recognition has been widely observed in various taxa. Cannibalism avoidance may be a strong driver for the evolution of kin recognition, as it may avoid a reduction in inclusive fitness. Kin recognition has recently been observed in a generalist phytoseiid, Amblyseius herbicolus (Acari: Phytoseiidae). This study experimentally examined the degree of relatedness needed between prey larvae and cannibal adults of A. herbicolus for the occurrence of kin discrimination. The adults were individually placed in enclosed arenas with two prey, a daughter and a more distant related larva, to observe their cannibalizing choice. The adults of A. herbicolus did not discriminate between close relatives (daughter versus niece) but preferably cannibalized more distant kin (i.e., first and second cousins once removed). Phenotype matching and familiarization seem prominent as recognition mechanisms used by A. herbicolus adults. The effect of learning on kin recognition through prior contact in A. herbicolus requires further investigation. Studies on other adaptive functions of kin recognition of A. herbicolus, such as cooperation and parental care, may provide meaningful insights.

Plastic female choice to optimally balance (k)in- and out-breeding in a predatory mite

Both close inbreeding and extreme outbreeding may negatively affect direct fitness. Optimal outbreeding theory suggests that females should preferentially mate with distantly related males. (K)in breeding theory suggests that, at similar direct fitness costs of close inbreeding and extreme outbreeding, females should prefer close kin to non-kin. Empirical evidence of plastic female choice for an optimal balance between close inbreeding and extreme outbreeding remains elusive. We tested the combined predictions of optimal outbreeding and (k)in breeding theories in predatory mites Phytoseiulus persimilis from two origins, Sicily and Greece, which suffer from both close inbreeding and extreme outbreeding depression. In three separate experiments, virgin females were presented binary choices between familiar and unfamiliar brothers, and between familiar/unfamiliar brothers and distant kin or non-kin. Females of Greece but not Sicily preferred unfamiliar to familiar brothers. Females of both origins preferred distant kin to unfamiliar and familiar brothers but preferred unfamiliar brothers to non-kin. Females of Sicily but not Greece preferred familiar brothers to non-kin. The suggested kin recognition mechanisms are phenotype matching and direct familiarity, with finer-tuned recognition abilities of Greece females. Overall, our experiments suggest that flexible mate choice by P. persimilis females allows optimally balancing inclusive fitness trade-offs.

Role of kin recognition in oviposition preference and cannibalism by the predatory mite Gynaeseius liturivorus

Animals often select oviposition sites to minimize the predation risk for eggs and juveniles, which are more vulnerable to predation than adults. When females produce eggs in clusters, the eggs and juveniles are likely to suffer from cannibalism. Although cannibalism among siblings is known to be lower than among non-siblings, there have been few investigations into the possibility that females select oviposition sites that reduce the risk of cannibalism for the offspring. To test this possibility, we examined oviposition preference by adult females of the predatory mite Gynaeseius liturivorus in response to the presence of her own eggs and to eggs of other females, offering plastic discs as oviposition substrates. Although females did not clearly show a preference for plastic discs on which they had oviposited, they avoided plastic discs on which other females had oviposited. When eggs of other females were artificially placed on clean plastic discs, adult female mites avoided these discs, suggesting that the eggs were used as cues for oviposition preference. Cannibalism among juvenile siblings was lower than among non-siblings. These observations show that adult females and juveniles of G. liturivorus discriminate kin relationships among conspecific individuals. Therefore, oviposition preference by adult female G. liturivorus may lead to the reduced risk of cannibalism among offspring.

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.

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.

Heritability and artificial selection on ambulatory dispersal distance in Tetranychus urticae: effects of density and maternal effects

Dispersal distance is understudied although the evolution of dispersal distance affects the distribution of genetic diversity through space. Using the two-spotted spider mite, Tetranychus urticae, we tested the conditions under which dispersal distance could evolve. To this aim, we performed artificial selection based on dispersal distance by choosing 40 individuals (out of 150) that settled furthest from the home patch (high dispersal, HDIS) and 40 individuals that remained close to the home patch (low dispersal, LDIS) with three replicates per treatment. We did not observe a response to selection nor a difference between treatments in life-history traits (fecundity, survival, longevity, and sex-ratio) after ten generations of selection. However, we show that heritability for dispersal distance depends on density. Heritability for dispersal distance was low and non-significant when using the same density as the artificial selection experiments while heritability becomes significant at a lower density. Furthermore, we show that maternal effects may have influenced the dispersal behaviour of the mites. Our results suggest primarily that selection did not work because high density and maternal effects induced phenotypic plasticity for dispersal distance. Density and maternal effects may affect the evolution of dispersal distance and should be incorporated into future theoretical and empirical studies.

Innate responses of the predatory mite Phytoseiulus persimilis to a herbivore-induced plant volatile

The responses of the predatory mite P. persimilis to herbivore-induced plant volatiles are at least partly genetically determined. Thus, there is potential for the evolution of this behaviour by natural selection. We tested whether distinct predator genotypes with contrasting responses to a specific herbivore-induced plant volatile, i.e. methyl salicylate (MeSa), could be found in a base population collected in the field (Sicily). To this end, we imposed purifying selection on individuals within iso-female lines of P. persimilis such that the lines were propagated only via the individual that showed either a preference or avoidance of MeSa. The responses of the lines were characterized as the mean proportion of individuals choosing MeSa when given a choice between MeSa and clean air. Significant variation in predator responses was detected among iso-female lines, thus confirming the presence of a genetic component for this behaviour. Nevertheless, we did not find a significant difference in the response to MeSa between the lines that were selected to avoid MeSa and the lines selected to prefer MeSa. Instead, in the course of selection the lines selected to avoid MeSa shifted their mean response towards a preference for MeSa. An inverse, albeit weaker, shift was detected for the lines selected to prefer MeSa. We discuss the factors that may have caused the apparent lack of a response to selection within iso-female line in this study and propose experimental approaches that address them.

Maternal manipulation of hatching asynchrony limits sibling cannibalism in the predatory mite Phytoseiulus persimilis

1. Sibling cannibalism is a common phenomenon in the animal kingdom but entails a high risk of direct and inclusive fitness loss for the mother and her offspring. Therefore, mechanisms limiting sibling cannibalism are expected to be selected for. One way of maternal manipulation of sibling cannibalism is to influence hatching asynchrony between nearby laid eggs. This has rarely been tested experimentally. 2. We examined the ability of ovipositing females of the cannibalistic predatory mite Phytoseiulus persimilis to influence the occurrence of sibling cannibalism among offspring by manipulating hatching asynchrony of nearby laid eggs. 3. In the first experiment, we assessed the occurrence of sibling cannibalism in relation to the hatching interval (24 h and 48 h) between nearby laid eggs. In the second experiment, we tested whether ovipositing females discriminate sites containing young (24-h old) and old (48-h old) eggs, fresh and old traces (metabolic waste products and possibly pheromones) left by the same female (24 h and 48 h ago), or young eggs plus fresh female traces and old eggs plus old female traces. Both experiments were conducted with and without prey. 4. Without prey, siblings were more likely to cannibalize each other if the hatching interval between nearby laid eggs was short (24 h). Cannibalism occurred less often when senior siblings (protonymphs) experienced a delay in the opportunity to cannibalize junior siblings (larvae). 5. Independent of prey availability, females preferentially added new eggs to sites containing old eggs plus old female traces but did neither distinguish between young and old eggs presented without own traces nor between fresh and old traces presented without eggs. 6. We discuss cue perception and use by P. persimilis females and contrast the outcome of our experiments and theoretical predictions of sibling cannibalism. We conclude that P. persimilis mothers increase hatching asynchrony of nearby laid eggs to prevent sibling cannibalism on the last produced offspring. Such a behaviour may be considered a simple form of maternal care increasing the survival prospects of offspring.

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

Integration of optimal foraging and optimal oviposition theories suggests that predator females should adjust patch leaving to own and progeny prey needs to maximize current and future reproductive success. We tested this hypothesis in the predatory mite Phytoseiulus persimilis and its patchily distributed prey, the two-spotted spider mite Tetranychus urticae. In three separate experiments we assessed (1) the minimum number of prey needed to complete juvenile development, (2) the minimum number of prey needed to produce an egg, and (3) the ratio between eggs laid and spider mites left when a gravid P. persimilis female leaves a patch. Experiments (1) and (2) were the pre-requirements to assess the fitness costs associated with staying or leaving a prey patch. Immature P. persimilis needed at least 7 and on average 14+/-3.6 (SD) T. urticae eggs to reach adulthood. Gravid females needed at least 5 and on average 8.5+/-3.1 (SD) T. urticae eggs to produce an egg. Most females left the initial patch before spider mite extinction, leaving prey for progeny to develop to adulthood. Females placed in a low density patch left 5.6+/-6.1 (SD) eggs per egg laid, whereas those placed in a high density patch left 15.8+/-13.7 (SD) eggs per egg laid. The three experiments in concert suggest that gravid P. persimilis females are able to balance the trade off between optimal foraging and optimal oviposition and adjust patch-leaving to own and progeny prey needs.